Nitrogen Outputs from a Field‐Size Agricultural Watershed

Abstract

AbstractNitrogen outputs from a 0.34‐ha field were measured for 1984 through 1987. The field consisted of Cowarts loamy sand (fine‐loamy, siliceous, thermic Typic Hapludult) managed in a summer row crop‐winter cover crop sequence. Outputs measured were subsurface flow, surface runoff, and denitrification. Monthly sampling of denitrification rates showed a high degree of temporal and spatial variability. Rates significantly increased with soil moisture and depth. Denitrification also increased after irrigation events. Nitrogen movement in subsurface flow was 97% NO3‐N and was about 10 times that in surface runoff. Most subsurface flow and movement of N (77 kg NO3‐N ha−1) occurred in December 1986 through February 1987 when 62 cm of precipitation fell in a 12‐wk period (December 1986–February 1987) following fertilizer application. This period accounted for one‐half the total subsurface nitrate movement for the 4‐yr study. Even though no crop was harvested in 1985, subsurface outputs of N were low in winter of 1985–1986. Soil NH4‐N levels were significantly higher after the year without a harvest, indicating that more of the N in crop residue was found in the soil pool.