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Research Strategies for the U.S. Global Change Research Program (1990)

Chapter: 6 Terrestrial Trace Gas and Nutrient Fluxes

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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"6 Terrestrial Trace Gas and Nutrient Fluxes." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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6 Terrestrial Trace Gas and Nutrient Fluxes OVERVIEW The composition of the global atmosphere is influenced strongly by the biosphere's activity. Although the importance of photosynthesis and respi- ration in controlling carbon dioxide and oxygen has long been known, the importance of biospheric processes controlling nitrogenous compounds such as nitrous oxide, nitric oxide, and ammonia, sulfur compounds such as hy- drogen sulfide, and various hydrocarbons has only recently been appreci- ated. Moreover, human activities (industrial, agricultural, and others) now affect these natural biospheric processes to such an extent that they may, in many circumstances, overtake some of them in importance. For the first time in the history of the earth, these natural and human-caused atmo- spher~c-biospheric processes may alter the global climate, with potential impacts on human welfare. This chapter was prepared for the Committee on Global Change from the contribu- tions of Paul Risser, University of New Mexico, Chair; Jim Brown, University of New Mexico; Stuart Chapin, University of California, Berkeley; David Coleman, University of Georgia; David Correll, Smithsonian Environmental Research Center; Mary Firestone, University of California, Berkeley; Robert Howarth, Cornell University; Daniel Jacob, Harvard University; Jerry Melillo, Marine Biological Laboratory; Robert Naiman, University of Minnesota; William Parton, Jr., Colorado State University; William Reiners, University of Wyoming; David Schimel, Colorado State University; Robert Sievers, University of Colorado; Richard Sparks, Illinois Natural History Survey; Jack Stanford, University of Montana; Peter Vitousek, Stanford University; and the National Research Council's Committee on Atmospheric Chemistry. Daniel Albritton, NOAA, participated as liaison representative from the Committee on Earth Sciences. 164

TERRESTRIAL TRACE GAS AlID NUTRIENT FLUXES 165 There have always been global changes caused by natural processes such as changes in solar activity, changes in the earth's orbit, volcanism, and plate tectonics. The global changes under consideration today, however, are affected by human activities and include a wide variety of causes and effects, such as stratospheric ozone depletion, tropospheric ozone forma- tion, global warming and sea level change, drought, deforestation, desertifi- cation, and reduction in biological diversity. Climatic change has occurred in the past on many occasions, but the projected rates now are much faster owing to the combination of natural and human-caused processes. A major challenge is to distinguish between these natural and human-influenced changes and to predict their specific and cumulative impacts on the biosphere and its inhabitants. Reliably predicting changes on the global scale of some of these pro- cesses requires an adequate understanding of the cycles of carbon, nitrogen, oxygen, sulfur, and phosphorus. These required understandings involve three important research components (CES, 1989~: · biogeochemical processes occurring within oceans and on the land, geophysical and biogeochemical processes that control the fluxes of compounds between the atmosphere and the aquatic and terrestrial bio- sphere, and meteorological and chemical processes that control the distribution and transformation of chemicals within the atmosphere. Changes in patterns and rates of terrestrial biogeochemical cycling caused by both natural and anthropogenic processes can cause changes in the glo- bal atmosphere; for example, the increase in carbon dioxide and other trace gases in the atmosphere can alter global temperature and rainfall patterns. Conversely, global changes can influence biogeochemical cycling; for ex- ample, global warming can cause an increase in the release of carbon diox- ide and methane from boreal forest and tundra soils. Thus the connections between the atmosphere and the terrestrial biosphere operate in both direc- tions. The bidirectional relationships between the atmosphere and the bio- sphere, and the complexity of these interactions, are the subject of this chapter. Problem Definition Although these atmosphere-biosphere interactions are now recognized as extremely important for environmental changes at the global scale, the physical and biological processes that control the flux rates and magnitudes to and from many ecosystem types are inadequately understood. This lack of un- derstanding is caused by the complexity of these biological and physiochemical systems, by the difficulty of measuring some of these flux exchanges in the

166 RESEARCH STRATEGIES FOR THE USGCRP field, and by the heretofore insufficient attention directed toward these cru- cial studies. Analytical methods must be developed for measuring trace gas and nutrient fluxes under ambient conditions, ecosystems need to be charac- terized in terms of the connections between nutrient pathways and trace gas sources and sinks, the physiological processes and biochemical controls of these processes need to be understood, and these processes must be well enough known to translate the results from local and regional scales to the global scale and to predict their behavior under various conditions of glo- bal change. General Approach The general approach for the research initiative proposed in this chapter is designed to provide an adequate understanding of trace gas fluxes and reservoirs and of the flows of nutrients. The following are addressed in the chapter: · Statement of the most crucial questions to be answered. · Identification of the processes and variables that have the highest pri- ority for attention. Description of the data that will be required to build and test algo- rithms for models describing and predicting these processes. Designation of the most appropriate geographical areas and the envi- ronmental conditions under which the studies should be conducted. Description of the experiments that must be conducted and the data that must be collected. · Method for organizing the resulting data and information into coher- ent data sets and models for describing the processes and for predicting their behavior under alternative conditions of global change. To proceed with this general approach, data and information must be provided from efforts discussed in other chapters of this report. Some of these interactions are shown in Figure 6.1. Arrows A and B refer, respec- tively, to the trace gases and nutrient fluxes that are essential components of the research programs proposed in this report. Conducting these studies will depend on (1) models and measurements describing chemical composition of and reactions in the atmosphere, (2) predictions of changing climate, (3) measurements of changes in land use, and (4) assessing the influence of other anthropogenic activities. Data and information about these input vari- ables will be generated from other coordinated studies in the U.S. Global Change Research Program (USGCRP). The key variables regulating the fluxes of trace gases to and from terres- trial ecosystems vary from gas to gas. Thus, measuring one set of variables for one gas may not be appropriate to the understanding of another gas. On .

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES ll 1 Atmospheric characteristics a-----------_____ , 2 Climate ______ ___ ________ , 3 Land use Ecosystem Function 1 ll - 167 Ecosystem Function J 1 1 Rae Ecosystem ~Ecosystem I I Structure ' Structure ,, 1 1 1 1 1 1 1 1 ll FIGURE 6.1 Coordination of the Mace gas (A) arid nutrient flux (B) studies with Rose described in other parts of die USGCRP (1, 2, and 3~. the basis of our current knowledge, it is possible to predict the necessary variables for modeling each gas. Table 6.1 is a first approximation of a summary of the variables needed to predict the exchanges of the major trace gases discussed in this report. The "influence" characteristics are expressed in general terms only. These variables may affect the fluxes through their influences on biomass loading, leaf resistances (e.g., stomata! opening), plant biological activity, soil chemical activity, microbial activity, or surface layer turbulence. Many of these variables can be mapped from satellite observations, others from land-based surveys. This chapter consists of two related topics, namely, the exchange of radiatively, chemically, and biologically active trace gas species between the atmosphere and terrestrial ecosystems and the fluxes of nutrients within and among landscape units. Trace gas emissions lead directly to local effects but also may move laterally and affect adjacent or more distant landscape units. Materials (e.g., nutrients and pollutants) move within eco- systems but also move laterally in the hydrological cycle when they constitute or are attached to airborne particles. Moreover, lateral flows of nutrients, especially nitrogen and phosphorus, affect the sources and sinks of trace gases. The interactions between Face gas and nutrient fluxes are included in the described research programs. The major global changes affecting the fluxes of water, sediment, nutri

168 RESEARCH STRATEGIES FOR THE USGCRP TABLE 6.1 Environmental Variables Regulating the Fluxes of Trace Gases from Terrestrial Ecosystems Variable Influence Mapping Strategy Surface temperature Solar radiation (PAR) Leaf area index Greenness index (chlorophyll) Vegetation type Plant stress Surface roughness Sensible heat flux Surface wind Soil moisture Soil type Soil chemistry Leaf resistance Plant biological activity Soil chemical activity Microbial activity Leaf resistance Plant biological activity Biomass loading Leaf resistance Plant biological activity Leaf resistance Plant biological activity Leaf resistance Plant biological activity Turbulence Land-based Turbulence Satellite Turbulence Land-based Soil chemical activity Land-based Microbial activity Soil chemical activity Microbial activity Soil chemical activity Microbial activity Satellite Land-based Satellite Land-based Satellite Satellite Satellite Land-based Land-based Lard-based Land-based ents, and pollutants are land use and climate. Of these, altered land use will cause larger changes in these fluxes in the next years and few decades than will climatic change. However, climatic change will also affect lateral water flows and nutrient cycling, which, in turn, will affect trace gas flux and indirectly the climate. Thus there are specific links between climatic change, water and nutrient fluxes, and feedbacks to trace gas flux. Many nutrient cycling studies to date have been conducted in relatively homogeneous areas (Likens et al., 1985~. Much less is known about the transfer of nutrients across boundaries between ecosystems, but such trans- fers may greatly affect trace gas fluxes (Schimel et al., 1989~. Therefore more careful attention should be given to these boundaries in terms of the fluxes that occur across boundaries and their controls. At the global scale, the most significant issue concerning biogeochemis- try is the effect of land use (e.g., cultivation and deforestations on the flows

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 169 of carbon, nitrogen, phosphorus, and sulfur from specific ecosystems and across the landscape. These losses from terrestrial systems occur by several atmospheric and soil surface and subsurface pathways and involve various interlocking biogeochemical cycles. Moreover, these exchanges ultimately affect the productivity and behavior of terrestrial, freshwater, and marine ecosystems. The purpose of this chapter is to describe the crucial research questions, to identify the types of experiments to be conducted, to assess the availabil- ity of existing data, and to identify the locations and types of ecosystems that should receive the highest priority for immediate attention. As such, the recommendations are more specific than those found in the report of the Committee on Earth Sciences (1989), but less specific than some research plans, e.g., the International Global Atmospheric Chemistry (IGAC) pro- gram (Galbally, 1989~. The research needs discussed in this chapter are intended to be comple- mentary to IGAC, a core project of the IGBP, and to address a crucial gap in understanding the fluxes of trace gases and materials to and from terres- trial systems. The focus of IGAC is principally on global atmospheric chemistry, with plans currently under development to include in the program the study of terrestrial sources of trace gases. RESEARCH NEEDS Trace Gases Carbon Dioxide Atmospheric carbon dioxide concentrations have been rising at 0.4 to 0.5 percent per year, apparently faster than ever before in the earth's history. Recently, they have increased even more rapidly. During the last decade, these increases have been associated with increasing amplitude of the an- nual cycle of atmospheric carbon dioxide and possibly surface air tempera- ture of the earth. It is necessary to know the causes and effects of the accelerated rate of increase in atmospheric carbon dioxide, because it is a radiatively active greenhouse gas that has contributed to global warming and will continue to do so and because it has direct effects on ecosystems. Research Priorities. The following research questions, listed in approxi- mate order of priority, must be addressed to determine the causes and con- sequences of increasing atmospheric carbon dioxide. The priorities reflect the perceived importance of each research topic in reducing the uncertainty with which we can predict future changes in carbon dioxide. Chapter 7 addresses ocean-atmosphere interactions.

