Illinois Fertilizer
Conference Proceedings
January 24-26, 2000
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R.G. Hoeft, E.D. Nafziger, R.L. Mulvaney, S.T. Mirek, L.C.
Gonzini, and J.J. Warren1
Nitrate levels in excess of the public health standard of 10 mg N/l in public water supplies along with concern about hypoxia in the Gulf of Mexico have drawn renewed interest to improving fertilizer N use efficiency. Prior research has indicated that some farmers may be unknowingly contributing to these problems. The objectives of this research were to determine the effect of rate and time of N application on nitrate-N concentrations in water from tile lines, and to evaluate the effect of previous N rate on current N needs and on recovery of fertilizer N by plants. Tile line monitoring systems that record water flow rates and collect water samples on a predetermined schedule have been installed at 10 experimental sites. At each site, N rate studies were conducted when the field was planted to corn.
While it is too early in the study to draw definite conclusions, the following observations were evident from the data:
There was no relationship between previous N rate and corn grain yield response to applied N.
Relative yield response was higher for spring and sidedress treatments than for fall-applied N.
Nitrate loss in tile line effluent was related to previous history of excess rates of N application.
Nitrate loss was frequently higher in the Oct.l to Sept. 30 time period following corn than following soybean.
Residual NO3-N levels were generally highest on fields having a history of excessive rates of N application.
Residual NO3-N levels were generally
higher with sidedress, lowest with fall, and intermediate with spring-applied
N.
Some Midwestern producers may be unknowingly contributing nitrates to water supplies. On-farm research identified 13 of 77 fields in which corn was non-responsive to fertilizer N. There was evidence to indicate that these fields had a previous history of high levels of fertilization and/or manure application (Brown et al., 1993). Based on these results, along with work by Torbert et al. (1992) showing that excess fertilizer N is assimilated into organic N compounds, and recent work by Stevens et al. (1997) demonstrating that these compounds mineralize more easily than native organic matter, we have theorized that these non-responding sites likely had adequate N release from the soil to meet the needs of the crop. Continued application of optimum or above-optimum N rates on these fields will enhance the potential for nitrate movement through tile line drainage. Buzicky et al. (1983) demonstrated that above-optimum application rates increased the loss of fertilizer-supplied N in tile drainage water and that the problem was even greater when the fertilizer was fall applied.
A 1995 survey of producers in the Sangamon River watershed indicated that nearly 70% were applying 40 lb N/acre or more above the recommended level for corn, with some overapplying by as much as 100 lb N/acre. A 1999 survey in the same watershed indicated that nearly 70% of the producers were within 20 lb N/acre of the recommended rate. The objectives of the project reported in this paper are to:
Ten experimental sites having clearly defined tile systems that drain only that field or a known portion thereof were identified in 1997. Tile line monitoring systems that record water flow rates and collect water samples on a predetermined schedule based on flow rate were installed at each location. At four of the locations, air and soil temperatures were recorded at 5-minute intervals, and precipitation was recorded at 30-minute intervals at all locations. Past cropping records including yield, time and rate of N application, and crop rotation were recorded for each site (Table 1). Other than in the small plot area of the field, the farmers have been encouraged to continue to apply the same rate of N and to continue to manage the field in the same manner as in the past.
Small plot nitrogen rate studies were conducted at seven of the sites in 1997, at the same seven in 1999. and at the other four sites in 1998. The 1999 treatments were repeated in the exact location in the field as was used in 1997. Ammonium sulfate was applied in 40 lb N/acre increments at rates ranging from 0 to 240 lb N/acre at seven locations, from 36 to 276 lb N/acre at two locations, and from 45 to 285 lb N/acre at two locations. The differential in N rates was due to the application of DAP by the farmer. The ammonium sulfate was applied at each location near the time that the farmer made his application. At five locations, 15N-labeled ammonium sulfate was applied to microplots within each N rate plot at the same rate of N. The 1999 15N plots were moved within the small plot area to allow us to determine 15N uptake from the 1999 application as well as residual uptake from the 1997 application. Corn was planted in mid- to late-April and thinned to 29,000 plants per acre at the V-4 growth stage. At maturity, grain was hand-harvested for yield determination.
At physiological maturity, whole plant samples were collected from the microplot
areas that had received the 15N-labeled fertilizer
and from the 0, 80, 160, and 240 lb N/acre rate plots that had received unlabeled
ammonium sulfate. The whole plant samples were analyzed for total N and 15N.
Following harvest, soil samples were collected to a 4-foot depth from all plots
and analyzed for both inorganic and organic N and 15N.
The water samples collected by the automatic samplers were analyzed for nitrate
and ammonium N and for total and soluble P. 15N-labeled
diammonium phosphate was applied in the fall of 1997 at locations 2822 and
3827 at the rate of 40 lb N/acre. In the spring of 1998, soil samples were
collected and analyzed for organic and inorganic 15N.
The weather differed considerably between the cropping seasons (Figure 1). Temperatures were cooler than normal and precipitation was somewhat lower than normal in 1997. In contrast, the winter and spring of 1998 and 1999 were warmer than normal, and precipitation was above normal, especially in May and June of 1998.
