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A Conceptual Framework for Analyzing Agricultural and Environmental Concerns C. VERNON COLE U.S. Department of Agriculture MAJOR ISSUES IN AGRICULTURAL DEVELOPMENT AND ENVIRONMENTAL RESEARCH A major requirement for the long-term stability of civilization is a dependable supply of food, feed, and fiber. Agricultural sys- tems developed over wide ranges of climate and soil resources have achieved remarkably high levels of production of food, feed, and fiber to provide relative security for an ever-increasing world population. With few exceptions, high levels of production are achieved by bring- ing agronomic practices into harmony with natural processes of the biosphere. Global and regional patterns of distribution of energy and wa- ter are the overriding controls on biological processes essential for agricultural production. Availability of essential nutrient elements imposes the next level of control, but the use of fertilizers has largely removed or mitigated this limitation. However, levels of fertilization which are out of balance with other processes can result in both short- and long-term adverse effects on the environment and soil productivity. The challenge to modern agriculture is to develop op- timum balances between inputs and outputs of production systems, while also providing a basis for continued increases in productivity with minimum adverse effects on the environment. The following issues are of major concern for both agricultural productivity and environmental quality: ⢠soil erosion; ⢠sedimentation, salinization and sodification;
water soluble pollutants; soil organic matter loss; toxic substances; genetically engineered organisms; climatic change resulting from increasing concentrations of trace gases and CO2 in the atmosphere. Agricultural and environmental issues are related and can be analyzed using a common conceptual framework. An appropriate framework should integrate processes of energy flow, nutrient cycling, and mass transport into a quantitative description of system function. In its simplest form, this approach involves determining the energy and mass balances of water and materials in a unit of land and determining rates and controls for the transformations in this unit. The mass balance of any component involves inputs, outputs, and changes in storage within the system. Major outputs are those to stream water and groundwater, to the atmosphere, and in harvested products. Thus, management of agro-ecosystems aims to maximize crop yield per unit input while minimizing losses from the system. A more complete understanding of the interactions of units of soil with their environment at appropriate temporal and spatial scales is needed. The question of scale must be addressed to solve long-term and large-scale agricultural and environmental problems. Because of new technologies, regional- and global-scale questions can now be addressed. These technologies include remote sensing, computerized data management systems, analytical and monitoring equipment, chemical and biochemical techniques and, perhaps most importantly, systems analysis. CONCEPTUAL FRAMEWORK Early conceptual models of ecosystems developed by Dokuchaev and Jenny emphasized the factors controlling development of soil properties, referred to as state factor controls. These have been extended to include key processes of flows of energy and matter to provide integrating structures for analysis of ecosystem dynam- ics. The relationships among driving variables (controlling factors), key processes, and ecosystem properties and structure are shown in Figure 1. Driving variables are externally imposed variables little affected by the state of the ecosystem. Important ecosystem processes include primary production, de- composition, and nutrient cycling above and below the ground. Heat
DRIVING VARIABLES PARENT MATERIAL CLIMATE BIOTA TOPOGRAPHY Rangeland Grazing Species Nutrient Input Fire MAN \ Cropland Cultivation Fallow Crop Selection Residue Management Water Management Nutrient Inputs Fire Harvest Forests Seeding and Planting Site Preparation Watershed Management Fire Harvest Initial time TIME Present or Future PROCESSES ENERGY INPUTS AND TRANSFORMATIONS ⢠Radiation ⢠Nutrient Cycling ⢠Primary Production Immobilization ⢠Decomposition Mineralization ⢠Weathering Translocation ⢠Transport Erosion Gaseous Leaching Runoff DEVELOPMENT OF ECOSYSTEM PROPERTIES ⢠Vegetation ⢠Consumers ⢠Soil Base Status Texture Organic Matter Phosphorus Sulfur Nitrogen Salinity FIGURE 1 properties. Relationships among driving variables, processes, and ecosystem