170 RESEARCH STRATEGIES FOR THE USGCRP · How might climatic warming and associated changes in precipitation and nutrient status alter the biological carbon storage in ecosystems, espe- cially those with large pools of stored soil carbon, through the redistribution of terrestrial ecosystems and through effects of enhanced carbon dioxide concentrations? Profitable approaches to carbon dynamics and global bud- get will be field and laboratory experiments, whole-ecosystem manipula- tions, and modeling, especially in tundra, boreal forest, and peat bog ecosystems, where there are large stores of organic carbon and where relatively large temperature changes may occur. · How do increased atmospheric carbon dioxide and associated changes in moisture, temperature, and nutrients affect plant litter quality and the associated changes in soil respiration and nutrient mineralization? What are the effects of the resulting changes in nutrient availability on plant and microbial processes and on the sensitivity of intact ecosystems to enhanced carbon dioxide? This issue is best approached with a combination of field and laboratory experiments. These studies should be done in a range of ecosystems (e.g., wet versus dry and fertile versus infertile) where the strength of feedbacks between nutrient cycling, litter quality, and plant response to carbon dioxide might be expected to differ. The role of soil nutrient status is important in the context of global change, because industrial and agricul- tural pollution have dramatically increased the nitrogen availability of some ecosystems. Interactions of water availability and carbon dioxide fertiliza- tion must be studied, because projected climatic changes will involve changes in both parameters. · How do ecosystem processes and different functional groups of plants (or specific key species) belonging to different ecosystems respond directly to changes in carbon dioxide and temperature in terms of rates of photosynthesis, allocation and net carbon balance, and indirectly in terms of competitive ability and such secondary processes as resistance to pathogens and herbivores? How are these carbon dioxide responses altered by interactions with other environmental stresses (e.g., drought, ozone, and nutrients)? Which ecosystems are the most sensitive? This research item differs from the item above in that it is plant-oriented rather than ecosystem-oriented and, as such, requires experiments at the level of individual plants. · How would altered hydrological regimes predicted by global climatic models affect ecosystem carbon balance through changes in productivity and respiration in the short term and the characteristics of and the geo- graphical distribution of ecosystem types in the long term? This issue must be approached through modeling and field and laboratory experiments. · Why don't the perceived sources and sinks match the interhemispheric carbon dioxide studies? Much is known about the major sources and sinks for carbon dioxide and the global pattern of carbon dioxide transport in the

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 171 atmosphere, but currently the perceived sources and sinks for carbon diox- ide do not match the interhemispheric carbon dioxide gradient. In addition, the processes affecting trace gases (e.g., carbon dioxide, methane) in the paleoecological record must be reconciled with current understanding of carbon dioxide sources and sinks (see chapter 3~. These issues should be addressed through the continuation and expansion of current programs to collect information that will most effectively differentiate among possible sources and sinks of carbon dioxide, e.g., enhanced plant growth and soil organic matter accumulation, fossil fuel burning, tropical burning associated with land clearing, and enhanced decomposition in boreal ecosystems. An expanded network for measuring atmospheric carbon dioxide and detailed isotopic measurements are needed to localize the major current sources and sinks for atmospheric carbon dioxide and to validate models that deal with the seasonal effects of terrestrial vegetation on atmospheric carbon dioxide. What are the consequences of landscape conversions, such as that of tropical forest to grassland, in terms of changes in stored soil carbon, evapotranspiration, energy balance, carbon balance, and nutrient status? Field measurements in appropriate ecosystems and modeling are needed to ad- dress this question. · What is the effect of climatic change on episodic events such as fire frequency, the amount of carbon released, the resultant change in vegeta- tion, and consequent changes in albedo, evapotranspiration, and plant pro- duction? How would the effects of human activities relate to those caused by climatic change? These effects should be addressed with satellite monitoring of disturbances such as fires and then related to surface moisture, tempera- ture, and biomass. Patterns in natural and modified savannas, the boreal forest, and the tropics are of particular interest. · What are the pools of biomass and soil carbon, net primary produc- tion, and ecosystem respiration in the world's ecosystems? All of the con- siderable field data need to be adequately collated and related to vegetation and soil maps for inclusion in global climate models. New data must be acquired by remote sensing of surface temperature, surface moisture, atmo- spheric water vapor concentration, and indicators of vegetation production and biomass. All of these research questions address carbon balance at the ecosystem or global level and therefore are readily incorporated into ecosystem, re- gional, and global models. The major challenge will be designing models and experiments that link studies at the ecosystem level with inputs and predictions at regional and global levels (see chapter 5~. It is important to recognize that as climate changes, so will the structure and species compo- sition of these ecosystems. Thus models of ecosystems for today's circumstances

172 RESEARCH STRATEGIES FOR THE USGCRP will be inadequate for ecosystems for tomorrow, and therefore the models and analyses must be adaptable to changing ecosystem characteristics. The evolutionary approach described in chapter 2 is critical to success. Methane The concentration of methane is increasing in the atmosphere at a rate of about 1 percent per year and has approximately doubled in the past few hundred years. Methane is a greenhouse gas that, on a molecule-for-mol- ecule basis, is about 20 times more effective than carbon dioxide in trapping heat. In addition to its role as a greenhouse gas, methane is an important sink for the hydroxyl radical in the atmosphere. The hydroxyl radical is the primary agent responsible for the oxidation and subsequent removal from the atmosphere of many reduced radiatively, chemically, and biologically important atmospheric gases. Depending on atmospheric nitrogen oxide concentrations and other chemical parameters, methane increases can change the atmospheric concentrations of the hydroxyl radical and hence change the atmospheric lifetimes and concentrations of several important gases, which would lengthen the time over which a species like methane contributes to radiative forcing of the climate system. Also, methane is an important source of water vapor in the stratosphere, and increases in stratospheric water vapor can have other significant global consequences. Although the major sources of atmospheric methane are for the most part known, there is great uncertainty about the relative importance of these sources and which combination of sources and sinks is responsible for the rapid buildup of this gas in the atmosphere. The mechanism of methane production is fairly well known and results from anaerobic microbiological processes in wetlands, rice fields, and ruminants. Less well known are the environmental, physical, and biological processes that control the release of methane to the atmosphere. There are also major uncertainties about the anaerobic and aerobic sinks of methane in soils and sediments. One of the major questions that needs to be addressed is how changes in climate (e.g., warming in the northern high latitudes) may affect the global methane cycle. Methane sources in the tundra and wetland regions of the subarctic are major natural sources of atmospheric methane. A better understanding of ecosystem processes that control methane fluxes to and from the atmo- sphere and the impact that environmental changes may have on these processes is required if reasonable predictions of future atmospheric concentrations of methane are to be made. In addition, much of the methane in the high- latitude north is sequestered in permafrost and sediments as clathrates, which could serve as very significant sources of atmospheric methane if warming occurs. Similarly, a rise in sea level or warming of the oceans could also release marine clathrates. While this methane sink has been identified, its

TERRESTRIAL TRACE GAS AND NUTRIENT FL=ES 173 characterization, e.g., the temperature dependencies of the chemical reac- tions, needs to be better quantified. Significant research is in progress on methane, and new studies should be coordinated with projects proposed by NASA, the International Global Atmospheric Chemistry Program, and IGBP. Research Priorities. The research activities required to better define bio- geochemical budgets and cycling of methane were detailed in a recent Dahlem conference (Schimel et al., 1989~. The major research activities required under the USGCRP are as follows: Process studies that relate methane production, consumption, and fluxes to environmental parameters, to human activities such as burning and live- stock farming, and to changes in ecosystem structure and function need to be conducted for ecosystems of known or potential methane sources (e.g., wetlands, tundra, rice agriculture, and landfills). The study of the response of high-latitude northern ecosystems to environmental change should be studied through large-scale manipulative field experiments. In major rice- growing areas (e.g., India and China) the effect of cultivation practices on methane production, destruction, and atmospheric fluxes requires attention. Also, atmospheric pollutants could affect trace gas fluxes. Thus process studies must include interactions with pollutants. Improved instrumentation for the direct measurement of methane fluxes over small- and large-scale regions must be developed in order to improve our understanding of the relationship between fluxes and ecosystem processes and dynamics. Emphasis should be placed on the integration, or scaling, of information obtained from simultaneous chamber, tower, and aircraft flux measurements. . Better spatial and temporal coverage of atmospheric methane concen- trations and isotopic composition (carbon and hydrogen) in source regions must be obtained in order to apportion the global sources of atmospheric methane and understand the ecological and environmental controls of meth- ane releases to the atmosphere. More extensive studies of isotopic compo- sition of methane as a function of source and production and destruction processes should be made in order to use atmospheric isotopic information to better understand the biogeochemical budgets and cycle of methane. With such data it will be possible to more accurately model and determine the regional fluxes of methane to the atmosphere. · In order to fully understand the atmospheric methane cycle, improved estimates of the atmospheric oxidation by the hydroxyl radical must be obtained. This requires a more complete understanding of atmospheric photochemistry than is currently available. Specifically, it is necessary to either directly or indirectly determine the concentration of hydroxyl radical in the atmosphere and the chemical processes that control this concentra- tion. Thus it will be necessary to determine if a portion of the methane

