Nitrogen is ubiquitous in the environment. It is one of the most important plant nutrients and forms some of the most mobile compounds in the soil-crop system. Nitrogen is continually cycled among plants, soil organisms, soil organic matter, water, and the atmosphere (Figure 6-1). Nitrogen enters the soil from many different sources and leaves the root zone of the soil in many different ways. This flux of nitrogen into, out of, and within the soil takes place through complex biochemical transformations.

The mounting concerns related to agriculture's role in nitrogen delivery into the environment are reflected in several detailed reviews (Follett and Schimel, 1989; Follet et al., 1991; Hallberg, 1987, 1989b; Keeney, 1986a,b; Power and Schepers, 1989). A brief review of the nitrogen cycle and nitrogen budget or mass balance considerations is necessary to understand the options for management improvements in farming systems to mitigate the environmental impacts of nitrogen.

The nitrogen cycle is critical to crop growth. 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

compounds that form the soil humus. At any one-time, most of the nitrogen held in the soil is stored in soil organic matter.

Mineralization processes transform the nitrogen in soil organic matter to ammonium ions (NH₄), releasing them into the soil. 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. It is readily converted into nitrate, through nitrification, at appropriate soil temperatures (above about 9°C [48°F]). Ammonium can create water quality problems for fish and aquatic life under certain temperature and dissolved oxygen conditions.

Nitrification processes transform ammonium ions, which are produced by mineralization or added to the soil, to nitrite (NO₂) and to nitrate (NO₃), which is easily absorbed by plant roots. Nitrification is typically mediated by soil bacteria and can take place rapidly with adequate soil moisture and temperature under oxidizing conditions in the soil. Except for some atmospheric processing, nitrification in the soil is the sole natural source of nitrate in the environment. Nitrate is soluble and mobile in water and is the form of nitrogen most commonly related to water quality problems. Nitrates that are not absorbed by plants or microorganisms or otherwise immobilized may readily move with percolating water and may leach through the soil to groundwater. Nitrates in the groundwater can move through springs and seeps or shallow flow systems to pollute surface waters, or they can leach into deeper aquifers.

Immobilization includes various processes through which ammonium ions and nitrates are converted to organic nitrogen (referred to as organic-N) and immobilized or bound up in the soil. Ammonium and nitrate ions can be taken up by plants or microorganisms in the soil, transforming the nitrogen into organic matter. Mineralized nitrogen can rapidly recycle through transformations to ammonium and nitrate and then back into the organic-N pool. This occurs primarily through the action of microbes.

Denitrification, another biological transformation, converts nitrate into nitrite and then to gaseous nitrogen (N₂) and nitrous oxide (N₂O). This is the major pathway that returns nitrogen from the soil environment to the atmosphere. Such losses are of environmental concern because these gases are among those that contribute to the so-called greenhouse effect and may affect the protective layer of ozone in the stratosphere.

Mineralization, nitrification, immobilization, and denitrification are interactive processes through which a nitrogen molecule may move many times. The processes are affected by oxidizing and reducing conditions and the availability of oxygen and organic carbon in the soil. These processes go on simultaneously; they can coexist in close proximity and vary temporally in the same setting. In the small pores within aggregates in the soil profile, oxygen may be depleted and reducing conditions may become dominant, resulting in denitrification. Yet, on the exteriors of aggregates, around macropores, oxygen may be available and nitrification occurs. Seasonally, in a setting where the soil is normally dominated by air-filled pores and oxidizing conditions, the soil may become saturated with water during recharge events, and reducing conditions and denitrification may dominate temporarily. It is the balance between these processes and their seasonal timing that determines how much nitrogen is available for crops and how much nitrogen may be lost from the soil to groundwater and surface water or the atmosphere.

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). The nitrogen in commercial fertilizer is directly added to soil systems in many forms, but the dominant forms are ammonium, nitrate, and urea. Some nitrogen, primarily as nitrate and ammonium, is also added with precipitation.

Nitrogen is taken up by crops and can be removed from the soil

system with the harvested portion of the crop (for example, grain) or can be left in the soil system as root mass or crop residues. Nitrogen can be lost to the atmosphere through denitrification or the volatilization of ammonia from the fertilizers and manures applied to the soil surface. It can also move through or over the soil with water to pollute surface water or groundwater.