and water play key roles through their effects on the rates of bi- ological and chemical processes. The rates of ecosystem processes determine the rate and direction of change of ecosystem structure, i.e., the composition of the biota. Ecosystem structure, in turn, has feedback effects on the rates of processes. The general hypothesis is that driving variables control the rate and direction of processes that determine the properties of an ecosystem. Specific hypotheses involve the particular effects of driving variables on various processes and thereby on the dynamics of ecosystem structure over time. This approach requires a thorough understanding of processes over a wide range of controls. It is assumed that the same suite of processes are operative over a wide range of systems and that it is controlled by the state variables that regulate their expression in a given system. In this context the biogeochemistry of carbon, nitro- gen, sulphur, and phosphorus provides linkages between processes and ecosystem properties. The effects of controlling factors are expressed and observed across a wide range of spatial scales from global down to regional, landscape, and field levels. Process controls are also documented across a wide range of scales of time, ranging from hours and days for microbial and plant growth processes to years and centuries for the development of steady-state levels of organic matter and pedogenesis. Information on the nature of process controls and the interactions of biogeochemical cycles provides linkages across various levels of investigation and has stimulated integrated approaches to analysis of both natural and agricultural systems. BIOGEOCHEMICAL CYCLES The transformations and gains or losses of key nutrient elements provide valuable diagnostics for analysis of changes in both natural and man-managed ecosystems. Examination of changes in elemental forms and concentrations in organic matter of soils developed along environmental gradients have given insight into interrelationships between carbon (C), nitrogen (N), sulphur (S), and phosphorus (P). Changes in C, N, and P show the close linkages between organic C and N in mature soils and P content in the original parent material (Cole and Heil 1981). Thus, given sufficient time, the supply of N comes into balance with the supply of P (assuming S is not lim- iting). Microbial growth processes are the principal arenas for the adjustment of the supply of N to the supply of P.
Although carbon transformations closely track energy fluxes, knowledge of carbon flows has limited value in predicting nutrient flows since each nutrient has specific reactions and storage compart- ments which must be considered. Flows determined for C are not directly transferable to N, nor are those for N the same as those for S or P. The mechanisms stabilizing organic C, N, S, and P are not necessarily common to all four elements. A dichotomous system in which N and parts of soil-S are stabilized as a result of direct association with soil-C has been proposed (McGill and Cole 1981). These forms mineralize as the result of C oxidationâclassical biolog- ical mineralizationâto provide energy. Sulphur and P in the form of esters, on the other hand, are stabilized directly by interaction with mineral components and are mineralized by enzymes in response to the need for a specific element. This latter process is called biochem- ical mineralization since it operates largely outside the cell. These concepts account for variability in soil organic matter composition and set the stage for predicting the relationship between the cycling rates of N, S, and P in soils. Since the behavior of S is intermediateâ between that of N and Pâa study of S transformations helps to explain differences in quality of soil organic matter. These elemental interactions need to be integrated with other concurrent pedogenic processes such as mineral weathering and leaching. The complex interactions of chemical and biochemical processes involved in nutrient cycling are illustrated by the conceptual model of phosphorus dynamics in a soil/plant system shown in Figure 2. Primary P minerals are slowly dissolved during weathering processes providing phosphate ions that enter into the solution P pool (Smeck 1985, Stewart and Sharpley 1987). Soil solution phosphate ions are shown to be in equilibrium with a quantity of labile inorganic P (P{) such that in any one soil the ratio of labile inorganic P to solution P maintains a constant ratio over the normal range of P concentration found in cultivated soils. A portion of the solution P will be precipitated as secondary P minerals and eventually converted to occluded or unavailable forms in more weathered soils. Plants take up phosphate ions from the soil solution, and the dy- namics of P uptake are well researched (Barber 1984). The depleted solution P pool is immediately replenished from labile and moder- ately labile P< forms. If these pools are depleted, less soluble species such as secondary P minerals regulate the solution P concentration. Uptake of solution phosphate by bacteria and fungi stimulated by the addition of microbial substrates such as litter and crop residues,
10 PLANTS s RESSTANTJA ORGANlC :E p : ;0 HC104 ^SECONDARY ,> OJ P ^MlNERALS to: VAGGREGATE ;c PROTECTED! ORGANlC FIGURE 2 Conceptual model of phosphorus cycling in soil plant systems. (Source: Stewart, J.W.B. et al. 1983) and release of soluble phosphate ions (P{) and labile and stable or- ganic (P0) forms are represented in Figure 2 as a revolving wheel to emphasize the central role of the microbial population in P cycling. When microbial cells are ruptured or lysed, a variety of organic and inorganic P compounds are released to the soil solution which react with inorganic and organic soil components to form Pi and P0 com- pounds of differing solubility or susceptibility to mineralization. The rate of mineralization of P0 forms depends largely on phosphatase activity which, in turn, can be controlled by solution P concentration (McGill and