174 RESEARCH STRATEGIES FOR THE USGCRP increase results from a reduction in hydroxyl radical concentrations (and hence a diminished oxidizing capacity of the atmosphere) through increases in the atmospheric concentrations of hydroxyl radical sinks, e.g., methane, carbon monoxide, and volatile organic compounds. Volatile Organic Compounds Depending on oxides of nitrogen concentrations, a number of volatile organic compounds (VOCs) are photochemical sources or sinks of tropo- spheric ozone a toxic gas and a greenhouse gas and as such they may play a significant role in global warming (see the section "Tropospheric Ozone" below). In addition, VOCs compete for oxidation by the hydroxyl radical with other atmospheric species, in particular methane; changes in the VOC budget could therefore affect the methane budget. The importance of the terrestrial biosphere as a source of VOCs is still poorly understood (Logan, 1985~. Only a fraction of the large number of biogenic VOCs have been identified in the atmosphere, and few data on emission rates are available. To identify and quantify the role of VOCs in atmospheric processes, it will be necessary to establish a comprehensive inventory of biogenic VOCs in the atmosphere, their emission rates from different types of ecosystems, and the environmental variables determining these emission rates. Addi- tional studies of the chemistry of biogenic VOCs need to be made in the laboratory. Research Priorities. The following research needs are listed in order of . . prlorlty: Accurate techniques for identifying and measuring individual and cu- mulative VOCs fluxes and atmospheric concentrations (down to the pptv range) must be developed. A top priority should be to develop analytical instrumentation that can be operated from aircraft or better sampling and Reconcentration techniques, since concentrations seem to be affected by sample storage. Once such instrumentation is available, large-scale field studies of atmospheric concentrations should be conducted to determine the regional and continental budgets of biogenic VOCs. Particular focus should be placed on tropical and mid-latitude forests, as biogenic VOCs may be strong modifiers of atmospheric photochemistry over these regions (Logan, 1985; Tingey et al., 1979~. · Improved measurements of fluxes are needed. The two methods cur- rently used are (1) branch enclosure measurements and (2) inversion of measured atmospheric concentrations using chemistry-transport models. These methods have provided valuable information, but they are not fully satisfac- tory. Branch enclosure measurements are intrusive, and the resulting emission data will be biased to the degree that the biological functioning of the

TERRESTRIAL TRACE GAS AND NUTRIENT FLU}fES 175 enclosed branch is impaired. Use of chemistry-transport models suffers from our poor understanding of the atmospheric reactivities of biogenic VOCs and of their decomposition products. Development of fast-response instrumentation for measuring atmospheric concentrations of biogenic VOCs is a top priority; such instrumentation would allow direct, nonintrusive measurements of fluxes by the eddy correlation technique. The sensitivities of biogenic VOC emissions to environmental factors need to be determined in the field and in the laboratory. Most data available so far are measurements of the effect of light and temperature on the emis- sions of isoprene and pinenes, for a few plant species (e.g., Tingey et al., 1979; Yokouchi and Ambe, 1984~. Relatively few data are available on the effects of other potentially important factors such as water stress, air pollution stress, and fertilization. Laboratory studies should be aimed at increasing our understanding of the fundamental biotic mechanisms controlling the emissions of biogenic VOCs by various types of vegetation. · The atmospheric reactivities of additional biogenic VOCs eventually need to be determined in the laboratory. Rate measurements of the reac- tions with the hydroxyl radical, ozone, and nitrogen trioxide made to date have allowed an assessment of the potential of specific VOCs to play a significant role in atmospheric chemistry. Reaction mechanisms for species found to be important should be investigated in detail, with a focus on the fate of the decomposition products (particularly the short-lived organic radicals). Environmental chamber experiments would provide a first assessment of the potential of biogenic VOCs as photochemical precursors of ozone and carbon monoxide, and as sinks for the hydroxyl radical and ozone. More precise kinetic investigations should also be conducted to understand the fundamental chemical mechanisms involved in the photochemical decom- position of biogenic VOCs. Sulfur Soils and terrestrial plants are known to emit a number of reduced sulfur species to the atmosphere including dimethylsulfide (DMS), hydrogen sul- fide (H2S), carbonylsulfide (COS), carbon disulfide (CS2), and methyl mer- captan (CH3SH). Coastal salt marshes and wet tropical soil might be significant sources of H2S (Delmas and Servant, 1983; Andreae, 1985; Andreae et al., 1988; 1989~. The available data indicate that these terrestrial emissions are in general much weaker than biogenic emissions from the oceans and are dwarfed by sulfur emissions from anthropogenic sources. Emissions from salt marshes could account for a significant portion of the global atmo- spheric sulfur budget, despite the small area involved (Andreae, 1985~. Emissions from tropical soils and vegetation appear to be responsible for the back- ground concentrations of sulfate observed over the Amazon Basin (Andreae

176 ° Plankton emit DMS RESEARCH STRATEGIES FOR THE USGCRP DMS forms small aerosols (a) Aerosols initiate cloud formation (a) Cloud cover alters albedo _ ......... . .......... Altered radiation influences plankton FIGURE 6.2 Proposed relationship between oceanic plankton and cloud cover. et al., 1990) and could thus regulate cloud structure over tropical forests under pristine atmospheric conditions. The terrestrial biosphere could be a factor for climate regulation through its effect on the atmospheric budget of COS. COS is the longest-lived sulfur species in the atmosphere; it can be transported to the stratosphere, where it provides a source of stratospheric sulfate, thereby affecting plan- etary albedo (Figure 6.2~. Preliminary studies have indicated rapid uptake of COS at the stomata of plants, suggesting that vegetation could provide the major global sink for COS (Golden et al., 1988~. The topic of acid deposition is discussed in the section "Nitrous Oxide and Reactive Nitrogen Compounds" (below). Research Priorities. priority. The following research needs are listed in order of · Measurements of biogenic sulfur emissions are needed from many more types of terrestrial ecosystems. Measurements of H2S, DMS, CS2, and CH3SH should focus on regions thought to be potentially significant re- gional and global sources: wet tropical regions, coastal marshes, boreal forest peatlands, and tundra bogs that could be affected by changes in per- mafrost; anaerobic environments such as rice paddies and landfills; and industrial sources. The global distribution of terrestrial biological sinks and sources of COS needs to be quantified. Environmental variables affecting biogenic sulfur emissions need to be better understood. Preliminary studies suggest that vegetative emissions depend on temperature and insolation (Andreae et al., 1989), but the under- lying mechanisms are unknown. Particular focus should be placed on un

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 177 derstanding the factors that regulate H2S emissions from salt marshes, as climatic change could alter dramatically the global surface area occupied by these ecosystems. . Possible increases in reduced sulfur emissions to the atmosphere as a function of nutrient and sulfate inputs need to be investigated, particularly for the above ecosystems. · Biomass burning has been proposed as a major source of atmospheric sulfur over tropical regions (Andreae et al., 1988), but very few data are available. Aircraft and land-based studies are needed to document the pos- sible importance of this source. · The role of terrestrial ecosystems as a sink for atmospheric COS needs to be investigated further. No data are available for the tropical forests, where vegetative uptake of trace gases in general appears to be particularly efficient. · At night, plants appear to constitute net sinks for H2S and DMS; research is needed to understand the mechanisms for this uptake. Tropospheric Ozone Concentrations of tropospheric ozone in the northern hemisphere appear to have risen steadily over the past few decades (Logan, 1985), and its photochemistry is the major source of the hydroxyl radical near the earth's surface. Tropospheric ozone a greenhouse gas is produced within the troposphere by oxidation of carbon monoxide and VOCs in the presence of nitrogen oxide. Ozone also enters the troposphere from the stratosphere. It is removed by photolysis, chemical reactions, and deposition to the earth's surface. The observed increase of ozone concentrations in the northern hemisphere is generally attributed to anthropogenic emissions of nitrous oxide, carbon monoxide, and hydrocarbons. New studies are required, but efforts are under way in the International Global Atmospheric Chemistry and the NASA Earth Observation System programs. Biosphere-atmosphere interactions may play an important role in the glo- bal climate and budget of tropospheric ozone. First, deposition through the stomata and on the cuticles of plants is thought to provide a major sink. Second, interactions of biogenic VOCs with nitrous oxide of anthropogenic origin could elevate ozone production substantially over preindustrial lev- els, thus contributing to the rise in ozone concentrations. Much has been learned, however neither of these processes is thoroughly understood, and there is a serious need for further research. Finally, ozone injury to vegetation may alter the function of ecosystems, and eventually their structure, with possible feedbacks on climate.

