6
Nitrogen in the Soil-Crop System

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

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



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Soil and Water Quality: An Agenda for Agriculture 6 Nitrogen in the Soil-Crop System 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 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

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Soil and Water Quality: An Agenda for Agriculture FIGURE 6-1 The nitrogen cycle. Source: Pennsylvania State University, College of Agriculture. 1989. Groundwater and Agriculture in Pennsylvania. Circular 341. College Station: Pennsylvania State University. Reprinted with permission from © The Pennsylvania State University.

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Soil and Water Quality: An Agenda for Agriculture 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 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 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 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.

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Soil and Water Quality: An Agenda for Agriculture Denitrification 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. Interactive Processes 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. 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). 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

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Soil and Water Quality: An Agenda for Agriculture TABLE 6-1 Nitrogen (N) Inputs, Outputs, and Balances in the United States under the Low, Medium, and High Scenarios   Metric Tons of N (Percent of Total Inputs)a Input or Output Low Scenario Medium Scenario High Scenario Input Fertilizer-N 9,390,000 (47) 9,390,000 (45) 9,390,000 (42) Manure-N 1,730,000 (9) 1,730,000 (8) 1,730,000 (8) Legume-N 6,120,000 (30) 6,870,000 (33) 8,560,000 (38) Crop residues 2,890,000 (14) 2,890,000 (14) 2,890,000 (13) Total input 20,100,000 (100) 20,900,000 (100) 22,600,000 (100) Output Harvested crops 10,600,000 (53) 10,600,000 (51) 10,600,000 (47) Crop residues 2,890,000 (14) 2,890,000 (14) 2,890,000 (13) Total output 13,500,000 (67) 13,500,000 (64) 13,500,000 (60) Balance 6,670,000 (33) 7,420,000 (36) 9,110,000 (40) NOTE: See the Appendix for a full discussion of the methods used to estimate nitrogen inputs and outputs. a Input, output, or balance as a percent of the total mass of inputs. 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

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Soil and Water Quality: An Agenda for Agriculture 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.

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Soil and Water Quality: An Agenda for Agriculture TABLE 6-2 Nitrogen Accumulation and Nitrogen Replacement Value Estimated for Alfalfa and Soybeans     Nitrogen Accumulation (kg/ha) Legume Scenario Total Nitrogen Input Nitrogen Replacement Valuea Alfalfa Low 230 45 Medium 250 65 High 380 195 Soybeans Low 175 10 Medium 200 35 High 220 55 NOTE: See the Appendix for a full discussion of the methods used to estimate nitrogen accumulation and replacement values. a The 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. 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. Nitrogen in Fertilizers 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,

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Soil and Water Quality: An Agenda for Agriculture TABLE 6-3 State and National Nitrogen Inputs and Outputs (metric tons)   Inputs Outputs State Fertilizer-N Recoverable Manure-N Legume-N Fixation Crop Residues Total Harvested Crop Crop Residues Total Balance Low Scenario Alabama 111,000 25,300 44,300 9,420 190,000 60,800 9,420 70,200 120,000 Alaska 1,850 0 0 61 1,910 620 61 681 1,230 Arizona 73,500 22,200 12,800 3,980 112,000 35,500 3,980 39,500 73,000 Arkansas 188,000 40,300 229,000 54,600 512,000 220,000 54,600 275,000 237,000 California 482,000 106,000 88,400 32,800 709,000 246,000 32,800 279,000 431,000 Colorado 126,000 61,300 66,600 48,000 302,000 175,000 48,000 223,000 78,500 Connecticut 6,450 4,880 1,900 81 13,300 3,800 81 3,880 9,430 Delaware 15,200 5,850 16,300 4,380 41,700 14,000 4,370 18,400 23,400 Florida 258,000 15,700 7,900 3,090 285,000 25,100 3,090 28,200 257,000 Georgia 175,000 31,400 56,500 20,000 283,000 105,000 20,000 125,000 158,000 Hawaii 14,900 1,030 10 0 15,900 110 0 110 15,800 Idaho 187,000 26,600 85,800 37,900 337,000 186,000 37,900 224,000 113,000 Illinois 805,000 37,200 676,000 380,000 1,900,000 1,120,000 380,000 1,500,000 402,000 Indiana 462,000 32,900 349,000 200,000 1,040,000 595,000 200,000 795,000 249,000 Iowa 780,000 87,400 688,000 394,000 1,950,000 1,200,000 394,000 1,590,000 356,000 Kansas 438,000 114,000 202,000 163,000 917,000 547,000 163,000 710,000 207,000 Kentucky 165,000 24,500 103,000 35,300 327,000 163,000 35,300 199,000 129,000 Louisiana 138,000 5,670 111,000 26,100 280,000 109,000 26,100 135,000 145,000 Maine 11,600 7,780 2,200 1,230 22,800 11,700 1,230 12,900 9,880 Maryland 38,800 21,100 36,500 12,600 109,000 47,100 12,600 59,700 49,300 Massachusetts 9,860 4,390 3,000 161 17,400 5,300 161 5,460 12,000 Michigan 220,000 37,900 163,000 64,000 485,000 238,000 64,000 302,000 183,000 Minnesota 525,000 79,400 450,000 220,000 1,270,000 737,000 220,000 957,000 317,000 Mississippi 142,000 13,400 145,000 23,800 324,000 115,000 23,800 138,000 186,000 Missouri 322,000 42,400 386,000 113,000 863,000 487,000 113,000 600,000 263,000

