Effect of soil water content on denitrification during cover crop decomposition

Abstract

Experiments were conducted to assess the effects of soil water content on denitrification during hairy vetch (Vicia villosa) decomposition. Hairy vetch plants were grown from seed to maturity in soil cores. Before and after kill, simulated rainfall was applied to cores weekly and leachate was analyzed for NO3/- and NH4/+. Denitrification incubations (ca. 48 h duration) were conducted 3, 17, 31, and 45 days after kill using the acetylene block method. Soil water content was varied systematically to give a range of percent water-filled pore space (WFPS) values from field capacity (60%) to saturation (100%). Little denitrification occurred on Day 3 (<2 nag N2O-N). Substantial denitrification occurred on Days 17, 31, and 45, with maxima of 44, 27, and 30 mg N2O-N produced in saturated cores, respectively, accounting for approximately 60 to 75% of the total inorganic N (NO3/- + NH4/+ + N2O) present in cores. There was an apparent linear relationship between denitrification and soil water content (WFPS), with a threshold for denitrification at ca. 60% WFPS. Cumulative N lost from cores during four denitrification incubations ranged from 1 to 48 Kg N ha-1, depending on percent WFPS. Rates of N mineralization were relatively linear after denitrification incubations (55 days). Cumulative N mineralized from unsaturated cores was ca. 190 Kg N ha-1 through 120 days after kill. These data indicate that substantial quantities of vetch-N may be lost during decomposition.