Losses of Nitrogen in Surface Runoff in the Blackland Prairie of Texas

Abstract

AbstractOur objective in this study was to determine NO3−‐N and total N losses in surface runoff from Houston Black clay, a swelling clay soil with a relatively low infiltration rate. The study was carried out on duplicate 4‐ha watersheds cropped to a rotation of grain sorghum (Sorghum bicolor [L.]), cotton (Gossypium hirsutum [L.]), and oats (Avena sativa [L.]), all fertilized at recommended rates of N application. The loss of NO3−‐N varied considerably during the study, depending on events before each runoff‐producing storm. Concentrations of NO3−‐N were usually highest just after fertilizer application when the soil was near field capacity and lowest when large amounts of water infiltrated into dry soil immediately before runoff. During runoff‐producing storms just after fertilizer application, the concentrations were lowest in the initial runoff and highest near the end of the runoff event. To compute NO3−‐N losses with reasonable accuracy on these soils, the shape of the entire NO3−‐N concentration curve needed to be well defined.In general, the results of this study indicate that a small and probably insignificant amount of N is lost to surface waters when crops are fertilized at recommended N rates in the Texas Blackland Prairie. For the entire 5‐year study, the mean concentration of NO3−‐N in runoff was 2.9 and 2.3 ppm NO3−‐N for the duplicate watersheds. The mean total loss of NO3−‐N was 3.2 kg ha−1 year−1. Losses of sediment associated N were about 5 kg N ha−1 year−1.