|HONG, NAN - UNIV OF MO
|SCHARF, PETER - UNIV OF MO
|DAVIS, J - USDA-NRCS
|Sudduth, Kenneth - Ken
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2006
Publication Date: 11/27/2006
Citation: Hong, N., Scharf, P.C., Davis, J.G., Kitchen, N.R., Sudduth, K.A. 2007. Economically optimal nitrogen rate reduces soil residual nitrate. Journal of Environmental Quality. 36:354-362
Interpretive Summary: Most nitrogen fertilizer in the U.S., and especially in the Mississippi River basin, is applied to corn. Corn is grown more widely on tile-drained land than other nitrogen receiving crops, creating a rapid pathway for nitrate transport to surface water. This makes corn the crop which loses the greatest amount of nitrate to water resources in the Midwest. Nitrate remaining in soil after harvest is extremely vulnerable to leaching into groundwater. Targeting nitrogen fertilizer application to match spatially variable crop nitrogen need may reduce nitrate left in the soil after harvest and has been suggested as a strategy to reduce nitrate movement to the Gulf of Mexico from the Mississippi River. Economically optimal nitrogen rate (EONR) is the fertilizer rate that maximizes profit and is slightly lower than the rate that maximizes crop yield. Applying fertilizer at the EONR is suggested to have both economic and environmental benefits, but little research has been published to address the environmental benefits. Our objective was to determine if residual soil nitrate after harvest was reduced when nitrogen fertilizer was applied at the EONR, as opposed to rates commonly used on corn production fields. In experiments on six MO fields, we found that residual soil nitrate after harvest was very similar from unfertilized corn and corn receiving nitrogen fertilizer at the EONR. Applying fertilizer nitrogen at the EONR reduced the leftover soil nitrate by at least 10 lbs/acre below common rates. Soil nitrate increased substantially as fertilizer rate increased above EONR. When the rate of nitrogen fertilizer exceeded EONR by 90 lbs/acre, we found on average nearly 80 lbs of nitrate-N in the soil at harvest time. Thus by applying fertilizer nitrogen at the EONR, environmental benefits are achieved and residual soil nitrate is reduced. Further, since EONR varied widely both among and within these six fields, EONR at both whole- and sub-field scales needs to be accurately diagnosed. These results justify the development of new technologies and procedures that will help farmers determine EONR early in the growing season. Farmers will directly benefit from such strategies by having increasingly lower fertilizer costs because nitrogen fertilizer costs have increased significantly in recent years. Also, if such strategies reduce excess applications of nitrogen fertilizer to fields, then the general public will benefit because nitrogen loss to lakes and rivers will be reduced, and the environment will be improved.
Technical Abstract: Post-harvest residual soil nitrate-N (RSN) is susceptible to loss by leaching and transfer to water resources. Practices that minimize the RSN can reduce N loss to the environment. Our objectives were (i) to determine if the RSN after corn (Zea mays L.) harvest can be reduced if fertilizer is applied at the economically optimal rate (EONR) as compared to current producers’ practices in the midwestern U.S., and (ii) to compare RSN levels if no fertilizer is applied to RSN levels if fertilizer is applied at a rate below, at, or above the EONR. Six experiments were conducted in producers’ fields in three major soil areas (Mississippi Delta alluvial, deep loess, claypan) over two years. At four transects in each field, six treatment rates from 0 to 280 kg/ha were applied, the EONR was determined, and the RSN was sampled to a 0.9 m depth from five treatment plots. The EONR at sampling sites varied from 49 to 228 kg/ha depending on site and year. Estimated average RSN at the EONR was 27 kg/ha in the 0.9 m profile. Applying fertilizer at the EONR reduced the RSN by at least 11 kg/ha compared to the producer’s rate. When EONR was > the N rate applied, average RSN was about 21 kg/ha and was not different than if the total N applied was zero or equal to the EONR. When a N application was 50 kg/ha more than EONR, average RSN increased to 39 kg/ha, but the RSN was not significantly greater than the RSN at EONR. When the N rate was between 50 and 100 kg/ha more than EONR, average RSN increased to 49 kg/ha and was significantly greater than RSN at EONR. When the N rate was 100 kg/ha > than EONR, average RSN significantly increased to an average of 91 kg/ha. Our results show that applying the EONR will likely produce environmental benefits over current practices.