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Soil and Water Quality: An Agenda for Agriculture   Inputs Outputs State Fertilizer-N Recoverable Manure-N Legume-N Fixation Crop Residues Total Harvested Crop Crop Residues Total Balance Montana 91,100 12,000 114,000 52,400 270,000 212,000 52,400 264,000 5,200 Nebraska 608,000 93,000 273,000 236,000 1,210,000 662,000 236,000 898,000 312,000 Nevada 4,880 2,220 22,500 521 30,100 29,900 521 30,400 (300) New Hampshire 2,040 1,980 1,700 24 5,740 3,210 24 3,230 2,510 New Jersey 20,800 3,170 11,700 3,040 38,700 14,900 3,040 17,900 20,800 New Mexico 24,000 14,500 18,300 5,410 62,200 36,800 5,410 42,200 20,000 New York 93,900 78,200 84,300 18,700 275,000 138,000 18,700 156,000 119,000 North Carolina 176,000 33,000 94,000 30,900 334,000 122,000 30,900 153,000 181,000 North Dakota 302,000 13,700 162,000 108,000 586,000 357,000 108,000 465,000 121,000 Ohio 356,000 40,100 327,000 134,000 856,000 468,000 134,000 602,000 253,000 Oklahoma 246,000 30,500 44,300 34,100 355,000 162,000 34,100 196,000 159,000 Oregon 114,000 13,500 34,100 16,600 178,000 93,900 16,700 111,000 67,600 Pennsylvania 47,500 79,200 90,900 29,700 247,000 156,000 29,700 186,000 61,200 Rhode Island 1,490 0 200 8 1,700 330 8 338 1,360 South Carolina 65,100 5,680 43,200 11,300 125,000 45,600 11,300 56,900 68,400 South Dakota 164,000 35,300 278,000 94,900 572,000 360,000 94,900 455,000 117,000 Tennessee 140,000 20,600 96,600 23,500 281,000 118,000 23,600 141,000 140,000 Texas 674,000 153,000 29,700 94,800 951,000 340,000 95,800 436,000 515,000 Utah 27,000 11,300 44,900 4,480 87,700 57,100 4,480 61,500 26,100 Vermont 4,480 17,800 9,500 256 32,000 17,400 256 17,700 14,400 Virginia 71,300 25,400 47,800 11,000 156,000 73,500 11,000 84,500 71,000 Washington 185,000 25,300 40,600 39,200 290,000 158,000 39,200 197,000 93,200 West Virginia 11,900 5,610 7,800 948 26,300 14,800 948 15,700 10,500 Wisconsin 243,000 161,000 271,000 84,000 759,000 441,000 84,000 524,000 235,000 Wyoming 20,900 9,570 50,500 5,180 86,200 53,700 5,180 58,900 27,300 United States 9,390,000 1,730,000 6,120,000 2,890,000 20,100,000 10,600,000 2,890,000 13,500,000 6,670,000 Medium Scenario Alabama 111,000 25,300 50,500 9,420 196,000 60,800 9,420 70,200 126,000 Alaska 1,850 0 0 61 1,910 620 61 681 1,230 Arizona 73,500 22,200 13,900 3,980 114,000 35,500 3,980 39,500 74,100 Arkansas 188,000 40,300 261,000 54,600 544,000 220,000 54,600 275,000 269,000

