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 l egumes is important to optimize management practices of green manure c ropping systems. Our objective was to quantify the rates of C and N mi neralization of cowpea [Vigna unguiculata (L.) Walp.] decomposing in c ontinuously moist soil and in soil air-dried and rewetted every 5 days . Fresh cowpea plant parts were kept at 30 degrees C for 68 days. Evol ved CO2 and mineral N released were measured periodically. Repeated dr ying and wetting did not significantly reduce C mineralization of cowp ea, 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 rep eatedly dried and wetted soil at 68 days. Carbon and N mineralization from cowpea were linearly related after an initial phase of rapid C lo ss. With repeated drying and wetting, a greater amount of N remained i n undecomposed plant material, that was retrieved periodically during incubation. Repeated drying and wetting of the soil appeared to increa se the resistance of certain N compounds of the plant to microbial dec omposition. 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 sail , but may contribute to long-term N fertility by increasing soil organ ic N content.