178 RESEARCH STRATEGIES FOR THE USGCRP Research Priorities. The following studies include those of the highest . . priorities. · The highest priority is to improve the data base for ozone deposition to various terrestrial ecosystems. Eddy correlation measurements from tow- ers and aircraft are delicate but provide at this time the best means to collect such data. Measurements from towers are particularly useful as they allow simultaneous monitoring of the environmental factors likely to influ- ence ozone deposition fluxes. These factors include micrometeorological variables (e.g., heat flux, humidity, temperature, friction velocity, and light intensity within the canopy) and parameters of ecosystem structure and function (e.g., leaf area index, surface roughness, and stomata! and cuticular resistances). Models should be designed to relate the measured ozone fluxes to fundamental meteorological and ecosystem properties (cf. Meyers and Baldocchi, 1988~. Tropospheric ozone is variable in distribution, especially near anthro- pogenic sources. Ozone affects the growth of vegetation directly and indi- rectly affects the ability of plants to respond to secondary influences such as drought, insects, and other pollutants. It is necessary not only to gener- ate ozone concentration and flux data but also to understand the effects of ozone on vegetation, especially in relation to other environmental condi- tions. . The physiological and chemical processes controlling ozone uptake, the release of other chemicals by plants stressed by ozone exposure, and the responses of various vegetation types to ozone exposure should be investi- gated in the laboratory and field. At the level of individual plants, data are needed for the rates of reaction of ozone at the plant mesophyll and at the plant cuticle. If most of the ozone uptake by plants takes place at the stomata, then changes in ecosystem function could have important implica- tions for ozone deposition. An interesting issue, particularly in light of the effects of enhanced carbon dioxide on stomata! closure, is the possibility of stomata! closure due to ozone stress. Such an effect would introduce a positive feedback to the rise in tropospheric ozone levels. On the other hand, chamber exposure to whole plants and segments of ecosystems dem- onstrates significant responses to elevated ozone levels. The generality of this process must be evaluated in both urban areas and more remote areas that are influenced by human activities. · Large-scale field measurements of atmospheric composition should be conducted to evaluate the contributions of biogenic VOCs and nitrous oxide to ozone production (see NRC, 1984; Lenschow and Hicks, 1989~. As pointed out in the section "Volatile Organic Compounds" above, biogenic VOCs could be important photochemical precursors or sinks of tropospheric ozone, although more data are needed to determine their concentrations and reactivities in the atmosphere. Long-range transport-photochemistry mod

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 179 els should be developed to interpret the observations of atmospheric compo- sition in terms of ozone production rates and estimate the contributions from biogenic emissions to these production rates. Carbon Monoxide Carbon monoxide is emitted directly to the atmosphere by human activ- ity (e.g., fossil fuel burning and biomass burning) and is also produced by atmospheric oxidation of hydrocarbons. Emission of carbon monoxide by the biosphere may be globally important, but the data base is very limited. Carbon monoxide has an atmospheric lifetime of a few months against oxidation by the hydroxyl radical, its principal sink. There is strong evidence that atmospheric concentrations of carbon mon- oxide are increasing in the northern hemisphere as a result of anthropogenic emissions. Because of competition between carbon monoxide and methane for oxidation by the hydroxyl radical, a rise in carbon monoxide could have consequences for a parallel rise in methane. Also, carbon monoxide is a photochemical precursor of ozone, so that enhanced production of tropospheric ozone could follow from higher carbon monoxide concentrations (Logan et al., 1981~. Thus increases in carbon monoxide concentration can contribute to global warming by causing increases in the atmospheric concentrations of two major greenhouse gases. Only a few measurements of biosphere-atmosphere exchange of carbon monoxide have been reported in the literature. It appears that soils can be both sources and sinks of carbon monoxide, the net direction of the flux depending strongly on the environmental conditions (Conrad and Seller, 1985~. Some preliminary measurements suggest that plants are a significant source of carbon monoxide (Seller, 1978), but the research is incomplete. Measured atmospheric concentrations of carbon monoxide in tropical for- ests of Africa and South America indicate evidence for a strong direct natural source of carbon monoxide (Marenco and Delaunay, 1985~. Oxida- tion of biogenic VOCs constitutes another potentially important natural source of carbon monoxide (Hanst et al., 1980~. Research Priorities. The available literature does not permit an assessment of whether emissions of carbon monoxide from soils and plants could sig- nificantly affect the carbon monoxide budget on a global scale. There is a need for exploratory research aimed at addressing this issue. The geographical data base for carbon monoxide natural emissions should be expanded, par- ticularly in tropical regions. Fast-response instrumentation for measuring atmospheric carbon monoxide concentrations is now available, so that eddy correlation flux measurements can be conducted from towers and from air- craft. Regions where biogenic fluxes of carbon monoxide can make a significant

180 RESEARCH STRATEGIES FOR THE USGCRP contribution to the atmospheric budget must be identified, and the environ- mental factors affecting carbon monoxide emission in these regions must be examined. Finally, fundamental laboratory studies are needed to understand the biotic and abiotic processes regulating the biogenic flux of carbon monoxide. Nitrous Oxide and Reactive Nitrogen Compounds The release and uptake of nitrogen-containing trace gases by ecosystems have important implications for atmospheric composition and nutrient fluxes and cycling. Nitrous oxide, a significant greenhouse gas with a long atmo- spheric lifetime, is the most important agent in natural ozone destruction in the stratosphere and has been increasing in atmospheric concentration at the rate of about 0.25 percent per year. Nitrous oxide is formed in soils by both Vitrification and denitrification processes in both natural and agricultural ecosystems. Nitric oxide, also produced in soil and released to the atmo- sphere through nitrogen cycling, is a chemically reactive gas that regulates tropospheric ozone production. Nitnc oxide is also released to the atmosphere in significant quantities through the burning of biomass. Ammonia is released to the atmosphere from plants, fertilized soils, and animal wastes. Ammonia is the primary basic gas in the atmosphere and can play a major role in controlling the acidity of precipitation. Ammonia can be a significant vector for the medium-range transport of nitrogen into and from ecosystems. Nitric acid is produced in the atmosphere through the oxida- tion of nitric oxide and nitrogen dioxide and is rapidly deposited through dry and wet deposition to the surface of the earth. Deposition of nitric acid (and of nitrate salts resulting from the neutralization of nitric acid) can be an important nutrient input to ecosystems and, since nitric acid is strong, can also contribute to ecosystem stress. The input of nitric acid and its nitrate salts (as well as sulfur oxides near sulfur sources) can be especially important to ecosystems near regions where anthropogenic nitrogen oxide emissions are large (e.g., northeastern United States, Central Europe). Sec- onda~y aLkyl nitrates and peroxyacetylnitrate (PAN) are formed in the atmosphere and can transport nitrogen from urban to rural areas, affecting ozone levels. It is necessary to obtain an improved understanding of how the budgets and cycling of nitrogen within ecosystems are connected to the atmospheric fluxes of nitrogen-containing gases. Research Priorities. The following priorities are identified: · Because of the important role of nitrous oxide as a greenhouse gas and the large uncertainties about sources, increasing our understanding of the ecological and environmental factors that control the atmospheric source strength of this gas for different geographical locations is a high priority.

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 181 The NSF Long-Term Ecological Research Program and the proposed IGBP Regional Research Centers and the Tropical Soils Biology and Fertility (TSBF) programs provide a framework for such studies. These studies will require direct flux measurements of nitrous oxide from different ecosystems over a wide range of ecological and environmental conditions. Tropical areas should initially receive a high priority for such studies, since these areas, which are believed to be important nitrous oxide source regions, are undergoing rapid changes in land use practices that are expected to signifi- cantly alter production and the fluxes of nitrous oxide to the atmosphere. · The topic of acid precipitation, including both nitrogen and sulfur compounds, requires significant additional study. In particular, there needs to be a far better understanding of the conditions under which this process results in fertilization of ecosystems and of the conditions that result in a toxicity from acid precipitation itself or from other pollutants. Moreover, the research should not only aim to understand the effects of acid precipita- tion on the biosphere and the consequences for trace gas and nutrient fluxes, but also study the secondary consequences such as the release of aluminum and other materials from soils receiving acid precipitation. Improved understanding about the amount of nitrous oxide produced through the fertilization of agricultural systems must be developed through process studies designed to understand the mechanisms relating nitrous ox- ide and ammonia fluxes and the type of fertilizer, agricultural practices, application method, crop structure, soil type, and prevailing climate. · Mechanistic ecosystem models that relate nitrous oxide fluxes to soil microbiology, micrometeorology, soil type, and environmental conditions must be developed, and the processes better understood. · The global distribution of biological nitric oxide and ammonia fluxes to the atmosphere needs to be better established, and the importance of these emissions in atmospheric photochemistry and precipitation chemistry defined. · Improved fast-response instrumentation for direct determination of fluxes by micrometeorological techniques of all nitrogen-containing trace gases over both small and large spatial scales must be developed. Such instrumentation will permit a more detailed assessment of the biological source strengths of atmospheric nitrous oxide, ammonia, and nitric oxide and allow a more accurate determination of the atmospheric input of nitrogen to selected ecological regimes. · The uptake of nitrogen oxides and ammonia by terrestrial plants needs to be investigated. Preliminary studies suggest that this uptake could provide a significant sink for several gas species in some regions. For example, trees in the Netherlands are known to be major sinks for ammonia released by agricultural operations, and such a process could be important in other regions as well. Other recent studies have suggested that vegetation can