Cole 1981). Stable P0 accumulates in both chemically resistant and aggregate protected forms. Organic P existing in chemically or physically protected forms may be slowly mineralized as a by-product of overall soil organic matter mineralization or by specific enzyme action in response to the need for P. Therefore, organic matter turnover as well as solution Pt- concentration and the demand for P by microbial and plant compo- nents will be factors controlling the lability of P0 (McGill and Cole 1981). A continuous drain on soil P pools by cultivation and crop removal will rapidly deplete labile P< and P0 forms. In summary, the P cycle is a system in dynamic equilibrium with interchanges governed by chemical, physical, and biological re- actions. Microbial activity is depicted as a wheel rotating in the soil
11 in response to energy, particularly C inputs, and having a central role in P transformations. Should the wheel be stopped or slowed down by lack of C inputs, the supply of P to plants will be limited to the quantity of labile P<. If the wheel is operating, then the plant is supplied with a larger quantity of labile P as solution P is con- stantly being replenished from labile P< and P0 forms. Over a longer timescale these same processes operate to evolve major changes in amount and forms of phosphorus in soils across toposequences and chronosequences (Smeck 1985). Similar conceptual models have been developed for other ma- jor nutrient elements (Follett et al. 1987). Simulation models are being developed to integrate information on nutrient and organic matter dynamics with information on all other factors controlling the functioning of complex natural and managed systems. REGIONAL ANALYSIS OF SOIL ORGANIC MATTER, NUTRIENT AVAILABILITY, AND SOIL PRODUCTIVITY: THE GREAT PLAINS A study of management effects on soil organic matter and pro- ductivity in the semi-arid Great Plains of North America exemplifies the application of this conceptual framework. The accumulation of organic matter in grassland soils over a wide range of temperature and moisture gradients in the Great Plains is an excellent example of state factor controls on development of an important ecosystem property. Soil organic matter is a key indicator of soil quality as it af- fects nutrient availability, soil stability, and susceptibility to erosion. Organic matter levels reflect past management history. Regional-scale investigations of nutrient and carbon dynamics in agricultural ecosystems have been conducted since 1979. The objectives of the research are to: ⢠evaluate past, present, and future management practices with re- spect to their impacts on soil organic matter in the Great Plains, and to understand key processes of organic matter formation in semi-arid soils; and ⢠develop the capability to predict the effects of management and climatic changes on organic C, N, S, and P across the Great Plains, with projections to similar soil/climate zones around the world. The dynamics of soil organic C and N in cultivated and grassland
12 systems were simulated with a mathematical model which success- fully represents the long-term impact of cultivation on soil organic matter C and N levels for a wheat fallow system, and simulates the impact of different combinations of straw, manure, and N addition in Swedish soils. A later version of this model simulates the dynamics of C, N, S, and P in the soil and plant system. It includes the controls of plant lignin content and soil texture on nutrient cycling and soil organic matter levels. This model has been used to simulate steady-state organic mat- ter, plant production, and decomposition levels for grassland soils across the climatic gradients of the Great Plains region. It has also been used to simulate cultivation effects on N, P, and S availability over the long term in Canadian wheat fallow systems and the im- pact of different fire frequency on C, N, and P dynamics in a native tallgrass prairie. Rangeland productivity was evaluated using data collected by the U.S. Soil Conservation Service for 12,000 range sites, and soil properties were examined from chemical characterization of 1,000 pedons. These data were combined with climatic data for regression analysis, yielding regional maps of productivity and organic matter levels illustrated in Figure 3. The analysis of changes in levels of organic C, N, S, and P in grassland soils placed under cultivation confirmed concepts of state factor control over soil processes that were developed in early studies of topo- and chronosequences formed over geological time periods. Major controls on key chemical, physical, and biological processes were identified to provide linkages for interpretation across different scales of time and space. The systems analytical approach is presented as a tool for or- ganizing and integrating information on key chemical, physical, and biological processes fundamental to the understanding of changes in natural and managed systems. When quantified with the use of mathematical simulation models, this approach enables prediction of management effects on productivity and the potential for adverse environmental effects. ACKNOWLEDGEMENT Helpful discussions with Dr. John Stewart, Director, Saskatch- ewan Institute of Pedology, during preparation of this paper are gratefully acknowledged.