Even under native prairies and forests, some nitrogen loss occurs through leaching, denitrification, erosion, and biomass. Biomass nitrogen can be lost because of a limited harvest, lost from senescing vegetation, or carried away by wind or smoke when the biomass is burned. Nutrient gains and losses in natural ecosystems are roughly in balance, however; and nitrogen losses from natural ecosystems into water are significantly lower than losses from agricultural ecosystems. Numerous studies on various scales have shown from 3-to 60-fold greater nitrate concentrations in surface water and groundwater in agricultural areas compared with those in forested or grassland areas (Hallberg, 1987, 1989b; Keeney, 1986a,b; McArthur et al., 1985; Omernik, 1976). Continued growth of plants in natural ecosystems depends on the cycling of nutrients between biomass and organic and inorganic stores (Miller and Larson, 1990).

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

residues voided in manures, and nitrogen supplied by legumes (alfalfa and soybeans). Outputs include nitrogen in harvested crops and crop residues. (See the Appendix for a full discussion of the methods used to estimate nitrogen inputs and outputs.) Only the manure that is collectible and that can be applied to croplands was considered. Some of the nitrogen in collectible manures is lost through volatilization, runoff, leaching, or other processes before it can be applied to croplands. The amount of nitrogen lost depends on the methods used to collect, store, and apply manures. In Table 6-1, only that portion of total nitrogen voided in manures that was estimated to be economically collectible and recoverable for use on croplands was used as the nitrogen inputs from manure.

Estimates of the rate of nitrogen fixation by alfalfa and soybeans vary widely. Estimates of rates of fixation by alfalfa range from 70 to 600 kg/ha/yr (62 to 532 lb/acre/yr) and from 15 to 310 kg/ha/yr (13 to 275 lb/acre/yr) for soybeans (Appendix Table A-4). Such large ranges in reported values are related, in part, to differences in soil nitrogen availability, climate, and crop variety. In addition, the amount of nitrogen fixed by alfalfa depends on the density and age of the stand. Estimates are further complicated because the fixed nitrogen is not immediately available for use by crop plants and some of the reduced need for nitrogen by crops following legumes is related to rotation effects other than the nitrogen supplied by fixation. Because of these difficulties, nitrogen replacement values are usually used to estimate the effect of legumes on the need for supplemental nitrogen by succeeding crops. The nitrogen replacement values include both the rotation effects and the influence of fixed nitrogen when determining the need for supplemental nitrogen.

Because of the wide range of estimates of nitrogen fixation by legumes (alfalfa and soybeans), the committee used three fixation-nitrogen replacement value estimates (low, medium, and high scenarios) to calculate nitrogen inputs. The nitrogen fixation rates and replacement values under the three scenarios are given in Table 6-2. The nitrogen replacement value, as used here, is the difference between the nitrogen input (fixed and accumulated nitrogen) and the nitrogen removed with the harvested legume crop (see Appendix Table A-5.)

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. A more detailed analysis of nitrogen inputs and outputs from agricultural lands helps to identify opportunities for reducing nitrogen losses from farming systems.

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. All of these nitrogen sources are subject to the transformations of the nitrogen cycle and all can contribute to environmental nitrogen losses. The importance of any particular source depends on the type of agricultural enterprise, its geographic location and climate, and the soil's microclimate. This variation is evident in Tables 6-3 and 6-4, which report state- and national-level nitrogen mass balances.

The nitrogen in fertilizers is the single largest source of nitrogen applied to most croplands. In 1987, 9.39 million metric tons (10.4 million tons) of nitrogen was applied nationwide in the form of synthetic fertilizers. For the low, medium, and high scenarios, the amount of synthetic fertilizer applied represents 47, 45, and 42 percent of nitrogen inputs, respectively. The importance of synthetic fertilizers as a nitrogen source (fertilizer-N) varies widely around the United States, depending on the crop and the region where that crop is grown. Three of the four major commodity crops—corn, wheat, and cotton—use 61 percent of U.S. fertilizer-N. Corn, which covers about 21 percent of U.S. cropland,

is by far the major nitrogen user in the United States, accounting for about 41 percent of the fertilizer-N applied (Vroomen, 1989).

Rates of application of fertilizer-N also vary by crop and region. Of the major commodity crops, little or no nitrogen is applied to soybean crops; but in 1988, an average of 153 kg of nitrogen/ha (137 lb/acre) was applied to corn crops nationwide. Corn crops receive the highest amounts of fertilizer-N, which have increased nationally from about 67 kg/ha (60 lb/acre) in 1965 to about 157 kg/ha (140 lb/acre) in 1985. Rates have declined slightly since 1985. The rates of fertilizer-N application to crops such as sorghum and potatoes are also significant, but these crops cover more limited areas (Vroomen, 1989). Geographically, fertilizer-N use parallels cropping patterns; 10 states—predominantly grain-producing states—account for nearly 60 percent of fertilizer-N use (Table 6-5).