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Soil and Water Quality: An Agenda for Agriculture   Inputs Outputs State Fertilizer-N Recoverable Manure-N Legume-N Fixation Crop Residues Total Harvested Crop Crop Residues Total Balance California 482,000 106,000 96,100 32,800 717,000 246,000 32,800 278,000 438,000 Colorado 126,000 61,300 72,400 48,000 308,000 175,000 48,000 223,000 84,300 Connecticut 6,450 4,880 2,100 81 13,500 3,800 81 3,880 9,630 Delaware 15,200 5,850 18,600 4,380 44,000 14,000 4,370 18,400 25,700 Florida 258,000 15,700 8,900 3,090 286,000 25,100 3,090 28,200 258,000 Georgia 175,000 31,400 64,400 20,000 291,000 105,000 20,000 125,000 166,000 Hawaii 14,900 1,030 15 0 15,900 110 0 110 15,800 Idaho 187,000 26,600 93,200 37,900 345,000 186,000 37,900 224,000 121,000 Illinois 805,000 37,200 770,000 380,000 1,990,000 1,120,000 380,000 1,500,000 496,000 Indiana 462,000 32,900 397,000 200,000 1,090,000 595,000 200,000 795,000 297,000 Iowa 780,000 87,400 780,000 394,000 2,040,000 1,200,000 394,000 1,590,000 450,000 Kansas 438,000 114,000 227,000 163,000 942,000 547,000 163,000 710,000 232,000 Kentucky 165,000 24,500 116,000 35,300 340,000 163,000 35,300 199,000 142,000 Louisiana 138,000 5,670 126,000 26,100 296,000 109,000 26,100 135,000 160,000 Maine 11,600 7,780 2,400 1,230 23,000 11,700 1,230 12,900 10,100 Maryland 38,800 21,100 41,300 12,600 114,000 47,100 12,600 59,700 54,100 Massachusetts 9,860 4,390 3,300 161 17,700 5,300 161 5,460 12,300 Michigan 220,000 37,900 181,000 64,000 503,000 238,000 64,000 302,000 201,000 Minnesota 525,000 79,400 507,000 220,000 1,330,000 737,000 220,000 957,000 373,000 Mississippi 142,000 13,400 165,000 23,800 344,000 115,000 23,800 138,000 206,000 Missouri 322,000 42,400 439,000 113,000 916,000 487,000 113,000 600,000 315,000 Montana 91,100 12,000 124,000 52,400 280,000 212,000 52,400 264,000 15,200 Nebraska 608,000 93,000 306,000 236,000 1,240,000 662,000 236,000 898,000 345,000 Nevada 4,880 2,220 24,400 521 32,000 29,900 521 30,400 1,600 New Hampshire 2,040 1,980 1,800 24 5,840 3,210 24 3,230 2,610 New Jersey 20,800 3,170 13,200 3,040 40,200 14,900 3,040 17,900 22,300

