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USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
* Corresponding author (jaynes{at}nstl.gov)
Received for publication August 25, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTSSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: H, high nitrogen fertilizer treatment • L, low nitrogen fertilizer treatment • M, medium nitrogen fertilizer treatment • MCL, maximum contaminant level
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSSUMMARY AND CONCLUSIONSREFERENCES |
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The relationship between corn yield and N fertilizer application rate has been the focus of intense research for at least 50 yr (Krantz and Chandler, 1954). Studies on N leaching losses and the relationship between N fertilizer application rate and NO3 leaching, particularly in artificially drained soils, have been conducted over a much briefer time (Angle et al., 1993; Rasse et al., 1999). In one of the first controlled studies, Baker et al. (1975) found that the concentration of NO3 in tile drainage water averaged 21 mg N L-1 and the losses averaged approximately 30 kg N ha-1 yr-1 for a N fertilizer application of 112 kg N ha-1 on corn grown in rotation with unfertilized oat (Avena sativa L.) or soybean. In continuous corn production, Randall and Iragavarapu (1995) found flow-weighted NO3 concentrations during an 11-yr period of applying 200 kg N ha-1 to average 13.4 and 12.0 mg N L-1 for conventional-tillage and no-tillage systems, respectively. In comparing the effect of N fertilizer rate, Baker and Johnson (1981) found that increasing the fertilizer rate from 100 to 250 kg N ha-1 on corn, grown in rotation with either soybean or oat, doubled the NO3 concentration in tile drainage from 20 to 40 mg N L-1. Similar results have been reported by Gast et al. (1978) for N fertilizer applied to continuous corn.
These studies have provided valuable information on the effect of N fertilizer rates on NO3 leaching. However, studies have been conducted on only a limited number of locations and typically on small, intensively managed agricultural plots (
200 to 300 m2) rather than production fields. Our objective was to quantify the effects of N fertilizer rate on corn and soybean yields and NO3 losses in subsurface drainage water within a production field on highly productive soils not previously studied.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSSUMMARY AND CONCLUSIONSREFERENCES |
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Flow-weighted composite water samples were collected in glass jars connected by a siphon tube to the sump pump. Water samples were returned to the laboratory on a weekly or shorter basis, depending on tile flow rate, and refrigerated until analysis. Water samples were analyzed for NO3 using a Lachat Autoanalyzer (Zellweger Analytics, Lachat Instrument Division, Milwaukee, WI). Nitrate was quantitatively reduced to nitrite and the nitrite concentration determined colorimetrically. The method quantitation limit was 1.0 mg N L-1 as NO3. Nitrate mass loss in tiles was calculated by multiplying the NO3 concentration for the composite sample times the volume of water discharged during the time the composite sample was collected.
The field was planted to corn in 1995, 1996, and 1998 and soybean in 1997 and 1999 (Table 2). Prior to this time, the field had been in a typical 2-yr corn–soybean rotation. Primary tillage consisted of either moldboard or chisel plowing. A field cultivator was used to prepare the soil for planting in the spring and the field was cultivated with a row crop cultivator several times during the early growing season for weed control. Plant counts were 66000 and 75000 ha-1 for corn in 1996 and 1998, respectively and 370000 ha-1 for soybean in 1997 and 1999. All operations other than nitrogen fertilization and harvesting were performed by the farmer–owner as part of his normal production practices.
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Grain yield was measured on a transect adjacent to the drain line within each of the nine subsurface drainage plots with either a modified Gleaner K combine (Allis–Chalmers, Milwaukee, WI) or a modified John Deere (Moline, IL) 4420 combine (Colvin, 1990). The transect was offset from the drain line by about 3 m to avoid the soil disturbed by tile installation, but the location was the same each year. Along a transect, a 20-m length was harvested, the combine stopped to allow grain to finish cycling through the combine, and the grain was weighed and moisture content measured. A three-row-wide strip for corn and a five-row-wide strip for soybean was harvested along each transect. A total of 225 yield values were collected by making 25 contiguous yield measurements on each transect. All grain weights were adjusted to a moisture content of 155 g kg-1 for corn and 130 g kg-1 for soybean. In 1998 and 1999, grain samples from each plot were collected and grain protein determined using near-infrared spectroscopy at the Iowa State University Grain Quality Laboratory.