The symbiotic bacteria associated with leguminous crops such as alfalfa and soybeans can fix and add substantial amounts of nitrogen to the soil. The amount of nitrogen fixed by alfalfa and soybeans under the low, medium, and high scenarios is approximately 6.1 million metric tons (6.6 million tons), 6.9 million metric tons (7.5 million tons), and 8.6 million metric tons (9.5 million tons), 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). Alfalfa has been reported to fix as little as

70 kg of nitrogen/ha (62 lb/acre) and as much as 600 kg of nitrogen/ha (532 lb/acre). Soybeans have been found to fix as little as 15 kg of nitrogen/ha (13 lb/acre) and as much as 310 kg of nitrogen/ha (275 lb/acre) (Appendix Table A-4). Some of that fixed nitrogen is removed when the crop is harvested, but some remains in the soil and is available for subsequent crops.

Estimates of the amount of nitrogen actually fixed by particular legumes are problematic because there are no unequivocal methods for measurement (see Appendix). Crop rotation with legumes, however, consistently produces a yield benefit to succeeding crops with reduced inputs of nitrogen. The contribution of legumes to the national nitrogen balance is very important (Tables 6-3 and 6-4). To minimize environmental losses of nitrogen and to optimize crop yields, an estimate of the legume contribution to nitrogen in the farming system must be considered.

The importance of the nitrogen in manure (manure-N) in the mass balance varies from region to region (Tables 6-3 and 6-4). When livestock is a component of the farming system, the contribution of manure-N to the mass balance can be significant.

Economically recoverable manure-N represents 9, 8, and 8 percent of total nitrogen inputs in the low, medium, and high scenarios, respectively. The mass of economically recoverable manure-N, however, is relatively low compared with the total mass of manure-N. Nationally, only 34 percent of the total nitrogen voided in manures is estimated to be economically recoverable for use elsewhere. The portion of manures that are economically recoverable can be increased through better management.

The amount of manure-N actually applied to croplands depends on the kind of manure and, particularly, the way that the producer handles the manure. 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) found that manure was applied to fields at rates ranging from 29 to 101 metric tons/ha (13 to 45 tons/acre), even though producers thought they were applying 45 metric tons/ha (20 tons/acre). Different animal manures contain different proportions of nitrogen, and the nitrogen occurs in various forms. A large portion of the nitrogen in manures may be found in the organic form and is not

immediately available for crops when it is applied. This nitrogen becomes available over time as it is mineralized and can contribute nitrogen over several crop seasons.

These and other special problems in managing nutrients in manures are discussed at greater length in Chapter 11.

Crop residue is the mass of plant matter that remains in the field after harvest (such as corn stover). The harvested portion of crops remove nutrients from the system, but most of the crop residues remain in the soil system and effectively enter the organic-N storage component. Although crop residues from a previous year may be factored as an input, the crop residues of the current crop year must be considered an output, and for a given field this often results in a relative balance. Hence, in routine management and nutrient-yield response evaluations, residues are often ignored as inputs and are implicitly factored into the soil-mineralization contribution.

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. In certain farming systems and at certain times, these other inputs can be important. Because of their variability and the difficulty in estimating the amount of nitrogen obtained from these sources on a state or national basis, they were not used to estimate the mass balances given in Tables 6-1, 6-3, and 6-4. There are other inputs sources, such as nitrogen in dry deposition, crop seed, foliar absorption, and nonsymbiotic fixation of nitrogen. These are minor or secondary inputs that are not typically manageable and are seldom measured. These sources have been implicitly included in nutrient-yield response evaluations and are explicitly ignored in most studies.

The amount of nitrogen found in rainfall varies from storm to storm and region to region. The total inorganic nitrogen deposited in rainfall ranged from 3.9 to 12.4 kg/ha/year (3.5 to 11.1 lb/acre/year) in studies done in Indiana, Iowa, Minnesota, Wisconsin, and Nebraska (Tabatabai

et al., 1981); and annual averages across the eastern United States in National Atmospheric Deposition Program Monitoring range from 3 to 7 kg/ha (3 to 6 lb/acre) (Schepers and Fox, 1989). These sources provide low amounts of nitrogen compared with the nitrogen inputs from fertilizers, manure, and legumes in intensively managed croplands. Hence, they are not typically considered in cropland nitrogen mass balances (Oberle and Keeney, 1990). They can be, however, an important source of nitrogen in rangelands and natural ecosystems (Schepers and Fox, 1989). Nitrogen inputs from precipitation are generally low, and they are often assumed to be about equivalent to the annual nitrogen losses through runoff and erosion (Meisinger, 1984).

