Nitrate‐Nitrogen Leaching Following Urea Fertilization and Irrigation

Abstract

AbstractWheat (Triticum aestivum) was grown in undisturbed columns of Gardena silt loam, a coarse‐silty, mixed Pachic Udic Haploboroll, to evaluate NO3‐‐N leaching from urea under irrigation. Optimum and excessive irrigation rates and 135 kg N/ha (as urea in single application vs. three increments applied during the growing period) were compared in factorial combination. Chloride was added to all columns for comparison with NO3‐‐N. Leachate samples were collected throughout the study and analyzed for NO3‐‐N, NH4+‐N and Cl‐. Columns were dismantled at crop maturity and sampled for residual NO3‐‐N. Total N was determined in wheat at maturity.Leaching fractions (LF) differed significantly between optimum and excessive irrigation but were not affected by applied urea. Cl‐ moved with the wetting front. Leaching patterns for NO3‐‐N and Cl‐ were similar, but a slight lag in NO3‐‐N was observed. Concentration of NO3‐‐N in drainage water from all columns was initially similar (30 ppm NO3‐‐N), but then decreased as leaching volume changed. Maximum NO3‐‐N leaching occurred with split urea and excess irrigation. Minimum leaching occurred with optimum irrigation and split urea. Total NO3‐‐N leachate load was directly correlated with leachate volume. Soil solution concentrations at crop maturity were as much as 170 µg NO3‐‐N/ml. Residual NO3‐‐N varied among treatments and revealed as many as three peaks in NO3‐‐N concentration in the profile, indicating that N leaching occurred following each urea application. The results indicate marked NO3‐‐N movement from urea under excessive irrigation.