182 RESEARCH STRATEGIES FOR THE USGCRP provide a sink for nitrogen dioxide, which would inhibit the export of soil- derived nitric oxide to the atmosphere. Nutrient and Material Fluxes Fleeces Across Terrestrial Systems Under current natural and human-influenced conditions, massive amounts of sediment, nutrients (including fertilizers), and pollutants are transferred across terrestrial portions of the biosphere and into streams and eventually the oceans. The increase in transport due to human activity above natural rates has assumed major proportions with global biogeochemical cycles. These transfers are caused by various land uses and are affected by changes in global climate, and, in turn, these fluxes affect global climate by influ- encing hydrology, trace gas exchange, and other processes. A basic research issue is to understand the flows of water, sediment, nutrients, and pollutants across terrestrial systems and into the air or aquatic systems and the responses of these systems. We must determine how global climatic change will affect these fluxes, especially the reciprocal interactions between the hydrosphere and the biosphere. To accomplish this objective, the fluxes of these materi- als will be quantified across representative ecosystems subjected to an array of land use activities. Site-specific information will be incorporated into hierarchical models for achieving global descriptions of the current conditions and for predicting the consequences of future changes in land use and cli- mate. The general research issue will be addressed by the following four steps: · Establish a network of accurate measurements of gaseous and hydro- logic nutrient fluxes across landscapes representing major ecosystems that have received minimal impacts from human activities. These experimental landscapes will be paired with those receiving various land management practices (Gosz et al., 1988; Jordan et al., 1986; Lowrance et al., 1985; Peterjohn and Correll, 1984~. Examples of land use changes that must be studied in appropriate regions include whole tree harvesting, introduction of multiple cropping and irrigation, conversions between forest-grassland-cropland, and various urbanization scenarios. Watersheds have proved to be powerful experimental approaches for biogeochemical studies (Likens et al., 1985) and should for a major component of these experiments. The watershed experimental designs permit an analysis of inputs and outputs to the re- search landscapes, and, in addition, the discharge streams become integrat- ing measures of biogeochemistry of the area under study. · Processes that control the fluxes of material will be described by ecosystem type. Models describing these processes will be driven by variables subject to climatic change, thus allowing a prediction of changes in flux rate as a

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 183 function of global climatic change. To describe these fluxes and to predict both the effects of land use and global change on these fluxes and their feedback on global change, an understanding of various control factors must be included. Field tests and models will be developed relating fluxes of water, sediment, nutrients, and pollutants to interactive fluxes of trace gases be- tween the biosphere and the atmosphere. Substantial progress has been made in the development of ecosystem models that simulate dynamics of material flows and nutrient cycling with plant production for specific systems (Parson et al., 1987; Pastor and Post, 1986~. These models have been tested and validated using site-specific data. Recently, these models have been used to simulate regional ecosystem dynamics by linking ecosystem-level models to regional geographic information systems (GISs). The GIS systems contain information about the spatial variations in the soils, land use, and climatic variables for a region. The linked GIS and ecosystem model systems (Burke et al., 1990; Welch et al., 1988) use the GIS system to provide the driving variables for simulation of the dynamics of the plant and soil system for the spatial grid. · Models will be developed for each region of the biosphere relating these material fluxes to land use and to climate variables. These models will then be used for identifying the regions and land uses most susceptible to climatic change. Research Priorities. The following three priorities should be addressed by comprehensive studies. · The fate and effects of nitrogen and other nutrients deposited on ter- restrial ecosystems. Nitrogen is added to terrestrial systems by natural processes (e.g., lightning) and in large amounts through fertilizer applica- tion and as air pollutants. Once on the landscape, nitrogen accumulates in the vegetation and soil, and eventually in the groundwater, streams, estuar- ies, and oceans (Gildea et al., 1986; Kempe, 1988; Vorosmarty et al., 1986~. Similar processes also add sulfur and other nutrients to terrestrial systems. The fate and effects of added nitrogen and other materials must be quanti- fied in each of the components of major ecosystems, especially as they undergo changes in climate and land use. Particular attention must be paid to nitrogen because of its relationship with the global carbon budget, i.e., the temperate zone is both the recipient of significant amounts of nitrogen and a major sink for carbon dioxide. Thus these local processes are also important at regional and global scales. · Nutrient transfers caused by cultivation. Under cultivation (e.g., crop- ping and forest harvest), nutrients are lost from terrestrial ecosystems. This loss of soil fertility has enormous impacts on the long-term productivity of the landscape and on the sustainable development of cropping systems.

184 RESEARCH STRATEGIES FOR THE USGCRP Fertility loss is caused by the interactions of cropping systems, soil condi- tions, and the prevailing climate. Despite some understanding of nutrient loss under standard agricultural practices in some well-studied regions (Beaulac and Reckhow, 1982; Bowden and Bormann, 1986; Lowrance et al., 1985; Robertson and Tiedje, 1987; Schimel et al., 1985), these processes are not known for many major ecosystem types. These losses are to the atmo- sphere, via surface flow and into the groundwater. The research questions that must be answered are how are these materials distributed, what are the pathways of these materials, and what are the consequences to the recipient ecosystems, i.e., how are these chemicals processed in the recipient terres- trial and aquatic systems? Moreover, these distribution and processing questions must be answered for systems undergoing changes in land use and climate. These are regional processes that must be aggregated to the global scale because land use patterns are effective at the local scale, but climatic changes occur at broader scales. Aeolian and alluvial erosion. In arid and semiarid regions, and possi- bly in polar regions, there is a significant redistribution of earth surface materials by aeolian and alluvial processes (Schimel et al., 1985~. Climatic change in the paleorecord has also been associated with these airborne and waterborne materials (see chapter 3~. That is, dust in the record is an indicator of climatic change. In addition, changes in phosphorus, iron, and other materials by these erosional processes also affect freshwater, estuary, and ocean systems. Thus these aeolian and alluvial processes are both an indicator of climatic change and a consequence of changing climate. Re- search approaches include coupling processes occurring at various spatial scales. For example, rain simulators and shelters could be used to deter- mine the effects of climatic change on erosional processes at the sources, and cesium-137 techniques may be applicable to determining the distribution patterns of the material that is moved by wind and water. It may be pos- sible to link a particle sensing network for airborne materials to changes in characteristics of the ecosystem that can be monitored by remote sensing. In all three of the research priorities, there is a need to link airsheds and watersheds. Landscapes are heterogeneous, and to answer.these questions it is necessary not only to determine the net exchange of materials within and among ecosystems but also to understand how materials are processed in these heterogeneous regions. The study of large drainage basins, with het- erogeneous land uses and natural features, will be an important research approach. These basins include biogeochemical processes in terrestrial eco- systems and in various impoundments, streams, and margins along different land use types. Materials are transferred, sequestered, and processed in dif- ferent ways within the heterogeneous basins. The behavior of these basins and their interactions with the atmosphere must be understood on the global scale.

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 185 Fluxes from Terrestrial to Coastal Marine Systems Estuaries and coastal seas are increasingly influenced by human activi- ties and are rapidly being degraded in many regions of the world. Although toxic substances in estuaries are of major concern, much of the degradation of estuaries and coastal seas can be traced to land use practices, eutrophica- tion, and the development of anoxic waters (Kemp et al., 1983, 1984; Larsson et al., 1985; Nixon, 1982; Officer et al., 1986; Price et al., 1985~. Net primary production in many estuaries is limited by nitrogen, and so eutrophication of these systems is caused by excessive inputs of nitrogen (Boynton et al., 1982; D'Elia et al., 1986; Howarth, 1988~. The export of nitrogen from terrestrial ecosystems ("nonpoint sources") accounts for half or more of the total nitrogen inputs to many major estuaries, such as Delaware Bay and Chesapeake Bay (Correll, 1987; Nixon and Pilson, 1983; Nixon et al., 1986~. Thus any change in the functioning of terrestrial ecosystems over large scales is likely to have a major effect on the downstream estuarine and coastal marine ecosystems, in many ways the ultimate receivers of sub- stances exported from these terrestrial ecosystems. There are two major concerns of eutrophication in the coastal zone. These are the effect on the coastal production of fisheries and wildlife and the effect on atmospheric fluxes of trace gases, particularly nitrous oxide and dimethylsulfide. Hypoxic waters now occur on the Louisiana coastal shelf near the Mississippi Delta over areas as large as, or larger than, similar phenomena reported on the East Coast (Turner et al., 1987~. The increase appears to be attributable to increased nitrogen delivery and reduced sedi- ment delivery by the Mississippi River. Sediments in the Mississippi have been trapped by upstream dams, beginning in the 1950s (Meade and Parker, 1985~. Decreased suspended sediments and increased water clarity and nitrogen are expected to increase phytoplankton production, which is light- and nitrogen-limited. Increased settling of phytoplankton, zooplankton, and organic material into bottom waters and removal of oxygen by decomposi- tion may explain the expansion of the hypoxic zones (Turner et al., 1987~. Virtually all large rivers of the world show similar patterns in sediment and nutrient delivery because of land use and dams. Resulting increases in phytoplankton production and settling in coastal zones may have a signifi- cant effect on the uptake of carbon dioxide from the atmosphere and long- term carbon storage. Coastal marine ecosystems have been postulated to be important sources of both nitrous oxide (Seitzinger et al., 1983) and dimethylsulfide (Andreae and Raemdonck, 1983~. Fluxes of both of these can be expected to increase if rates of net primary production increase (Seitzinger et al., 1983; Andreae and Raemdonck, 1983~. The movement of nitrogen and other substances from terrestrial ecosys- tems to estuaries and coastal marine ecosystems may be altered by at least three factors: (1) changes in land use and stream flow by stream regulation,

