Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2003
Publication Date: March 1, 2004
Citation: JAYNES, D.B., DINNES, D.L., MEEK, D.W., KARLEN, D.L., CAMBARDELLA, C.A., COLVIN, T.S. USING THE LATE SPRING NITRATE TEST TO REDUCE NITRATE LOSS WITHIN A WATERSHED. JOURNAL OF ENVIRONMENTAL QUALITY. 2004. V. 33(2). P. 656-668. Interpretive Summary: Excessive nitrate leaching from row crop production areas within the Midwest corn belt has led to serious surface water quality problems and contributed to the expansion of the hypoxic zone in the northern Gulf of Mexico. Identifying and implementing best management practices for producing crops that reduce nitrate losses to surface waters could help alleviate this water quality problem. Plot and small field trials have shown that applying a small amount of nitrogen fertilizer near planting time followed by sidedressing additional nitrogen fertilizer in late spring can significantly reduce nitrate leaching while maintaining corn yields when compared to fall application of nitrogen. This reduction may be even greater when a pre-sidedress soil test is used to determine the amount of fertilizer that needs to be sidedressed to meet crop needs. In this study, we collaborated with eight farmers to adopt the split application and pre-sidedress soil test to manage nitrogen fertilizer within a 1000-acre watershed. We compared the water quality leaving this treated watershed to water quality leaving adjacent watersheds where nitrogen fertilizer was applied using current farmers' practices (primarily fall application with no soil testing). Nitrate concentrations in water leaving the treated watershed were significantly reduced by about 30% compared to the control watersheds after four years, although nitrate concentrations still occasionally exceeded the maximum contaminant level set for drinking water by the USEPA. Corn yields were not significantly reduced within the treated watershed, but the pre-sidedress soil test did result in an overall reduction in nitrogen fertilizer applied. These findings will be of great interest to regulators and farmers seeking to maximize farming profits while reducing agricultural impacts on water quality.
Technical Abstract: Excessive nitrate contamination of surface waters can require costly treatment of water for human consumption and is implicated in the formation of a hypoxic zone in the northern Gulf of Mexico. Nitrate contaminated drainage water from tile drained watersheds is the primary source of NO3 in surface waters within the Midwest corn belt. Current crop production practices will need to be changed if surface water quality is to be improved while maintaining economically viable crop yields. Beginning in 1997, we implemented the late spring nitrogen test (LSNT) method for managing spring sidedress application of N-fertilizer for corn (Zea mays L.) production within a 400 ha, 16-field, tile drained sub-basin of the Walnut Creek watershed in central Iowa. Impacts on surface water quality were determined by comparing nitrate concentrations from the treated sub-basin to two adjacent sub-basins with N-fertilizer programs typical for the area - primarily fall-applied, anhydrous ammonia. Surface water discharges were measured continuously and water samples collected weekly for nitrate analysis from 1992 through 2000 for each of the three sub-basins. In 1998 and 2000, the LSNT method resulted in significantly lower N fertilizer rates compared to the farmers' standard program. There was no difference in average N-fertilizer application rates in 1997 and 1999. Time-series models indicated that by early 1998, the nitrate concentrations in surface water leaving the LSNT treatment sub-basin started to decline compared to the control sub-basins. Within the second year of the LSNT implementation, the nitrate concentrations coming from the treated sub-basin were significantly lower than the concentrations coming from both control sub-basins. Annual average flow-weighted nitrate concentrations for the last two years of the study were 11.3 mg-N/L for the LSNT sub-basin and 16.0 mg-N/L for the control sub-basins. Based on these results, wide spread adoption of the LSNT program for managing N-fertilizer where fall N application is typically practiced, could result in a 30% or greater decrease in nitrate concentrations in surface water.