A soil core method for estimating N-mineralization and denitrification during cover crop decomposition

A soil core method is described for monitoring rates and extent of N-mineralization and denitrification from intact leguminous cover crops (e.g., hairy vetch) as a function of soil water content, The method also allows for estimates of N-fixation in order to perform N-mass balances, Field conditions were simulated by growing cover crops in soil cores from seed to biomass levels comparable to the field, followed by harvest/kill. Soil cores were wetted periodically using a rain simulator, After simulated rain events, samples of leachate were obtained and soil water content adjusted by application of a vacuum (15 kPa) to the bottom of cores, The use of a PVC/silica filter (bubble point = 30 kPa) allowed for cores to be drained to field capacity without pulling ambient air through the soil, N-fixation (before harvest/kill) and N-mineralization (after harvest/kill) were determined by comparing NO3- leached from vetch with fallow cores. Denitrification was determined by periodically sealing cores, injecting and recirculating acetylene throughout cores, and quantifying N2O production after 48 h. Preliminary experiments with hairy vetch (Vicia villosa) indicate that plants are reasonably efficient at taking up soil NO;, intact roots decompose fairly rapidly in soil (<6 weeks), and there is potential for substantial losses of soil NO; as a result of denitrification at soil water contents greater than or equal to 70%.