Soil cores were taken randomly after harvest from each N treatment plot on 13 Nov. 1996, 1 Oct. 1997, 25 Oct. 1998, and 3 Nov. 1999. The soil cores were taken to a depth of 1.2 m by pushing a 38.1-mm-diameter steel soil probe, fitted with a removable acetate liner, into the soil with a hydraulic ram. The soil core and liner were removed from the steel probe, capped on each end, and stored at -10°C until NO3 extraction. The frozen soil cores were cut into 150-mm-long sections, removed from the liners, thawed, and mixed by hand. Three 20-g subsamples were taken for determination of soil water and NO3 content. Water content was determined by change in weight from drying a soil sample at 104°C for 48 h. Nitrate concentrations were measured colorimetrically (Keeney and Nelson, 1982) using flow injection analysis technology and Manifold no. 12-107-04-01-B (Lachat Instruments, Milwaukee, WI).
Exploratory analysis was conducted on all data sets using SAS (SAS Institute, Cary, NC). Nonparametric methods were used where appropriate when distributions failed the Kolmogorov–Smirnov test of normality (Mood et al., 1974). For tile NO3 concentration data, conventional analysis of variance was explored initially, but diagnostics revealed several problems. The time series records constructed from the treatment means were not stationary and each detrended series was autocorrelated. Hence, an autoregressive time response curve methodology was employed. Following Meek et al. (2000), models were developed for the differences H-L, H-M, and M-L in the paired NO3 sequences. For each contrast, the difference was modeled with splined polynomial segments. Time was cast as days past the first recorded tile flow (9 Apr. 1996). Polynomial terms included up to quartic powers of time. A single knot was fixed at 600 d past the onset of treatments. Up to two autoregression terms were selected from the set of terms that included up to the first eight lags. The 95% confidence interval was estimated over the entire period of comparison and used to determine the periods of significant difference by using inclusion of the zero line (no difference) within the confidence band as the indicator.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSSUMMARY AND CONCLUSIONSREFERENCES |
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Yearly mass of NO3 lost in the tile water was computed by multiplying the flow volume during composite sample collection by the average NO3 concentration for that time period and summing over the year. A simple analysis of variance (ANOVA) comparison of the mass loss by treatment and block for all years showed no significant difference (P = 0.05) between blocks and a significant difference between the mass loss from the H treatment (48 kg N ha-1) versus the M (35 kg N ha-1) and L (29 kg N ha-1) treatments, but no difference between the M and L treatments. Average yearly mass losses of NO3 by treatment (Table 3) varied from 13 kg ha-1 for the L treatment in 1997 to 61 kg ha-1 for the H treatment in 1996. The greatest mass losses were in 1996 and 1998, the years that corn was grown and N fertilizer applied. However, these were the same years having the greatest tile discharge volumes (Fig. 3), which accounts for some of the differences in mass loss among years.
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500 kg ha-1).
Grain Yield
Grain yield distribution for each year was negatively skewed and failed normality tests, thus nonparametric tests were used to compare the treatments. Nitrogen fertilizer treatment had a significant affect on yield for both years that corn was grown (Fig. 6). In 1996, the corn yield for the L treatment was significantly lower than the M and H treatments, while the M and H treatment yields were not significantly different. In 1998, corn yields for each treatment were significantly different.
