Tracking the Release of Soil Nitrate and Labile C in A Legume-Maize Rotation in Zimbabwe

This study compared the effect of a weedy fallow (5.2 t/ha biomass), a velvet bean (Mucuna pruriens) cut for hay (7.2 t/ha biomass) and a green-manured M. pruriens (6.49 t/ha biomass) on the dynamics of soil N and C in a maize crop. An on-farm, farmer participatory experiment was established on a farmer’s field in Wedza District, Zimbabwe. Soil mineral N and labile carbon were determined at intervals upto 120 cm depth, at maize planting and at 1 and 2 weeks after planting. Before planting, the soil mineral N content ranged from 28 kg N/ha after weed fallow to 107 kgN/ha following M. pruriens. Total nitrate concentration was highest in the 0-15 cm depth of the M. pruriens treatments in the pre-planting sampling, but following rainfall and maize planting, nitrate concentration declined rapidly. By 2 weeks after planting, 7.5 and 13.5 kg N/ha remained in the 0-120 cm soil depth of the weedy fallow and green-manured M. pruriens, respectively. Improving synchrony of nutrient release and uptake is critical when applying high quality residues which breakdown relatively slowly. This could result in significant inputs of C, release nutrients more slowly and reduce soil nutrient losses.