When averaged over all locations, the grain yield increase to applied N as a percent of the yield of the non-N fertilized plot was 102% in 1997, 146 % in 1998, and 68% in 1999 (Figure 2). Grain yield on the non-N fertilized plots was 72, 89, and 114 bushel per acre to 1997, 1998, and 1999, respectively. Differential in grain yield response to applied N was most likely due in large part to the influence of climatic variability on soil N release and/or loss. Mineralization in 1998 and 1999 was likely high due to the warm, moist soil temperatures, whereas the cooler soil temperatures in 1997 resulted in lower mineralization rates. The enhanced mineralization that occurred in 1998 was offset by denitrification and leaching that resulted from the excess rainfall that occurred in late May and early June. This was supported by the fact that N uptake by corn on the non-N fertilized plots was lower for nearly all locations in 1998 than in 1997 (Table 2). Precipitation was slightly higher than normal in April and August of 1999. However, the potential for this to cause leaching and denitrification was relatively low, as there were no large precipitation events in April (that month was characterized with frequent, low-volume rainfalls) and most of the N would have been taken up by the corn crop by the time the heavy rains were received in August.
There appeared to be no relationship between previous N rate and optimum N application per unit of land area (Figure 3) or per unit of production (Figure 4). Optimum N needed per unit of land area and per unit of production was less at nearly all locations in 1999 than in 1997. When averaged across locations and years, the amount of N required per bushel of yield obtained at the optimum N rate was slightly higher for fall and sidedressed than for spring-applied treatments (Figure 5).
Residual soil nitrate-N levels ranged from 6 to 66 lb/acre in 1997 and from 2 to 100 lb/acre in 1998 in the top 4 feet of soil in the untreated plot (Table 3). This wide range was not related to prior N rate, nor to time of N application, although the highest levels occurred in the sidedress fields. Increased N application rate resulted in an increase in residual N levels at all locations, but based on the 15N results, much of the increase could be attributed to soil release rather than fertilizer (Figure 6). At all but one location, the NO3-N levels were higher in 1998 following soybeans than in 1997 following corn (locations 1705 through 3717).
Flow-weighted nitrate-N concentrations from tile lines tended to be highest on fields planted to corn, and they tended to be greatest on fields that had a history of higher N rates in the past (Figure 7 and Figure 8). Notable exceptions to this statement included site 2704, which had a low concentration of nitrate-N in the tile line in both years (even though it had relatively high soil nitrate levels), and site 3827. which had a high NO3-N concentration in the tile water and had high soil NO3-N levels in the fall of 1998.
Total N loss per acre in tile line flow followed a pattern associated with previous N history (Figure 9 and Figure 10). The only exception was site 2704. which had a low N loss with a previous high rate of application. The high flow-weighted nitrate concentrations and high loss per unit of land area at site 2720 could be attributed to the fact that this site had the highest excess application rate of all sites. This site had the lowest residual soil nitrate-N concentration in the soil profile in the plot area in 1997, but the highest level in samples taken from outside the plot area in 1998, which would imply that the excess N had been immobilized during the corn year and was rapidly mineralized during the soybean year.
Recovery of the N in April from fall-applied DAP ranged from 45 to 55% of the total N applied, with approximately two-thirds of that recovered being present as inorganic N and the other third present as organic N (Table 4). Total recovery from the fall-applied DAP was slightly higher—45%—than from the low rate (80 lb N/acre) of ammonium sulfate—35%—but comparable to recoveries from the higher rates of ammonium sulfate—48 and 49%.
Table 1. Characteristics of the experimental sites.
Figure 1. Climate Data for Champaign, IL.
Figure 2. Effect of nitrogen rate on corn yield.
Figure 3. Effect of time of N application and previous N history on optimum N per unit of land area.
Figure 6. Effect of N rate on soil and fertilizer derived residual NO3-N at 3 locations in 1997.
1R.G. Hoeft and E.D. Nafziger are Professors, Dept. of Crop Sciences; R.L. Mulvaney is Professor, Dept. of Natural Resources and Environmental Sciences; S.T. Mirek is Graduate Research Assistant, Dept. of Crop Sciences; and L.C. Gonzini and J.J. Warren are Senior Research Specialists, Dept. of Crop Sciences, univ. of IL.
Brown, H.B., R.G. Hoeft, and ED. Nafziger. 1993. Evaluation of three N recommendation systems for corn yield and residual soil nitrate. In R.G. Hoeft (ed.) Proc. II. Fen. Conf. pp 43-50.
Buzicky, G.C.. G.W. Randall, R.D. Hauck, and A.C. Caldwell. 1983. Fertilizer N losses from a tile-drained Mollisol as influenced by rate and time of 15N depleted fertilizer application. p. 213. In Agronomy abstracts. ASA, Madison, W l.
Stevens, W.B., R.G. Hoeft, and R.L. Mulvaney. 1997. Effect of N fertilization on accumulation and release of readily-mineralizable organic N. In R G. Hoeft (ed.) Proc. II. Pert. Conf. pp 65-78.
Torbert, H.A., R.G. Hoeft, R.M. VandenHeuvel, and R.L. Mulvaney. 1992. Effect of moisture regime on recovery and utilization of fertilizer N applied to com. Commun. Soil Sci. Plant Aria]. 23:1409-1426.
