Illinois Fertilizer
Conference Proceedings
January 25-27, 1999
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R.G. Hoeft, E.D. Nafziger, R.L. Mulvaney, L.C. Gonzini, and
J.J. Warren2
Some Midwestern producers may be unknowingly contributing nitrates to water supplies. Onfarm 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 survey of Champaign County producers indicated that nearly 70% were applying 40 lb N/acre or more above the recommended level for corn. The reasons for such overapplication are numerous, but one frequently mentioned reason is risk aversion. The objectives of the project reported in this paper are to:
1. Ascertain the effect of rate and time of N application on nitrate-N concentrations in water from tile lines.
2. Evaluate the effect of previous N rate on current N needs.
3. Evaluate the effect of previous N rate on recovery of fertilizer N by plants.
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 five-minute intervals. At all of the locations, precipitation was recorded at 30-minute intervals. 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 were encouraged to continue to apply the same rate of N and manage the field in the same manner as in the past.
Small-plot nitrogen rate studies were established at seven of the sites in 1997 and at four sites in 1998, using ammonium sulfate in 40 lb N/acre increments. The total N fertilizer application ranged from 0 to 240 lb N/acre at seven locations, from 36 to 276 16 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 farmers. 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. 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 handharvested 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 24016 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 depth of four feet from all plots and analyzed for both inorganic and organic N and ESN. The water samples collected by the automatic samplers were analyzed for nitrate and ammonium N. 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 two cropping seasons (Figure 1). Temperatures were cooler than normal throughout the spring and early summer of 1997. In contrast, the winter and spring of 1998 were much warmer than normal, and soils never froze during the entire winter. Precipitation was near normal for the 1997 growing season, but above normal for the 1998 season. Even though the seasons were quite different, yield response to applied N was greater than normal in both seasons (Figure 2) Normally, when soil temperatures are cool in the spring and early summer, mineralization rates are low. This did not appear to be the case in 1997, as total N uptake on the 0 N treated plots plus residual N in the soil profile were relatively high (100 lb N/acre or higher) at three of the locations (Table 2). The warm winter and early spring of 1998 probably resulted in much higher nitrification rates than normal, and when that was followed by the abnormally high precipitation throughout the early summer, it likely resulted in increased leaching and denitrification rates.
The high N requirement was likely due to the climatic conditions mentioned above and to the high yields obtained. There appeared to be no relationship between previous N rate and optimum N per unit of land area (Figure 3), per unit of production (Figure 4), or relative yield increase (Figure 5). When averaged across locations and years, the percent yield increase of the optimum N rate over the control was higher for spring and sidedress treatments than for fall-applied N (Figure 6). Similarly, the amount of N required per bushel of yield obtained at the optimum N rate was higher for fall-applied than spring or sidedressed treatments (Figure 7).
Fertilizer N recovery at the optimum N rate was calculated by dividing the difference between the total N uptake on the 0 lb N/acre treatment and the total N uptake at the optimum N rate by the optimum N rate (Figure 8). The recovery of fertilizer N measured in this way appeared to be higher for spring-applied than for fall-applied or sidedressed N. The lower recovery from the fall application may have been associated with the wet soils that resulted in denitrification loss. Even though that may have occurred, the yield obtained with the fall application was as high as from the spring applications. The lower recovery associated with the sidedress treatments was most likely the result of the lower yields obtained at those locations.
Fertilizer N uptake was less as measured by the 15N technique than the difference method at the three 15N locations (Figure 9). Contrary to the data shown (Figure 8), N uptake was greater on the sidedress than on preplant treatments. This differential in results between the two techniques is probably due to the fact that addition of N stimulates plant and root growth, and thus the treated plants are better able to utilize soil N than is the case with the control plots.
Residual soil nitrate-N levels ranged from 5 to 69 lb/acre in the top four feet of soil in the untreated plot (Figure 10). This wide range was not related to prior N rate, nor to time of N application. The relatively low fertilizer N uptake observed at site 3715 may have been due to the high residual soil nitrate level at that location. 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 to fertilizer (Figure 11).
In 1997, flow-weighted nitrate-N concentrations from tile lines tended to be highest on those fields planted to corn, and they tended to be greatest on those fields that had a history of higher N rates in the past (Figure 12). Notable exceptions to this statement included site 2704, which had a low concentration of nitrate-N in the tile line even though it was planted to corn and had a relatively high historical N application, and site 3827, for which a high nitrate-N concentration occurred despite being planted to soybeans and having a previous history of near optimum applications. Since site 3827 receives N as a sidedressing, it is possible that there may have been more carryover N available for leaching during the winter and early spring following the corn crop. Total N loss per acre in tile line flow followed a pattern associated with previous N history (Figure 13). The only exception was site 2704, which had a low N loss with a previous high rate of application.
The relationship between flow-weighted nitrate-N concentration and previous N history or time of N application was not evident in 1998 (Figure 14). There appeared to be a relationship between current crop and nitrate-N concentration, with corn generally having the highest levels. Total N loss per unit of land area tended to be associated with previous N rate when soybeans were grown in 1998 (Figure 15). As in 1997, the greatest loss per unit of land area occurred when corn was grown, except at sites 1705 and 2720, which had high N loss values even though soybean was being grown. The results from site 2720 are rather confusing. The high flowweighted nitrate concentrations and high loss per unit of land area could be attributed to the fact that this site had the highest excess application rate of all sites. However, this site had the lowest residual soil nitrate-N concentration in the soil profile of all sites, which would imply that the excess N had been immobilized 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. Approximately two-thirds of that recovered was inorganic N, with the other third being organic N (Table 3). Total recovery from the fall-applied DAP-35%-was slightly higher than from the low rate (80 lb N/acre) of ammonium sulfate but comparable to that recovered from the higher rates of ammonium sulfate-48% and 49%. The percentage of inorganic N recovered from ammonium sulfate was somewhat higher than that from DAP.
Table 1. Characteristics of the experimental sites.
Table 2. Total uptake and residual NO3-N in the top four feet of soil at three locations in 1997.
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 8. Effect of time of N application and previous N history on fertilizer N recovery.
Figure 9. Fertilizer N recovery in plants at physiological maturity at 3 locations in 1997.
Figure 11. Effect of N rate on soil and fertilizer derived residual NO3-N at 3 locations in 1997.
Figure 13. Relationship between previous N rate, current crop, and total N lost per acre in tile line water - 1997.
Figure 14. Relationship between previous N rate, current crop, and flow weighted NO3-N concentration in tile water - 1998.
Figure 15. Relationship between previous N rate, current crop, and total N lost per acre in tile line water - 1998.
1 Presented at the Illinois Fertilizer and Chemical Association Conference, Peoria,IL Jan. 25, 1999.
2 R.G. Hoeft and E.D. Nafziger are Professors, Dept. of Crop Sciences; R.L. Mulvaney is Professor, Dept. of Natural Resources and Environmental Sciences, and L.C. Gonzini and J..1. Warren are Senior Research Specialists, Dept. of Crop Sciences, Univ. of IL.
Brown, H.B., R.G. Hoeft, and E.D. Nafziger. 1993. Evaluation of three N recommendation systems for corn yield and residual soil nitrate. In R.G. Hoeft (ed.) Proc. Il. Fert. 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 ESN depleted fertilizer application. p. 213. In Agronomy abstracts. ASA, Madison, WI.
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. Il. Fert. 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 corn. Commun. Soil Sci. Plant Anal. 23:1409-1426.