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4% higher than yields in 1997. Thus, there was no carryover effect of N fertilizer treatment on soybean yields, which is in agreement with other N rate studies (Stone et al., 1985; Bundy et al., 1993). Following Cerrato and Blackmer (1990), we can assume a quadratic plus plateau model for the response of corn yield to N fertilizer treatment and calculate a fertilizer to corn price ratio of 4.5 using prices of $0.40 kg-1 for fertilizer N ($0.18 lb-1) and $88.40 Mg-1 for corn grain ($2.25 bu-1). For the price ratio of 4.5, the economic optimum N fertilizer rate in 1996 was between 67 (L) and 135 (M) kg ha-1 and between 114 (M) and 172 (H) kg ha-1 in 1998. However, even at the lowest N fertilizer rate, the NO3 concentration in the tile water still exceeded the 10 mg N L-1 MCL when corn was grown (Fig. 6). In the years that soybean was grown, NO3 concentrations in the tile water exceeded the MCL for all but the L treatment. The average NO3 concentration in the drainage increased with increasing N fertilizer rate, and while the addition of N fertilizer produced a curve of diminishing returns for corn yield, it produced a curve of increasingly greater NO3 concentrations in drainage.
Nitrogen Mass Balance
We can calculate a partial mass balance of N within the field for each year (Karlen et al., 1998) by assuming the conservation of mass:
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A residual > 0 would indicate that inputs of N exceed losses from the field and N is available for other processes such as increasing soil organic matter. A residual < 0 would indicate that inputs do not balance outputs and that additional N must be coming from sources not included in the calculation to account for the observed losses. A residual = 0 would indicate that the field is in balance between N inputs and outputs and thus the production system is sustainable from a N perspective.
Inputs of N included in the computation were the application of fertilizer, N contained in rain, and N fixed by soybean (Table 3). Fertilizer inputs were known. Nitrogen additions with precipitation were calculated using measured precipitation data and the average NO3 concentration in rain of 1.5 mg N L-1 measured by Hatfield et al. (1996) for central Iowa. While these calculations ignore contributions from other forms of N, the resulting estimates are greater than atmospheric deposition rates measured for the Midwest by the National Atmospheric Deposition Program for NO3 and NH4 combined (Burkart and James, 1999). Nitrogen fixation was calculated using the relationship between soybean grain yield (Mg ha-1) and N fixed (Nfixed) given by Barry et al. (1993):
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Outputs of N included N removed with the grain harvest and NO3 in tile drainage. Protein content of grain was measured in 1998 for corn to be 64, 70, and 74 g kg-1 for the L, M, and H treatments, respectively. We used these values and an assumed protein to N ratio of 6.25:1 (David et al., 1997) to compute N removed in corn. Measured protein content of soybean for 1999 was 395 g kg-1 for all treatments and was used to compute N removal with soybean for 1997 and 1999 assuming a protein to N ratio of 6.25:1. For each treatment and year, N loss in tile drainage represented about one-third of the total N loss and grain removal about two-thirds of the total loss, which are about the same proportions found for a 48173-ha watershed of the Embarras River in Illinois (David et al., 1997).
Not accounted for in the partial N balance were N inputs from wet and dry deposition of N forms other than NO3, weathering of the soil mineral fraction, or decomposition of soil organic matter or crop residues. Nitrogen outputs not considered were N in deep percolation below the tiles, N loss in surface runoff, and N volatilized from the soil or plant (Francis et al., 1993, 1997) or through denitrification (Parkin and Meisinger, 1989).
Annual changes in NO3 stored in the soil were calculated for 1997 through 1999 by subtracting the residual soil NO3 mass measured the previous fall from the mass measured in the fall of the year in question. Change in residual soil NO3 was not calculated for 1996 because residual soil NO3 was not measured after harvest in 1995. Losses of NH4, NO2, and other forms of N in tile drainage were considered negligible as most N in tile drainage has been found to be in the NO3 form (Willrich, 1969). Changes in N due to changes in crop residues were considered to be negligible because crop residues should be stable over the rotation if biomass production is consistent over time. Changes in N due to changes in soil organic matter were also not considered and probably represent the largest source of error in the mass balance given the large reserve of N represented by the soil organic fraction in the field (>17000 kg N ha-1) and the potential effect small changes in this reserve would have on the annual N budget.
