Carbon and nitrogen mineralization from cowpea plants part decomposing in moist and in repeatedly dried and wetted soil

Knowledge of C and N mineralization kinetics during decomposition of legumes is important to optimize management practices of green manure cropping systems. Our objective was to quantify the rates ofC and N mineralization of cowpea [Vigna unguiculata (L.) Walp.] decomposing in continuously moist soil and in soil air-dried and rewetted every 5 days. Fresh cowpea plant parts were kept at 30°C for 68 days. Evolved CO2 and mineral N released were measured periodically. Repeated drying and wetting did not significantly reduce C mineralization of cowpea, which averaged 62% of initial plant C for the two moisture regimes at 68 days. Nitrogen mineralization from cowpea, however, was reduced significantly from 46% of initial plant N in moist soil to 29% in repeatedly dried and wetted soil at 68 days. Carbon and N mineralization from cowpea were linearly related after an initial phase of rapid C loss. With repeated drying and wetting, a greater amount of N remained in undecomposed plant material, that was retrieved periodically during incubation. Repeated drying and wetting of the soil appeared to increase the resistance of certain N compounds of the plant to microbial decomposition. Further, repeated drying and wetting of the soil severely inhibited growth and/or activity of nitrifiers. Periodic drying of the soil as occurs in the field will reduce N mineralization from legume green manures compared to the decomposition in continuously moist soil, but may contribute to long-term N fertility by increasing soil organic N content.