The amount of nitrogen in irrigation water is often quite low and is not normally considered in nitrogen mass balances. However, in areas where irrigated and fertilized crop production has been practiced for some time, the nitrogen in the form of nitrate (nitrate-N) in the groundwater used to irrigate crops has become a significant nitrogen source. In the Central Platte River Valley in Nebraska, nitrate contamination of the shallow groundwater has been increasing at a rate of 0.4 to 1.0 mg/liter/year (0.4 to 1.0 ppm/year) (Exner, 1985; Exner and Spalding, 1976, 1990; Spalding et al., 1978). The contamination is related to the nitrogen output losses from intensive nitrogen fertilization in irrigated corn production. In many areas the nitrate-N concentrations in the groundwater have increased from 2 mg/liter (2 ppm) to between 10 and more than 20 mg/liter (10 to >20 ppm) (Exner and Spalding, 1990). With increased nitrate-N concentrations, irrigation water can become an important source of nitrogen. For example, 30 cm (12 inches) of irrigation water with a nitrate-N concentration of 20 mg/liter (20 ppm) would result in an application of 60 kg of nitrogen to each hectare (54 lb/acre) irrigated.

Schepers and colleagues (1986) noted that in the Central Platte River Valley, the groundwater used to irrigate corn contributed an average of 46 kg of nitrogen/ha (41 lb/acre), or 31 percent of the nitrogen applied as fertilizer. The groundwater used to irrigate potatoes in Wisconsin contributed an average of 57 kg/ha (51 lb/acre), 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). In some natural resource districts in Nebraska, the nitrate-N in the irrigation water must now be

accounted for and is used to reduce the amount of fertilizer-N applied (Central Platte Natural Resources District, 1992; Schepers et al., 1991).

Mineralization is a relatively slow process that is dependent on temperature and moisture; only 2 to 3 percent of the organic nitrogen stored in soil is mineralized annually (Buckman and Brady, 1969; Oberle and Keeney, 1990). This 2 to 3 percent, however, is the basis for natural ecosystem nutrient cycling and, depending on the amount of organic matter in the soil, can supply a significant portion of the nitrogen needed by crops each year.

Despite the relatively slow rate of mineralization, this process can be an important factor in determining the year-to-year variability in the amount of nitrogen available to crop plants. The 2 to 3 percent mineralization rate is an average and the moisture and temperature regimes that are optimal for plant growth are also optimal for nitrogen mineralization and nitrification that make the nitrogen stored in soil organic matter available to plants. In years when conditions are optimal, more nitrogen may be released; this natural interaction is an important contributor of nitrogen in climatically optimal years that produce bumper crops. However, the mineralization of nitrogen from the soil, related to inherent soil fertility, has been implicitly included in nutrient-yield response and management evaluations for different soils. New tools are needed to measure the actual nitrogen available from mineralization and other residuals to account for and take advantage of annual variability.

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. Nitrogen is also leached as nitrate to locations deeper in the soil or to groundwater. Other minor outputs can include gaseous losses such as N₂O evolution during nitrification; decomposition of nitrous acid, or losses directly from maturing or senescent crops (Bremner et al., 1981; Meisinger and Randall, 1991; Nelson, 1982). Some nutrients are taken up by weeds or immobilized by microbes, but these nutrients primarily enter the organic-N storage pool. These minor outputs are secondary factors and have typically been implicitly included in nutrient-crop yield response models.

The nitrogen found in harvested crops represents the greatest and most important output of nitrogen from croplands. For 1987, the nitrogen harvested in crops and residues was estimated at more than 13 million metric tons (14 million tons) (Table 6-1). The amount of nitrogen harvested in crops and residues was estimated to be 67, 64, and 60 percent of total inputs under the low, medium, and high scenarios, respectively. The balance of total nitrogen inputs not accounted for in crops or residues was 6.7 million, 7.4 million, and 9.1 million metric tons (7.4 million, 8.1 million, and 10 million tons) under the low, medium, and high scenarios, respectively. These balances represent 33, 36, and 40 percent of total nitrogen inputs, respectively.