Woody legume fallow productivity, biological N2-fixation and residual benefits to two successive maize crops in Zimbabwe

Three woody legumes were planted as two-year 'improved fallows' to evaluate their residual nitrogen (N) effects on two subsequent maize crops under minimum and conventional tillage management. Maize monoculture and cowpea-maize-maize sequence treatments were included as controls. N-2-fixation was estimated using the N-15 natural abundance method to allow the N contribution from the fallows to be partitioned into N2-fixation and soil N capture. Acacia angustissima accumulated the largest amount of both below-ground and above-ground biomass during the 2-year growth period. Using Hyparrhenia grass as the reference plant, the proportion of N-2-fixed in litter was 56, 55, 84 and 58% for Acacia, Sesbania sesban, Cajanus cajan, and cowpea, respectively, resulting in inputs of biologically fixed N of 122, 84, 96 and 28 kg N ha(-1). Maize growth following the legumes for two subsequent cropping seasons was in most cases not directly related to the N inputs. The first year maize crop was severely infested with cutworms (Agrotis sp.) in the Sesbania and Acacia plots, while the second season maize was affected by drought particularly in the Acacia plots where there was high moisture demand by the re-growing trees. During the second season, N uptake by maize following Sesbania was significantly higher than the rest of the treatments. Conventional tillage resulted in better yields than minimum tillage across all treatments and in both seasons. It can be concluded that (1) improved legume fallow system results in net positive soil N balances, (2) despite large inputs of litter and continued recycling of N during the cropping phase, legume species that re-grow after cutting can result in depressed yields in drought seasons, and (3) tillage is beneficial even after improved legume fallows.