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Soil and Water Quality: An Agenda for Agriculture   Inputs Outputs State Fertilizer-N Recoverable Manure-N Legume-N Fixation Crop Residues Total Harvested Crop Crop Residues Total Balance New Mexico 24,000 14,500 19,900 5,410 63,800 36,800 5,410 42,200 21,600 New York 93,900 78,200 91,700 18,700 283,000 138,000 18,700 156,000 126,000 North Carolina 176,000 33,000 107,000 30,900 347,000 122,000 30,900 153,000 194,000 North Dakota 302,000 13,700 178,000 108,000 602,000 357,000 108,000 465,000 137,000 Ohio 356,000 40,100 369,000 134,000 899,000 468,000 134,000 602,000 296,000 Oklahoma 246,000 30,500 49,000 34,100 360,000 162,000 34,100 196,000 164,000 Oregon 114,000 13,500 37,500 16,600 181,000 93,900 16,700 111,000 70,500 Pennsylvania 47,500 79,200 99,500 29,700 256,000 156,000 29,700 186,000 69,800 Rhode Island 1,490 0 230 8 1,730 330 8 338 1,390 South Carolina 65,100 5,680 49,400 11,300 131,000 45,600 11,300 56,900 74,600 South Dakota 164,000 35,300 307,000 94,900 601,000 360,000 94,900 455,000 146,000 Tennessee 140,000 20,600 110,000 23,500 294,000 118,000 23,600 141,000 153,000 Texas 674,000 153,000 33,000 94,800 955,000 341,000 95,800 436,000 518,000 Utah 27,000 11,300 48,800 4,480 91,600 57,100 4,480 61,600 30,000 Vermont 4,480 17,800 10,300 256 32,800 17,400 256 17,700 15,200 Virginia 71,300 25,400 53,800 11,000 162,000 73,500 11,000 84,500 77,000 Washington 185,000 25,300 44,200 39,200 294,000 158,000 39,200 197,000 96,800 West Virginia 11,900 5,610 8,600 948 27,100 14,800 948 15,700 11,300 Wisconsin 243,000 161,000 296,000 84,000 784,000 441,000 84,000 525,000 259,000 Wyoming 20,900 9,570 54,800 5,180 90,500 53,700 5,180 58,900 31,600 United States 9,390,000 1,730,000 6,870,000 2,890,000 20,900,000 10,600,000 2,890,000 13,500,000 7,420,000 High Scenario Alabama 111,000 25,300 57,000 9,420 203,000 60,800 9,420 70,200 133,000 Alaska 1,850 0 0 61 1,910 620 61 681 1,230 Arizona 73,500 22,200 21,000 3,980 121,000 35,500 3,980 39,500 81,200 Arkansas 188,000 40,300 289,000 54,600 572,000 220,000 54,600 275,000 297,000 California 482,000 106,000 146,000 32,800 767,000 246,000 32,800 279,000 488,000 Colorado 126,000 61,300 110,000 48,000 345,000 175,000 48,000 223,000 122,000 Connecticut 6,450 4,880 3,200 81 14,600 3,800 81 3,880 10,700 Delaware 15,200 5,850 20,800 4,380 46,200 14,000 4,370 18,400 27,900

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Soil and Water Quality: An Agenda for Agriculture economically optimum rates of nitrogen fertilization ranging from 128 to 379 kg/ha (114 to 338 lb/acre). Using the standard model the predicted rate was 22 percent greater than the best model indicated by more thorough statistical evaluation of the results. This illustrates a source of potential error that contributes to excess nitrogen use. Although refinement of such crop response models is hardly as simple as it seems, refinement of such models is important for refining nitrogen input recommendations (Bock and Sikora, 1990). Determining Realistic Yield Goals Ideally, the nitrogen fertilizer application recommendation should be based on the amount of nitrogen that must be made available during the growing season to produce the crop. However, estimates of the amount of nitrogen that the crop needs must be made before the crop is grown and before the weather and other factors that will affect the year's yield are known. Hence, the producer establishes a yield goal: a preseason estimate of the crop yield the producer hopes to realize. The yield goal is then used to project the amount of nitrogen that should be applied on the basis of the projected amount needed to achieve the yield goal. The importance of setting realistic yield goals as the basis for making both economically and environmentally sound recommendations has been highlighted many times (see, for example, Bock and Hergert [1991]; Peterson and Frye [1989]; University of Wisconsin-Extension and Wisconsin Department of Agriculture, Trade and Consumer Protection [1989]; U.S. Congress, Office of Technology Assessment [1990]). 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. (See Chapter 2 for more discussion of yield goals.) The most reliable way to set yield goals is to base goals on historical yields, for example, during the past 5 years, actually achieved on a field-by-field basis. Use of a yield achieved under optimal weather conditions that lead to a bumper crop as the goal will lead to the overapplication of nitrogen during most years. This practice increases production costs and residual nitrogen; in addition, many soils, except those low in organic matter, may supply the added nitrogen needed during a bumper crop year because the warm and moist conditions that lead to a bumper crop also increase the amount of nitrogen mineralized from soil organic matter (Schepers and Mosier, 1991). Another part of the problem is that some producers set yield goals for

