Field-scale nitrogen balances associated with long-term continuous corn production

Excessive residual soil NO3-N indicates economic Inefficiency fur producers and Increases the potential for nonpoint leaching of N to mater resources. Our objective was to construct an approximate field-scale N budget Fur continuous corn (Zea mays L.) grown on deep loess soils in four 30- to 60-ha field-scale watersheds in western Iowa. Preplant and postemergence soil NO3-N levels were determined for the 0- to 30-cm, 30- to 60-cm, and 60- to 90-cm depths in April before N fertilizer was applied and again at the V6 growth stage, A simple root-zone water balance and N removal by corn grain were determined. The 4-yr average showed approximately 100 kg ha(-1) of NO3-N in the upper 90 rm of the root zone before an average of 168 kg of fertilizer N ha(-1) was applied, Grain removal accounted for 30 to 70% of the fertilizer N, There were significant differences in grain yield and N removal among the four watersheds. Factors contributing to those differences included different fertilizer rates, tillage practices, and application times, During the four study years, an average of 50% of the applied N was available for leaching, denitrification, and/or NH3 volatilization, High levels of residual soil NO3-N following continuous corn production coupled with steady percolation of precipitation that infiltrated but was not used by the corn crop appear to be the two major factors supplying NO3-N to groundwater baseflow that enters streams draining the watersheds. Bared on the seasonal water balance, the most successful N management strategies will be those that minimize the amount of residual NO3-N remaining in the soil profile at the end of the growing season.