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

Abstract

Abstract Excessive residual soil NO 3 ‐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 NO 3 ‐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 NO 3 ‐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 NH 3 volatilization. High levels of residual soil NO 3 ‐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 NO 3 ‐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 NO 3 ‐N remaining in the soil profile at the end of the growing season.