Soil and Water Quality An Agenda for Agriculture (1993) / Chapter Skim
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6 Nitrogen in the Soil-Crop System
6 Nitrogen in the Soil-Crop System
Pages 237-282
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From page 237... ...
The balance between inputs and outputs and the various transformations in the nitrogen cycle determine how much nitrogen is available for plant growth and how much may be lost to the atmosphere, surface water, or groundwater. Nitrogen is an important component of soil organic matter, which is made up of decaying plant and animal tissue and the complex organic 237
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From page 238... ...
238 / Soil and Water Quality: An Agenda for Agriculture Atmospheric ~ nitrogen Industrial Nation Fertilizer \ Volatilization Manure Crop \ removal Legumes Biological fixation Soil surface 1 Mineralization \ Ammonium Nitrification Nitrate NO2 Leaching , \ Crop residues \1 \ Crop L uptake ~; ~ Organic matter Immobilizatio / as/ Denitrification Runoff and lateral surface flow Groundwater FIGURE 6-1 The nitrogen cycle. Source: Pennsylvania State University, College of Agriculture.
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From page 239... ...
Ammonium is relatively immobile in the soil, being strongly adsorbed to clay minerals and organic matter. Ammonium may be delivered to surface water, attached to sediment or suspended matter, or in solution.
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From page 240... ...
NITROGEN MASS BALANCE A molecule of nitrogen may enter the soil system as organic-N from crop residues or other plant or microbial biomass, from animal manures or organic wastes (for example, sewage sludge or food processing residues) , and through the action of leguminous plants such as alfalfa that take nitrogen from the atmosphere and incorporate it into the plant's tissue (nitrogen fixation)
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From page 241... ...
Table 6-1 estimates the major, manageable, national nitrogen inputs and outputs for harvested croplands in 1987. Inputs of nitrogen include nitrogen applied to croplands as synthetic fertilizers, nitrogen in crop
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From page 242... ...
Estimates of nitrogen outputs in harvested crops and crop residues are also reported in Table 6-1. The difference between nitrogen inputs and outputs is reported as nitrogen balances.
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From page 243... ...
aThe nitrogen replacement value includes the amount of fixed nitrogen available to a succeeding crop and the reduced need for supplemental nitrogen that may be a result of rotation effects. Nitrogen Inputs The nitrogen delivered in rainfall; obtained from fertilizers; mineralized from soil organic-N, crop residues, manure, or legumes; or even delivered in irrigation water contributes to the nitrogen budget of a particular agricultural field.
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From page 245... ...
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From page 247... ...
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From page 253... ...
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From page 255... ...
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From page 256... ...
, respectively. These estimates represent 30, 33, and 38 percent of nitrogen inputs, respectively (depending on the rate of fixation and the nitrogen replacement values used for alfalfa and soybeans)
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From page 257... ...
Application rates vary dramatically from farm to farm, and manure is often applied by using manure-spreading equipment that makes careful calibrations of the nitrogen application rate difficult. In Lancaster County, Pennsylvania, for example, Schepers and Fox (1989)
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From page 258... ...
Other Nitrogen Inputs Synthetic fertilizers, legumes, and manures are the most important sources of nitrogen inputs to soil-crop systems. Nitrogen is, however, added to soil-crop systems in rainfall and irrigation water and through mineralization from soil organic matter.
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From page 259... ...
, or 25 percent of the nitrogen added as fertilizer (Saffigna and Keeney, 1977~. Surface waters used as sources of irrigation water usually contain much lower concentrations of nitrogen (Schepers and Fox, 1989~.
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From page 260... ...
Nitrogen Outputs The primary desired output is nitrogen taken up in harvested crops and crop residues. Nitrogen is lost to the atmosphere by volatilization and denitrification and is washed away in runoff in solution, attached to eroded particulates or organic matter.
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From page 261... ...
Nitrogen balances are positive under all three scenarios (Table 6-1~. At the national level, the nitrogen applied to croplands in synthetic fertilizers is roughly the same as that obtained in harvested crops (not including crop residues)
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From page 262... ...
For example, as many large-scale balances would suggest, the harvested crop nitrogen output is slightly greater than the fertilizer nitrogen input. However, more than 35 percent of the nitrogen output in harvested crops is accounted for by various legumes that receive very little nitrogen fertilizer.
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From page 263... ...
Various cropland studies show that the postharvest residual of available nitrogen in the soil, both in the fall and following crop season, is proportionately related to the amount of nitrogen applied (e.g., Bundy and Malone, 1988; Jokela, 1992; Jokela and Randall, 1989~. In the context of climatic variability and related crop yield variability, some residual nitrogen and some losses into the environment are inevitable.
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From page 264... ...
. The amount of nitrate-nitrogen lost in leaching to drainage tiles installed beneath topsoil was related in a nearly linear fashion to the amount of nitrogen applied for lands with application rates that exceeded 50 kg/ha (45 lb/acre)
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From page 265... ...
more than the recommended amount of nitrogen was applied. Data from the Big Spring Basin in Iowa trace the relationship of increasing residual nitrogen and groundwater nitrate concentrations over time (Figure 6-2~.