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Soil and Water Quality: An Agenda for Agriculture their whole-farm rather than for each field and often fertilize each field similarly. Not only must yield goals be set realistically but, to optimize management, they should also be set on a field-by-field and preferably a soil-by-soil basis (Carr et al., 1991; Larson and Robert, 1991). Synchronizing Applications with Crop Needs The need to improve nitrogen management by synchronizing applications with periods of crop growth has been often highlighted (see, for example, Ferguson et al. [1991]; Jokela and Randall [1989]; Peterson and Frye [1989]; Randall [1984]; Russelle and Hargrove [1989]; University of Wisconsin-Extension and Wisconsin Department of Agriculture, Trade and Consumer Protection [1989]; U.S. Congress, Office of Technology Assessment [1990]). Nitrogen is needed most during the period when the crop is actively growing. Nitrogen applied before that time is vulnerable to loss through leaching or subsurface flow because of the mobility of nitrates in the soil system. Larger applications of nitrogen are generally used if nitrogen is applied in the fall, in particular, to make up for the nitrogen that is lost or that becomes unavailable in the soil during the period between application and crop growth. However, timed or multiple applications must be carefully evaluated for their economic and environmental efficacy. Simply increasing the number of applications presuming that this will improve crop uptake efficiency may ignore many other factors that affect crop growth (Killorn and Zourarakis, 1992; Timmons and Baker, 1991). Production and environmental advantages to simple changes in timing of application may be climate and site-specific. 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. As discussed, typical soil test methods are inadequate. In practice, nitrogen recommendations rely on evaluating general soil types and using the state's (extension-experiment station) recommended rate for a given yield goal for the soil types in that region. This approach, in part, has led to the blanket nitrogen applications that are part of the current problems and inefficiencies.

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Soil and Water Quality: An Agenda for Agriculture Practical and accurate soil and plant testing methods that allow refined assessment of crop nitrogen needs, in relation to the nitrogen available (through soil mineralization, available residual, rotation, and manure additions) in a particular growing season, are needed to reduce the uncertainty and risk involved with nitrogen fertilizer applications. Various plant and tissue tests, such as petiole testing for potato (Wescott et al., 1991), have proved valuable to more efficient nitrogen management for high value vegetable and citrus crops. Such methods must be refined and implemented for the major row crops, such as corn, that consume the majority of nitrogen applied to croplands. Many methods are being tested across the Corn Belt (see, for example, Binford et al., 1992; Binford and Blackmer, 1993; Blackmer et al., 1989; Cerrato and Blackmer, 1991; Fox et al., 1989; Magdoff, 1991a; Meisinger et al., 1992; Motavelli et al., 1992; Piekielek and Fox, 1992; Roth et al., 1992; Tennessee Valley Authority, National Fertilizer Development Center, 1989). One of the methods showing promise is the presidedress soil nitrate test (PSNT). The soil testing is done at a specified time after crop emergence and measures the amount of nitrate-N available in the upper 0.3 to 0.6 m (1 to 2 feet) of the soil profile. The PSNT provides a measure of whether or not supplemental nitrogen is actually needed given the estimate of nitrogen that is already available to the crop. 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). Implementation of soil or tissue tests requires that producers sidedress a significant portion of their nitrogen. Few producers, however, currently sidedress their nitrogen applications. Further work is needed on an early spring test that might be useable for preplant applications. In this regard, development of monitoring and modeling systems to help estimate nitrogen availability from the soil and annual carryover, related to climatic, soil, and crop conditions are also needed. Such systems could help to provide forecasts to producers about carryover and availability for them to consider in their annual nutrient and fertilizer application plans. Obstacles to Better Nitrogen Management The measures described above, if implemented, would greatly improve the efficiency with which nitrogen is now used in current

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Soil and Water Quality: An Agenda for Agriculture cropping systems. Many of these measures could be implemented immediately; others require the development of refined tools such as better soil tests or improved crop response models. In the short-term, efforts to improve nitrogen management in current cropping systems should be the priority and should have the potential to result in immediate gains in both economic and environmental performance. In the long-term, however, it is unclear whether such improvements in nitrogen management alone will be sufficient to reduce nitrogen loadings to levels where damages are acceptable. There are elements of the nitrogen problem that suggest that, in the long-term, changes in cropping systems that will allow producers to capture more of the available nitrogen may be necessary to adequately reduce nitrogen loadings to surface water and groundwater in some environments. Economic Obstacles Producers face a management dilemma because the effectiveness and the efficiency of nitrogen management cannot be assessed, economically or environmentally, until the growing season is over. A crop that produces poor yields because of inclement weather will result in poor nitrogen use efficiency and uptake, potentially leaving large amounts of nitrogen to be lost to the environment, no matter how carefully a management plan was designed. Since producers must make nitrogen applications without being able to predict weather and crop yields, the potential for being wrong is always present and will always occur in some years. Current recommendations of crop nitrogen needs are based on long-term assessments designed to average the many sources of variance in the nitrogen-yield response. This method also averages the recoveries of residual nitrogen carried over from a previous year or the greater amounts that may be mineralized and available under optimal climatic conditions. 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. Figure 6-3 shows the yield response of corn to nitrogen for various soils under continuous corn, and Figure 6-4 shows the nitrogen-yield response for corn for three crop rotations on the same soils. The relationships in Figures 6-3 and 6-4 illustrate the benefits of nitrogen fertilization, up to a certain point, in increasing crop yield, particularly in continuous corn.