13 ⢠CLlMATlC PLANT PRODUCT1ON / DECOMPOSlTION 22 PARAMETER SOlL C (kg m'2) (SANDY) 1.0 FIGURE 3 The upper panel shows the ratio of productivity (NPP) to the climatic decomposition parameter (CDP). This parameter integrates the effects of temperature and moisture on heterotrophic activity. The ratio NPP/CDP indicates the potential for carbon stabilization in the soil. The lower panel shows predicted soil carbon levels to 20 cm for sandy-textured grassland soils.
14 REFERENCES Barber, 8.A. 1984. Soil nutrient bioavailability. New York: Wiley-Interscience, John Wiley and Sons. Cole, C.V., and R.D. Heil. 1981. Phosphorus effects on terrestrial nitrogen cycling. In F.E. Clark and T. Rosswall, Eds. Terrestrial nitrogen cy- cles, processes, ecosystem and management impact. Ecol. Bull. Stockholm. 33:363-374. Cole, C.V., J.W.B. Stewart, H.W. Hunt, and W.J. Parton. 1987. Cycling of carbon, nitrogen, sulfur and phosphorus: controls and interactions. Proceedings XIII Congress of the International Society of Soil Science (in press). Cole, C.V., J. Williams, M. Shaffer, and J. Hanson. 1987. Nutrient and organic matter dynamics as components of agricultural production systems models. In R.F. Follett, J.W.B. Stewart, and C.V. Cole, Eds. Soil fertility and organic matter as critical components of production systems. Soil Sci. Soc. Am. and Am. Soc. Agron. SSA Special Pub. No. 19. Chapter 9:147-166. Madison, WI. Follett, R.F., J.W.B. Stewart, and C.V. Cole, Eds. 1987. Soil fertility and organic matter as critical components of production systems. Soil Sci. Soc. Am. and Am. Soc. Agron. SSA Special Pub. No. 19. Madison, WI. McGill, W.B., and C.V. Cole. 1981. Comparative aspects of organic C, N, S, and P cycling through soil organic matter during pedogenesis. Geoderma 26:267-286. Parton, W.J., J.W.B. Stewart, and C.V. Cole. 1987. Dynamics of C, N, P, and S in cultivated soils: a model. Biogeochemistry (in press). Smeck, N. E. 1985. Phosphorus dynamics in soils and landscapes. Geoderma 36:185-199. Stewart, J.W.B., C.V. Cole, and D.G. Maynard. 1983. Interaction of biogeo- chemical cycles in grassland ecosystems. In B. Bolin and R.C. Cook, Eds. The major biogeochemical cycles and their interactions. Sussex: John Wiley & Sons. SCOPE 21. Chapter 8:247-269. Stewart, J.W.B., and A.N. Sharpley. 1987. Controls on dynamics of soil and fertilizer phosphorus and sulfur. In R.F. Follett, J.W.B. Stewart, and C.V. Cole, Eds. Soil fertility and organic matter as critical components of production systems. Soil Sci. Soc. Am. and Am. Soc. Agron. SSSA Special Pub. No. 19. Chapter 6:99-119. Madison, WI.