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From page 266... ...
Reducing the mass of residual nitrogen added to the soil-crop system can improve both economic and environmental performances. Reducing the mass of residual nitrogen in the soil-crop system can be accomplished by accounting for all sources of nitrogen added to the system, refining estimates of crop nitrogen requirements, refining yield goals, synchronizing the application of nitrogen with crop needs, and increasing seasonal nitrogen uptake in the cropping system.
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From page 267... ...
estimated nitrogen balances for southeastern Minnesota and found that nitrogen from alfalfa, soybeans, and manure provided, on average, 95 kg/ha (85 lb/acre) or 64 percent of the nitrogen applied in commercial fertilizers.
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From page 268... ...
237 (8) NOTE: Potential reductions are estimated by adjusting fertilizer application rates to account for the nitrogen supplied by alfalfa by fixation or indirectly from manure produced by livestock fed alfalfa.
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From page 269... ...
Of the 29 fields, 86 percent had greater concentrations of soil nitrate than were needed for optimal yields; 56 percent had at least twice the critical amount needed, and 21 percent had at least three times this amount. Crop response studies in this region have consistently shown that no fertilizer-N or only a small starter nitrogen application is needed to produce optimal or maximum yields after a multiyear alfalfa stand.
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From page 270... ...
This work is done through field trials by growing the crop using various nitrogen application rates, usually on research plots, and measuring the changes in crop yields. The variability in crop response to nitrogen is accounted for by multiple plot replications of the same nitrogen application rates to integrate the local variability imposed by soil (and imposed by the research methods used on small plots)
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From page 271... ...
, determination of optimal fertilization rates involves the fitting of some form of statistical model to the observed crop yield responses to the various rates of fertilizer application over time. Economically Optimum Rate of Nitrogen Application The concept of the economically optimum rate of nitrogen application was developed early in the assessment of the use of fertilizers to enhance crop production (Heady et al., 1955; National Research Council, 1961~.
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From page 272... ...
Setting realistic yield goals is particularly important for reducing residual nitrogen. An unrealistically high yield goal will result in nitrogen applications in excess of that needed for the yield actually achieved and will contribute to the mass of residual nitrogen in the soil-crop system.
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From page 273... ...
When timing is coupled with new tools, such as the presidedress soil nitrate test, to gauge the amount of nitrogen available, and hence the additional amount actually needed, significant economic and environmental benefits may be possible. New Tools for Nitrogen Management New tools and management methods are needed to accurately assess available residual nitrogen and to reduce the producer's uncertainty in estimating a crop's nitrogen needs.
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From page 274... ...
In a project to implement and evaluate the PSNT with fertilizer dealers in Iowa, replicated on-farm trials produced equivalent crop yields but reduced nitrogen applications an average of 42 percent using the PSNT to refine nitrogen applications. The test saved money for producers and significantly reduced environmental loading of nitrogen (Blackmer and Morris, 1992; Hallberg et al., 1991~.
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From page 275... ...
In addition, the nature of the crop response to nitrogen and its resulting effect on the economically optimal rate of nitrogen application also constrain the extent to which improvements in nitrogen management alone may reduce nitrogen losses from current cropping systems. The first stage in current management is to establish the nitrogen requirements of a crop under various soil and climatic conditions.
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From page 276... ...
240 280 FIGURE 6-3 Yield response of corn to nitrogen applied to three soils. Fayette silt, Fayette silt loam (fine-silty, mixed, mesic Typic Hapludalfs)
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From page 277... ...
As the rate of nitrogen application increases, less is recovered in the harvested grain (or in plant residues) and more nitrogen remains as residual nitrogen, potentially to be lost into the environment.
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From page 278... ...
EN, fertilizer-N; Ccc, continuous corn; CSb, corn-soybeans-(corn-oats) ; Cm, corn-oats-(meadowalfalfa brome mix)
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From page 279... ...
Because of the form of the nitrogen-yield response, the potential for nitrogen losses is very sensitive at high nitrogen application rates when plant uptake of nitrogen is limited. Decreasing the economically optimum yield goal by 5 percent reduces the unrecovered fertilizer-N by about 20 to 30 percent for the continuous corn arid reduces the unrecovered amount even more for the corn-soybean rotation.
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From page 280... ...
In many environments, it is likely that techniques for managing residual nitrogen will need to be used along with refined input management, or nitrate losses may remain unacceptably high. Cropping Systems as a Nitrogen Management Tool The development of cropping systems that prevent the buildup of residual nitrogen during the dormant season has been a focus of research in the past 10 years.
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From page 281... ...
In many cases, recovery of cover crop nitrogen has been found to be lower than recovery of fertilizer-N (Doran and Smith, 1991~. It is critical to determine whether cover crops continually recycle the nitrogen that they absorb or whether they merely act as a temporary sink for the residual nitrogen that ultimately ends up in groundwater or surface water.
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