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Soil and Water Quality: An Agenda for Agriculture Figure 6-4 also illustrates the need for less supplemental nitrogen in crop rotations with legumes; there is no yield benefit to nitrogen use following alfalfa. Figure 6-3 illustrates the inherent variability among soils in their capacity to supply nitrogen from mineralization. These factors vary not only among soils and cropping systems but from year to year as well. Figure 6-5 reexamines the relative efficiencies of the typical nitrogen-yield response relationship from the data in Figure 6-4. The nature of the relationship between the nitrogen application rate and FIGURE 6-3 Yield response of corn to nitrogen applied to three soils. Fayette silt, Fayette silt loam (fine-silty, mixed, mesic Typic Hapludalfs); Plano silt, Plano silt loam (fine-silty, mixed, mesic Typic Argiudolls); and Plainfield ls, Plainfield loamy sand (mixed, mesic Typic Udipsamments). Source: S. L. Oberle and D. R. Keeney. 1990. A case for agricultural systems research. Journal of Environmental Quality 20:4–7. Reprinted with permission from © American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America.

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Soil and Water Quality: An Agenda for Agriculture FIGURE 6-4 Yield response of corn to fertilizer for three crop rotations. Ccc, continuous corn; CSb, corn-soybeans-(corn oats); Cm, corn-oats-meadow-meadow (meadow-alfalfa brome mix). Source: Adapted from A. M. Blackmer. 1984. Losses of fertilizer N from soil. Report No. CE-2081, Ames, Iowa: Iowa State University, Cooperative Extension Service.; A. M. Blackmer. 1986. Potential yield response of corn to treatments that conserve fertilizer-N in soil. Agronomy Journal 78:571–575; and J. R. Webb. 1982. Rotation-fertility experiment. Pp. 16–18 in Annual Progress Report Northwest Research Center. Ames, Iowa: Iowa State University. yield for continuous corn illustrates that, at some point, additional increments of nitrogen application become less efficient. For every additional kilogram of nitrogen applied, less grain is produced, and hence, less of that increment of nitrogen is taken up by the plant. This result is illustrated by the shaded area and dashed lines in Figure 6-5. 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. The shaded areas in Figure 6-5 represent a range of values, for perspective. The apparent nitrogen recovery is calculated from the grain yield of a particular increment on the continuous corn yield curve, using

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Soil and Water Quality: An Agenda for Agriculture FIGURE 6-5 Nitrogen recovery related to fertilization rate. FN, fertilizer-N; Ccc, continuous corn; CSb, corn-soybeans-(corn-oats); Cm, corn-oats-(meadow-alfalfa brome mix). Source: Adapted from A. M. Blackmer. 1984. Losses of fertilizer-N from soil. Report No. CE-2081, Ames, Iowa: Iowa State University, Cooperative Extension Service.; A. M. Blackmer. 1986. Potential yield response of corn to treatments that conserve fertilizer-N in soils. Agronomy Journal 78:571–575; and J. R. Webb. 1982. Rotation-fertility experiment. Pp. 16–18 in Annual Progress Report Northwest Research Center. Ames, Iowa: Iowa State University.