186 RESEARCH STRATEGIES FOR THE USGCRP (2) changes in atmospheric inputs to the terrestrial ecosystems, and (3) changes in climate. Each of these is briefly discussed below. 1. Land use. That changes in land use alter nutrient and sediment export from terrestrial ecosystems is well known in a qualitative sense, and yet this has only been well studied in a relatively few areas. Most studies have concentrated either on the effects of disturbance on element export from forested ecosystems (Bormann and Likens, 1979) or on export from agroecosystems (Beaulac and Reckhow, 1982; Lowrance et al., 1985~. Some studies on element export from urban and suburban environments exist, but are of a site-specific nature. Some fairly sophisticated models are available for analyzing element export as a function of land use (Delwiche and Haith, 1983; Haith and Shoemaker, 1987), but these have been only partially tested, and their treatment of export from suburban and urban environments tends to be simplistic. One potential difficulty in applying such models to ele- ment export from terrestrial ecosystems to estuaries is that they do not allow for processing of substances within rivers, including riparian zones, natural main stem lakes, and man-made reservoirs, floodplains, and deltas (Costanza et al., 1990; Howarth et al., 1990; Mulholland, 1981; Vorosmarty et al., 1986~. 2. Atmospheric inputs. A recent technical report from the Environmen- tal Defense Fund (Fisher et al., lg88) concluded that nitrate in acid precipi- tation falling on terrestrial ecosystems can be a major source of nitrogen reaching estuaries. While this is a reasonable hypothesis, very little is known about the retention and export of nitrogen falling on terrestrial eco- systems in precipitation. Likens et al. (1985) found no clear relationship between nitrate inputs in precipitation and nitrate exports in stream flow over a period of 14 years in the Hubbard Brook watershed. Also, nitrogen exports from agricultural lands are not clearly related to nitrogen inputs in fertilizer (Beaulac and Reckhow, 1982), suggesting that export is also unlikely to be tightly related to input in precipitation. As discussed in the previous section, the factors regulating nitrogen retention and export from terrestrial ecosystems clearly deserve further study (Hooper et al., 1988~. 3. Climate. It seems likely that the export of elements and sediment from terrestrial ecosystems to coastal marine ecosystems can be altered by changes in climate, but this has received little study. Increased erosion and increased runoff resulting from a wetter climate seem likely to greatly in- crease element export, although nitrogen export might be decreased if deni- trification within soils increases markedly, a result of more waterlogged conditions. Results from a land-use, carbon- and sediment-export model for the Hudson River watershed suggest that carbon and sediment export may be more sensitive to the seasonal and day-to-day patterns in precipitation than to annual amounts (Howarth et al., 1989~. This may be true for other elements. In addition to the direct effects of climatic change on element

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 187 export through changing erosion and runoff, climatic change is likely to alter the structure and function of terrestrial ecosystems, which also is likely to alter element exports. Research Priorities. Three approaches to addressing issues related to sub- stance movement from terrestrial ecosystems to coastal marine ecosystems are recommended: · Establish a national surface water chemistry sampling network. The USGS operated a national surface water chemistry sampling network from the early 1970s until the early 1980s in conjunction with some of their gauging stations, but chemical sampling has largely been discontinued or has been extremely rare at most stations since the early 1980s. Such a network should be reestablished and expanded so as to better discern tem- poral variation in element export from terrestrial ecosystems to aquatic sys- tems and so as to detect differences in export from different types of ecosystems and in different climatic regimes. The recently completed USGS 4-year pilot project to test concepts for a National Water Quality Assessment (NAWQA) program represented a diversity of hydrological environments and water quality conditions, and as such, would provide a useful initial model (Hirsch et al., 1988). · Establish detailed watershed studies to examine element and substance export as a function of land use and climate. These studies would measure exports of carbon, nitrogen, phosphorus, water, and sediment at the water- shed scale with a much greater sampling frequency than used in the national surface water chemistry network. Watersheds should be selected so as to represent different land uses (e.g., undisturbed forests, agriculture, and sub- urbia) in given climatic types. Within any given climatic types, the watersheds representing different land uses should be in close proximity and should have similar parent materials. These studies should be run long enough to determine the effects of year-to-year variability in climate on element ex- port and resulting effects on aquatic systems. Studies should be established in three or four different climatic regimes. · Develop improved models for the export of substances from terrestrial ecosystems and for movement of these substances to estuaries. Models for the movement of surface and subsurface water, sediment, and elements from various terrestrial ecosystems should be improved. Such models could be tested using data collected from the proposed national surface-water chem- istry sampling network (first item above) and from the proposed watershed export studies (second item above). The goal of these models should be to allow better prediction of the potential effects of land use change, effects from alterations in atmospheric inputs to terrestrial ecosystems, and effects from climatic change on both terrestrial and aquatic systems.

188 RESEARCH STRATEGIES FOR THE USGCRP METHODS AND INSTRUMENTS Models For most of the potentially important trace gases, there is still unaccept- able uncertainty in the identity and global distribution of the main sources and sinks. Although the ultimate goal is to understand the fluxes of gases, nutrients, and pollutants, an essential first step is to quantify how concen- trations of each important material vary geographically and temporally and then to relate these patterns to the distribution of ecosystem types and an- thropogenic sources. This can be accomplished by a combination of moni- toring to measure the material concentrations and the associated biotic and physical variables and modeling to develop the predictive multivariate rela- tionships. Models will be most effective when developed in parallel with standard- ized monitoring stations located in representative ecosystem types (includ- ing human-modified ecosystems containing anthropogenic sources) around the world. It would be most efficient if these stations measured the concen- trations of all potentially important trace gases and other materials as well as the values of additional climatic and geological variables with sufficient accuracy and frequency to assess seasonal and interannual variation. Addi- tional data on environmental variables, such as local vegetation types and land use categories, can be obtained from remote sensing and other sources (e.g., government census records). It is recommended that the modeling and monitoring be conducted so as to quantify spatial variation on the scale of patterns of vegetation and land use change across the landscape. A scale of approximately 1 km is recommended as a general guide, but the sample distribution pattern should be based on the statistical distributions of the putative driving variables. It is anticipated that to understand global fluxes and their controls three scales of models (micro, meso, and macro) will have to be developed (see also chapters 2 and 5~. In addition, because there will often be important spatial heterogeneity within each of these scales, it will be necessary to develop techniques to aggregate or synthesize the outputs of the models at smaller scales to use as inputs for models at larger scales. Microscale Fluxes across the interface between the atmosphere and vegetation, soil, and water surfaces. Small-scale dynamics will vary with gas species or other materials and with environmental variables that affect sources, sinks, chemical reactions, and micrometeorological conditions. At this level it is possible to rely on basic knowledge of microbial physiology, transport processes, and chemistry. These models are often portable with minimal reparameterization but do require detailed input data. Small-scale experiments and models will be particularly important for elucidating the

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 189 mechanisms that control production and deposition and for predicting the changes in controlling factors and the resulting fluxes that may accompany different types of global change. For considerations of global changes in trace gases, it may not always be necessary to produce detailed, mechanistic models at this level. For ex- ample, for gases of primarily anthropogenic origin and readily identifiable sources, mesoscale models may suffice. But at least for some gas species with unknown biogenic sources and/or with moderate half-lives in the at- mosphere, detailed studies of the near-surface dynamics will be essential. A likely example is carbon monoxide, which appears to be produced in significant quantities in at least some tropical forests and to exhibit vertical changes in concentration between the soil and the canopy. Mesoscale-Fluxes within patches of similar ecosystem type. At the scale of approximately 1 km, it should be relatively easy to use remotely sensed and ground-based data to classify ecosystem types, including heavily human-modified ones such as different kinds of agricultural, suburban, and urban systems. It should also be practical, for example, to monitor spatial and temporal variation in gas concentrations at this scale (i.e., in the lower atmosphere above the vegetation). What is needed are predictive process models to characterize the sources, sinks, chemical transformations, and fluxes of gases and other materials that occur within three-dimensional cells at this scale. Moreover, we need to know, for example, how much biologi- cal detail is needed to characterize fluxes from physiognomic types of veg- etation. Figure 6.3 illustrates the five essential components of a mesoscale model: (1) vertical exchange with the soil, water, or vegetation that covers the earth's surface (inputs characterizing these production and deposition pro- cesses come from the microscale models, appropriately aggregated if necessary to account for surface heterogeneity); (2) vertical exchange with the upper atmosphere; (3) horizontal exchange, via wind, with adjacent patches of the same or different ecosystem type; (4) circulation and chemical reactions within the cell that affect the concentration and flux; and (5) environmental forcing functions, such as changes in vegetation, temperature, cloud cover, or the concentrations of other materials, that are likely to change the dy- namics of the gas or nutrient species in question. Models must be customized to account for the unique features of each gas nutrient or pollutant species. The local production and deposition com- ponents and the circulation and chemical reactions within the cell will tend to be species specific. · Mesoscale and macroscale Linking trace gas process models to earth system models. The final state in modeling global fluxes is to aggregate the mesoscale cells and incorporate the trace gases and other materials into atmosphere-biosphere interface models. This is necessary not only to account

190 RESEARCH STRATEGIES FOR THE USGCRP exchange with the upper atmosphere input loss horizontal exchange import export ~ / I \ exchange production with the surface I a / internal circulation and chemical reactions - - \ environmental forcing functions l 1 1 ~1 1 ---1- dep: Finks FIGURE 6.3 Main ingredients of a mesoscale model for trace gas fluxes. for regional variations in concentration and long-distance transport of an- thropogenic gases and other materials, but also to understand the global fluxes of any species for which the primary sources and sinks may be spatially isolated (e.g., between tropical forests and oceans). The problems of predicting atmospheric circulation on a scale of ap- proximately 100 km have not been solved, but there is currently a major research effort to improve and test GCMs. These models can be used (Matthews and Fung, 1987) to predict the large-scale dispersal of trace gases if atmospheric reactions are included. A much more difficult problem would seem to be the development of techniques for aggregating the outputs of mesoscale models over heterogeneous landscapes (see chapter 5) to obtain accurate inputs for the GCMs. For example, construction of an atmospheric boundary layer model should include (1) drag coefficients that vary with topography and vegetation especially when topographic variation is relatively

