A Nitrogen Decision Support Success Story--Lowering Stream Nitrate While Maintaining Full Yield.

Excessive nitrate leaching from the U.S. Corn Belt has created serious water quality problems and contributed to the expansion of the hypoxic zone in the Gulf of Mexico. We evaluated the yield and water quality effects of implementing the late spring nitrate test (LSNT) for corn (Zea mays L.) grown within a 400-ha, tile-drained sub-basin in central Iowa. Surface water discharge and NO₃ concentrations from the treated sub-basin and two adjacent sub-basins receiving primarily fall-applied, anhydrous ammonia were compared. In two of four years, the LSNT method significantly reduced N fertilizer applications compared with the farmers' standard practices. Average corn yield from LSNT fields and non-limiting N fertilizer check strips was not significantly different. Autoregressive (AR) models using weekly time series in surface water NO₃ concentration differences between the LSNT and control sub-basins indicated no consistent significant differences during the pre-LSNT (1992-1996) period. However, by  the second year (1998) of the treatment period (1997-2000), NO₃ concentrations in surface water from the treated sub-basin were significantly lower than the concentrations coming from both control basins. Annual average flow-weighted NO₃ concentrations for the last two years (1999-2000) were 11.3 mg N L^-1 for the LSNT and sub-basin, and 16.0 mg N L^-1 for the control sub-basins. Based on these values and the AR models, widespread adoption of the LSNT program for managing N fertilizer where fall N application is typically practiced could result in a ≥ 30% decrease for NO₃ concentrations in surface water. These results show that a substantial water quality improvement is possible by delaying N application by side-dressing and tailoring the rate applied to simply meet crop requirements for the current year. Grain yields were influenced more by hybrid selection, plant population, and weed management than differences among soil map units or N fertilizer rates. Despite the obvious water quality benefits of using the LSNT, perceived risk caused in sub-basin farmers to revert to their old practices and therefore, water quality gains were soon lost. Strategies for encouraging farmer adoption of more friendly N management practices, including possible changes in landscape management, will be discussed as future venues to prevent such reversals and encourage more efficient N management.