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Soil and Water Quality: An Agenda for Agriculture various standard assumptions. The first assumption is that the grain contains 1.5 percent nitrogen (and weighs 25 kg/ha [56 lb/bu]); the second assumption is that the corn plant requires 0.54 kg of nitrogen per kg of grain (1.2 lb/bu) produced, and the nitrogen is proportioned at 60 percent nitrogen into the grain and 40 percent into the stover. These and the other assumptions given below provide a set of curves enclosed by the shaded envelopes in Figure 6-5. The upper boundary (line 1 of the shaded areas), indicating the lower recovery of fertilizer-N for a given fertilizer application rate, was estimated by subtracting the nitrogen recovered by the unfertilized corn (yield, about 4 metric tons/ha [64 bu/acre]) from the total nitrogen recovered for a given fertilizer application rate. The nitrogen recovered by the unfertilized corn provides a measure of the average amount of nitrogen provided from the soil system (mineralization, including crop residues, and precipitation). The lower boundary provides a conservative estimate that is based on the total amount of nitrogen recovered in the grain but uncorrected for yields from unfertilized areas. The dashed lines (near the line 1 boundary in Figure 6-5) show the upper bound estimated from the corn yields in the corn-soybean rotation. The values for the incremental fertilizer recovery illustrate how fertilizer-N recovery declines rapidly as the crop approaches optimum and maximum yields. At the maximum yield, recovery effectively reaches zero; at the economically optimum yield, recovery of the last increment of fertilizer-N is less than 10 percent. Even under the more conservative second assumption, less than 50 percent of fertilizer-N is recovered at the economically optimum yield for continuous corn. Hence, even with economically optimum yields, there is considerable potential for nitrogen losses into the environment. 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 and reduces the unrecovered amount even more for the corn-soybean rotation. Attempts to push for a last small yield increment can greatly contribute to nitrogen losses. The fate of this nitrogen can follow many paths in the nitrogen cycle; some is immobilized, but other portions may be leached into groundwater or otherwise lost. Seasonal Obstacles In addition to the economic incentives, elements of nitrogen dynamics in the soil-crop system may constrain the gains from improved management of nitrogen inputs alone.

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Soil and Water Quality: An Agenda for Agriculture The application of nitrogen in the spring is followed by immobilization of nitrogen by plants and microbes in the spring and summer. This immobilization period is followed by mineralization of the nitrogen from plant and microbial tissues in the fall. The seasonal dynamics are such that nitrate levels in the soil are very low during the late summer and early fall (Boone, 1990; Magdoff, 1991b). Following harvest, crop residues, root tissues, and microbial cells begin to mineralize and nitrify, often leading to high soil nitrate concentrations that are susceptible to loss through leaching or runoff during the fall, winter, and spring (Gold et al., 1990). Thus, the nitrate that is lost from cropping systems is not simply nitrogen that has not been used by the crop but includes nitrogen that has been cycled through plant and microbial tissues during the growing season. Fine-tuning nitrogen input management will reduce losses of nitrogen but may not provide sufficient reductions in nitrate losses from mineralization of crop residues, root material, and microbial cells following harvest. In some settings, the only way to manage this residual nitrogen may be to keep it tied up in plant or microbial tissues by preventing mineralization or to provide a sink for this nitrogen in plants or microbes once it is mineralized. Mineralization can be inhibited by controlling the substrate quality of the residue (for example, residues with a high carbon-to-nitrogen ratio do not release much nitrogen). Use of cover crops or relay crops to take up the nitrogen mineralized following harvest is a mechanism for storing nitrogen in plants. 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. The major emphasis has been on the use of cover crops planted after crop harvest (for reviews, see Hargrove, 1988, 1991). Although cover crop techniques have demonstrated abilities to reduce erosion, surface runoff, and leaching into groundwater, several problems limit their widespread use and effectiveness. Langdale and colleagues (1991) report that the cover cropping systems are better developed in the southeastern United States than in other parts of the country and that because of the fragmentation of research efforts and the short-term economic policy structure of the U.S. agricultural system, cover crop use in other regions is prohibitive. The drawbacks and

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Soil and Water Quality: An Agenda for Agriculture concerns associated with cover crop use include depletion of soil water by cover crops, slow release of the nutrients contained in cover crop biomass, and difficulties in establishing and killing cover crops, especially in northern areas of the United States (Frye et al., 1988; Lal et al., 1991; Wagger and Mengel, 1988). Several aspects of the effects of cover crops on total crop system function are poorly understood. Cover cropping changes organic matter pools and microbial nutrient cycling patterns, affecting crop nutrient uptake and fertilizer use efficiency. It likely takes several years for these changes to stabilize and create a new equilibrium of organic matter and nutrient dynamics in soil (Doran and Smith, 1991). More important, the fate of the nutrients absorbed by subsequent cover crops is not clear. Studies with isotopically labeled nitrogen, as well as more conventional nitrogen budget studies, have shown that less than 50 percent of the nitrogen contained in cover crop tissues is absorbed by subsequent crops (Ladd et al., 1983; M. S. Smith et al., 1987; Varco et al., 1989). 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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