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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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This page in the original is blank.
Representative terms from entire chapter:
residual nitrogen
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