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 191 small, (2) turbulent exchange coefficients that depend on these drag coeffi- cients and on the thermal structure of the lower atmosphere, and (3) moisture- balance equations that depend on current and changing vegetation characteristics (Walker et al., 1990~. High priority should be placed on developing mesoscale models of trace gas flux, and then aggregating or synthesizing them over space so that they can be used as inputs into GCMs. More work on this type of predictive modeling is required, and for the full suite of gases and materials of inter- est. To avoid doing such modeling in an information vacuum, it will be necessary to accompany this effort with systematic monitoring of spatial and temporal variation in trace gas concentrations and material fluxes as a function of ecosystem type and with initial descriptive models that quantify the environmental correlates of this variation. Lower priority should be placed on developing microscale and macroscale models, because there is already considerable effort at these levels. But the pace of research at micro- and macro-levels must also be increased if we are to produce predictive models of global trace gas fluxes in time to deal with these and other pressing problems of global change. Instrumentation for Measuring Fluxes One of the current key limitations in formulating a predictive under- standing of global processes is the inability to measure unequivocally the abundances of many trace species that are centrally involved in those processes. This is particularly true for the measurement of chemical fluxes (emission or deposition), where the number of gas species for which it is generally accepted that reliable techniques exist are only a few. A wide variety of emission sources, deposition surfaces, and chemical species are involved in global fluxes. Natural emissions of chemically or climatically important compounds occur from terrestrial and oceanic sources (e.g., nonmethane hydrocarbons and methane from wetlands). Deposition surfaces that figure strongly in major removable processes range from veg- etative uptake (e.g., of carbon dioxide) to physical attachment (e.g., nitric acid depositing on wetted soils). The chemical variety of the emitted or depositing compounds (inert species and reactive radicals) implies that the likelihood of even semiuniversal detectors is unlikely. Flux measurements of trace gases (molecules per unit area per unit time) require a determination of the atmospheric concentrations over time. Mea- surement of low concentrations of many chemical compounds requires highly sensitive detectors and rigorous analytical quality assurance. The need to obtain a representative flux from a large spatial area generally implies use of remote or aircraft sensing. Many natural emissions are quite sensitive to moisture, temperature, and other such factors, thereby introducing substan

192 RESEARCH STRATEGIES FOR THE USGCRP tial spatial and temporal variability not well studied in most contemporary investigations. Because of the challenges that flux measurements pose and because of the necessity to substantially improve the current capabilities, a focused program for new methodologies and instrumentation is needed. To date, flux measurements have been made in enclosures, along gradi- ents, and via eddy correlation. The enclosure method establishes the flux from a small area based on increases in concentration of the compound in the container. The gradient method generally employs towers to determine differences in the target compound or element across a spatial gradient (e.g., as a function of altitude, in conjunction with meteorological analy- ses). The eddy correlation method, often used with towers or aircraft, relates small-scale concentration variations to variations in air motion. The usefulness of these methods depends on the research question and the scale of the investigation. The key to success in the gradient and eddy correlation methods is the availability of rapid-response sensors. Detectors are needed that can make reliable measurements of the concentration of a species at the part-per- trillion level and with a measurement rate of less than once per second. Thus the development of new physical and chemical sensors with those characteristics is the key to improving the status of flux measurements. Improvements and new innovations in instruments that measure more than one species simultaneously are especially needed, since covariation pro- vides key insight into the biogenic processes involved. It is imperative that rigorous intercomparison experiments precede the widespread and large-scale application of flux measurement techniques. The atmospheric chemistry community has developed an approach that provides a valuable unbiased indication of measurement capabilities. The main fea- tures of the experiments that have proved the most informative are the following: · several different methods for measuring the same species are involved; · "mature" instruments (i.e., those that have been used in published investigations) are compared; · measurements are made at the same time and place and under typical and documented environmental conditions, insofar as possible; · the expected accuracy and precision are hypothesized in advance of the intercomparison; and · all results and conclusions are published in the open literature. CROSS-CUTTING ISSUES Trace gas sources and sinks are affected by the intrinsic characteristics of ecosystems, by changes in land use, and by changing climatic conditions.

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 193 Understanding both the reservoirs and the fluxes under these three condi- tions is necessary for documenting and predicting global change. These studies require careful stratification to ensure a parsimonious set of mea- sured conditions with the greatest experimental efficiency. Thus trace gas studies will profit from a comprehensive experimental design that addresses several gases simultaneously. Similarly, in many cases nutrient flux studies can be conducted with trace gas studies. Finally, as discussed in chapter 5, water-energy balance of the biosphere will require instrumented watersheds. These, too, can be combined with the trace gas and nutrient flux studies. A global data base of direct measurements of trace gas fluxes to and from ecosystems is not achievable in the foreseeable future. No technology is available that would allow such measurements to be made remotely from satellites, and global surveys using ground-based or aircraft platforms would involve tremendous costs and logistical difficulties. The best approach at this time for constructing a global data base of trace gas fluxes is to map the environmental variables known to regulate those fluxes from each ecosys- tem. The functional dependences relating trace gas fluxes to these variables can then be quantified by ground-based and aircraft studies focusing on specific ecosystems, and the resulting data can then be aggregated as appro- priate. Laboratory and small-scale experiments must be conducted for the purpose of relating trace gas fluxes to input variables that can be measured via remote sensing techniques. Since measuring flux rates directly is difficult at global scales and grids of concentration data are much more feasible, it will be necessary to de- velop mathematical methods for inverting from concentration data to flux rates, and to be able to do so at local to regional to global scales. Additional constraints on the inversions may be derived from remote sensing and the development of large-scale soil and land use data bases. Also, there is a need to refine statistical techniques that identify adequate sample sizes in relation to the cost of acquiring data and the required sampling intensity. Large manipulation experiments will be necessary under selected condi- tions that represent important types of ecosystems, e.g., agricultural sys- tems. In other instances, where the initial conditions are variable and het- erogeneous, comparative measurements may be more reasonable. Moreover, careful analysis will be required of existing data, both to synthesize what is already known and for designing efficient experiments. Connecting large- scale manipulation experiments with experimental Regional Research Cen- ters will contribute to research economy and assist in the extrapolation of results to regional and global scales. Nutrient transfer studies measuring the lateral fluxes of nutrients should be organized to include hierarchical descriptors of land use arrangements and other driving variables. Most of the experimental conditions are in place, and thus the challenge is to establish the field measurements and not

194 RESEARCH STRATEGIES FOR THE USGCRP to develop new large-scale experimental conditions. The most difficult step in some of the studies will involve the prediction of how these fluxes will change with alterations in the regional and global climate. Thus the sce- narios for global climatic change must be solidified as a basis for these experiments and for subsequent models. Great economies can be achieved by careful coordination of the nutrient transfer and Face gas flux measurements. Making the measurements at the same sites will minimize logistic expenses and assist in the development of microscale and mesoscale models and of correlative indicators of system dynamics and responses. In many instances, the first step in these studies will be to develop initial models to determine unknown parameters and to identify the experiments most likely to yield critical important information. Thus models will be important at all stages of the studies, from beginning synthesis of known information and experimental design to final synthesis of new information and for scaling among time and space scales. These studies on trace gases and the fluxes of materials involve a wide spectrum of traditional disciplines and will require a significant number of scientists over one to two decades. In addition, the investigations will depend on a thorough understanding of human systems, especially in terms of land use practices. Thus the scientific community must be sure that there are educational programs that include this wide spectrum of disciplines. There must also be undergraduate and graduate programs that encourage the best of our students to participate in these studies. REFERENCES Andreae, M.O. 1985. The emission of sulfur to the remote atmosphere: Back- ground paper. Pp. 5-26 in J.N. Galloway et al. (eds.), The Biogeochemical Cycling of Sulfur and Nitrogen in the Remote Atmosphere. D. Reidel, Dordrecht, The Netherlands. Andreae, M.O., and H. Raemdonck. 1983. Dimethyl sulfide in the surface ocean and the marine atmosphere: A global view. Science 221:744-747. Andreae, M.O., et al. 1988. Biomass burning emission and associated haze layers over Amazonia. J. Geophys. Res. 93:1509-1527. Andreae, M.O., H. Berresheim, H. Bingemer, D.J. Jacob, and R.W. Talbot. 1990. The atmospheric sulfur cycle over the Amazon Basin. 2. Wet season. J. Geophys. Res., in press. Beaulac, M.N., and K.H. Reckhow. 1982. An examination of land use-nutrient export relationships. Water Res. Bull. 18:1013-1024. Billings, W.D. 1987. Carbon balance of Alaskan tundra and taiga ecosystems: Past, present and future. Quaternary Science Reviews 6:1265-1277. Bormann, P.H., and G.E. Likens. 1979. Pattern and Process in a Forested Ecosys- tem. Springer-Verlag, New York.

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196 RESEARCH STRATEGIES FOR THE USGCRP Haith, D.A., and L.L. Shoemaker. 1987. Generalized watershed loading functions for stream-flow nutrients. Water Res. Bull. 23:471-478. Hanst, P.L., J.W. Spence, and E.O. Edney. 1980. Carbon monoxide production in photooxidation of organic molecules in the air. Atmos. Environ. 14:1077- 1088. Hirsch, R.M., W.M. Alley, and W.G. Wilber. 1988. Concepts for a national water- quality assessment program. U.S. Geological Survey Circular 1021. 42 pp. Hooper, R.P., A. Stone, N. Christopherson, E. de Grosbois, and H.M. Seip. 1988. Assessing the Birkenes model of stream acidification using a multi-signal calibration methodology. Water Resour. Res. 24:1308-1316. Howarth, R.W. 1988. Nutrient limitation of net primary production in marine ecosystems. Annul Rev. Ecol. Syst. 19:89-110. Howarth, R.W., J.R. Fruci, and D. Sherman. 1990. The influence of land use on functioning of estuarine ecosystems: The Hudson River estuary as a case study. Ecological Applications, in press. Jordan, T.E., D.L. Correll, W.T. Peterjohn, and D.E. Weller. 1986. Nutrient flux in a landscape: The Rhode River watershed and receiving waters. Pp. 57-76 in D.L. Correll (ed.), Watershed Research Perspectives. Smithsonian Press, Washington, D.C. Kemp, W.M., W.R. Boynton, R.R. Twilley, J.C. Stevenson, and J.C. Means. 1983. The decline of submerged vascular plants in upper Chesapeake Bay: Summary of results concerning possible causes. Mar. Technol. Soc. J. 17:78-89. Kemp, W.M., W.R. Boynton, R.R. Twilley, J.C. Stevenson, and L.G. Ward. 1984. Influences of submerged vascular plants on ecological processes in upper Chesapeake Bay. Pp. 367-394 in V.S. Kennedy (ed.), The Estuary as a Filter. Academic Press, New York. Kempe, S. 1988. Estuaries their natural and anthropogenic changes. Pp. 251-285 in T. Rosswall, R.G. Woodmansee, and P.G. Risser (eds.), SCOPE 35: Scales and Global Change. John Wiley and Sons, Chichester, England. Larsson, U.R., R. Elmgren, and F. Wulff. 1985. Eutrophication and the Baltic Sea: Causes and consequences. Ambio 14: 10- 14. Lenschow, D.H., and B.B. Hicks (eds.~. 1989. Global Tropospheric Chemistry: Chemical Fluxes in the Global Atmosphere. Report of the Workshop on Measurements of Surface Exchange and Flux Divergence of Chemical Species in the Global Atmosphere. National Center for Atmospheric Research, Boul- der, Colo. Likens, G.E., F.H. Bormann, R.S. Pierce, and J.S. Eaton. 1985. The Hubbard Brook Valley. Pp. 9-39 in G.E. Likens (ed.), An Ecosystem Approach to Aquatic Ecology. Springer-Verlag, New York. Logan, J.A. 1985. Tropospheric ozone: Seasonal behavior, trends and anthropo- genic influence. J. Geophys. Res. 90:10463-10482. Logan, J.A., M.J. Prather, S.C. Wofsy, and M.B. McElroy. 1981. Tropospheric chemistry: A global perspective. J. Geophys. Res. 86:7210-7254. Lowrance, R.R., R.A. Leonardd, and L.E. Asmussen. 1985. Nutrient budgets for agricultural watersheds in the southeastern coastal plain. Ecology 66:287-296. Marenco, A., and J.C. Delaunay. 1985. Experimental evidence of natural sources of CO from measurements in the troposphere. J. Geophys. Res. 85:5599- 5613.