The mass balance residual for N during corn years indicates a net loss of N from the field for the L treatment in 1996 and 1998 (Table 3). That is, the L treatments must be consuming N from an unaccounted source such as soil organic matter to produce the grain yields and leaching losses observed. As such, the L fertilizer rate would not be sustainable because of long-term net losses of soil organic matter and ultimately yield.
For the M treatment in 1996, the N mass balance residual was positive, indicating greater N inputs than losses. In 1998, the M treatment resulted in a negative N mass balance residual, indicating that more N was lost than added. The H treatment resulted in a positive N mass balance for both years, indicating a surplus of N available to the soil. Thus, a N fertilizer rate between 114 to 135 kg ha-1 is required to balance the inputs and outputs of N with the current yields and drainage losses observed for this system.
The mass balance residual for N during the soybean years was always negative, indicating a greater loss of N than known inputs. Although computed N fixed by soybean was nearly 200 kg ha-1 in 1997 and 1999, this input of N was more than offset by N removal with soybean grain and in tile drainage. These mass balance calculations for soybean are in agreement with observations in other studies (Harper, 1974; Peoples and Craswell, 1992; Barry, et al., 1993; Vanotti and Bundy, 1995; David et al., 1997). Thus, although it is common to speak of an equivalent N fertilizer credit of 40 kg ha-1 after growing soybean for grain, the mechanism for this credit appears to be something other than an overall increase in soil N (Vanotti and Bundy, 1995; Green and Blackmer, 1995).
For the 2-yr corn–soybean rotation used here (1996–1997 and 1998–1999), the net mass balance residual for N is negative for all treatments except H in 1996–1997. Thus, for the rotation, all treatments (except possibly H) are not sustainable from a N balance perspective and are probably resulting in oxidation of soil organic matter to supply the missing N.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSSUMMARY AND CONCLUSIONSREFERENCES |
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Economic optimum N fertilizer rates for corn grain production were between the L and M treatments (67 to 135 kg ha-1) in 1996 and between the M and H treatments (114 and 172 kg ha-1) in 1998. Soybean grain production was unaffected by N fertilizer treatments during the corn year of the rotation. To balance the inputs and outputs of N to the system for the 2-yr rotation, N fertilizer rates needed to be at least at the H rate (202 kg N ha-1 in 1996 and 172 kg N ha-1 in 1998) given the current tile drainage losses experienced by this system. However, NO3 concentrations in tile drainage consistently exceeded the MCL for drinking water in the years corn was grown for all N fertilizer treatments and also during the years soybean was grown on the M and H treatments.
Thus, it appears that economic corn production cannot be sustained within this field under the current rotation and management scheme without producing tile drainage water that exceeds the MCL for NO3. The problem is not simply one of N fertilizer use, but of a corn–soybean production system created by artificial soil drainage and intensive tillage. This result is similar to findings found by others studying corn production on different soils in the Midwest U.S. Corn Belt, both on smaller plots (Baker and Johnson, 1981; Drury et al., 1993; Randall and Iragavarapu, 1995; Kanwar et al., 1997) and on watersheds (David et al., 1997; Jaynes et al., 1999). Thus, NO3 concentrations exceeding the MCL appear endemic to artificially drained soils cropped to corn within the Midwest.
Sustainable agriculture is defined as an integrated system of plant and animal production practices that will, over the long term, "enhance environmental quality and the natural resource base upon which the agricultural economy depends" (Section 1603 of the National Food, Agriculture, Conservation, and Trade Act of 1990). By including water quality as well as productivity as endpoints in the concept of sustainable agricultural production systems, dramatic changes will be required in management practices on artificially drained soils before corn production can be considered sustainable.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTSSUMMARY AND CONCLUSIONSREFERENCES |
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REFERENCES |
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