Field‐Scale Nitrogen Balances Associated with Long‐Term Continuous Corn Production

Abstract

AbstractExcessive residual soil NO3‐N indicates economic inefficiency for producers and increases the potential for nonpoint leaching of N to water resources. Our objective was to construct an approximate fieldscale N budget for 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 cm 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. Based 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.