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 197 Matthews, E., and I. Fung. 1987. Methane emission from natural wetlands: Global distribution, area, and environmental characteristics of sources. Global Biogeo- chemical Cycles 1:61-86. Meade, R.H., and R.S. Parker. 1985. Sediment in rivers of the United States. Pp. 49-69 in National Water Summary 1984. U.S. Geological Survey Water Supply Paper 2275. Meyers, T.P., and D.D. Baldocchi. 1988. A comparison of models for deriving dry deposition fluxes of O3 and SO2 to a forest canopy. Tellus 40:270-284. Mooney, H.A., P.M. Vitousek, and P.A. Matson. 1987. Exchange of materials between terrestrial ecosystems and the atmosphere. Science 238:926-932. Mulholland, P.J. 1981. Deposition of riverborne organic carbon in floodplain wetlands and deltas. Pp. 142-172 in Carbon Dioxide Effects Research and Assessment Program: Flux of Organic Carbon by Rivers to the Oceans. Re- port of a Workshop held in Woods Hole, Mass., September 21-25, 1980. Committee on Flux of Organic Carbon to the Ocean, Division of Biological Sciences, National Research Council. U.S. Department of Energy, Office of Energy Research, Washington, D.C. 397 pp. National Research Council. 1984. Global Tropospheric Chemistry: A Plan for Action. National Academy Press, Washington, D.C. National Research Council. 1988. Toward an Understanding of Global Change: Initial Priorities for U.S. Contributions to the International Geosphere-Biosphere Program. National Academy Press, Washington, D.C. 213 pp. Nixon, S.W. 1982. Nutrient dynamics, primary production, and fisheries yields of lagoons. Pp. 357-371 in Oceanologica Acta. Special publication, Proceedings of the International Symposium on Coastal Lagoons, SCOR/IABO/UNESCO, 1981, Bordeaux, France. Nixon, S.W., and M.E.Q. Pilson. 1983. Nitrogen in estuarine and coastal marine ecosystems. Pp. 565-648 in E.J. Carpenter and D.G. Capone (eds.), Nitrogen in the Marine Environment. Academic Press, New York. Nixon, S.W., C. Oviatt, J. Frithsen, and B. Sullivan. 1986. Nutrients and the productivity of estuarine and coastal marine ecosystems. J. Limnol. Soc. South Africa 12:43-71. Officer, C.B., R.B. Biggs, J. Taft, L.E. Cronin, M.A. Tyler, and W.R. Boynton. 1986. Chesapeake Bay anoxia: Origin, development, and significance. Sci- ence 232:22-27. Parton, W.J., D.S. Schimel, C.V. Cole, and D. Ojima. 1987. Analysis of factors controlling soil organic levels in grasslands in the Great Plains. Soil Sci. Soc. Am. J. 51:1173-1179. Pastor, J., and W.M. Post. 1986. Influence of climate, soil moisture and succession on forest carbon and nitrogen cycles. Biogeochemistry 2:3-27. Peterjohn, W.T., and D.L. Correll. 1984. Nutrient dynamics in an agricultural watershed: Observations of the role of a rip arian forest. Ecology 65:1466- 1475. Price, K.S., D.A. Flemer, J.L. Taft, and G.B. Mackiernan. 1985. Nutrient enrich- ment of Chesapeake Bay and its impact on the habitat of striped bass: A speculative hypothesis. Trans. Am. Fish. Soc. 1 14:97-106. Robertson, G.P., and J.M. Tiedje. 1987. Deforestation alters denitrification in a lowland tropical rain forest. Nature 336:441-445.

198 RESEARCH STRATEGIES FOR THE USGCRP Schimel, D.S., M.A. Stillwell, and R.G. Woodmansee. 1985. Biogeochemistry of C, N and P in a soil catena of the shortgrass steppe. Ecology 66:276-282. Schimel, D.S., W.J. Parton, F.J. Adamsen, R.G. Woodmansee, R.L. Senft, and M.A. Stillwell. 1986. The role of cattle in the volatile loss of nitrogen from a shortgrass steppe. Biogeochemistry 2:39-52. Schimel, D.S., M.O. Andreae, D. Fowler, I.E. Galbally, R.C. Harriss, D. Ojima, H. Rodhe, T. Rosswall, B.H. Svensson, and G.A. Zavarzin. 1989. Research priorities for studies on trace gas exchange. Pp. 321-331 in M.O. Andreae and D.S. Schimel (eds.), Exchange of Trace Gas Between Terrestrial Ecosys- tems and the Atmosphere. John Wiley and Sons, Chichester, England. Seller, W. 1978. The influence of the biosphere on the atmospheric CO and H2 cycles. Pp. 773-810 in W.E. Krumbein (ed.), Environmental Biogeochemistry and Geomicrobiology, Vol. 3, Methods, Metals, and Assessment. Ann Arbor Science Publishers, Ann Arbor, Mich. Seitzinger, S.P., M.E.Q. Pilson, and S.W. Nixon. 1983. Nitrous oxide production in nearshore marine sediments. Science 222:1244-1246. Strain, B.R., and J.D. Cure. 1985. Direct effects of increasing carbon dioxide on vegetation. Duke University Press, Durham, N.C. Tingey, D.T., M. Manning, L.C. Grothaus, and W.F. Bums. 1979. The influence of light and temperature on isoprene emission rates from live oak. Physiol. Plant. 47:112-118. Tissue, D., and W.C. Oechel. 1987. Responses of Eriophorum vaginatum to elevated CO2 and temperature in the Alaskan tussock tundra. Ecology 68:401-410. Trabalka, J.R., and D.E. Reichle (eds.~. 1986. The Changing Carbon Cycle: A Global Analysis. Springer-Verlag, New York. Tumer, R.E., R. Kaswadji, N.N. Rabalais, and D.F. Boesch. 1987. Long-term changes in the Mississippi River water quality and its relationship to hypoxic continental shelf waters. Pp. 261-266 in M.P. Lynch and K.L. McDonald (eds.), Estuar~ne and Coastal Management. Tools of the Trade. Proceedings of the Tenth National Conference of the Coastal Society, October 12-15, 1986, New Orleans, La. 391 pp. Vorosmarty, C.J., M.P. Gildea, B. Moore, B.J. Peterson, B. Bergquist, and J.M. Melillo. 1986. A global model of nutrient cycling: II. Aquatic processing, retention and distribution of nutrients in large drainage basins. Pp. 32-56 in D.L. Correll (ed.), Watershed Research Perspectives. Smidlsonian Environmental Research Center, Edgewater, Md. 421 pp. WaLker, B.H., S.J. Turner, R.J. Prinsley, D.M. Stafferd Smith, and H.A. Nix (eds.~. 1990. Proceedings of the Workshops of the Coordinating Panel on Effects of Global Change on Terrestrial Ecosystems. I. A Framework for Modeling the Effects of Climate and Atmospheric Change on Terrestrial Ecosystems, Woods Hole, Mass., April 15-17, 1989. II. Non-modeling Research Requirements for Understanding, Predicting, and Monitoring Global Change, Canberra, August 29-31, 1989. III. The hnpact of Global Change on Agriculture and Forestry, Yaounde, November-December 1989. IGBP Report No.11. Stockholm, Sweden. Welch, R., M. Remillard, and R.B. Slack. 1988. Remote sensing and GIS tech- niques for aquatic resource evaluation. Photogramm. Eng. Remote Sensing 54: 177-185.

TERRESTRIAL TRACE GAS AND NUTRIENT FLUXES 199 Williams, W.E., K. Garbutt, P.A. Bazzaz, and P.M. Vitousek. 1986. The response of plants to elevated CO2. IV. Two deciduous-forest tree communities. Oecologia 69:454459. Yokouchi, Y., and Y. Ambe. 1984. Factors affecting the emission of monoterpenes from red pine (Pinus densiflora). Plant. Physiol. 75:1009-1012.

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This book recommends research priorities and scientific approaches for global change research. It addresses the scientific approaches for documenting global change, developing integrated earth system models, and conducting focused studies to improve understanding of global change on topics such as earth system history and human sources of global change.

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