Tropical forage legumes provide large nitrogen benefits to maize except when fodder is removed

Abstract. Integration of tropical forage legumes into cropping systems may improve subsequent crop nitrogen (N) supply, but removal of legume biomass for forage is likely to diminish these benefits. This study aimed to determine: (i) under irrigated conditions, the potential N inputs that can be provided by different tropical forage legumes to a subsequent cereal crop; and (ii) the residual N benefits once fodder had been removed. Available soil mineral N following tropical forage legumes lablab (Lablab purpureus), centro (Centrosema pascuorum), butterfly pea (Clitoria ternatea) and burgundy bean (Macroptilium bracteatum) and grain legume soybean (Glycine max) was compared with a maize (Zea mays) control when legume biomass was retained or cut and removed (phase 1). An oat (Avena sativa) cover crop was then grown to ensure consistent soil-water across treatments (phase 2), followed by a maize grain crop (phase 3) in which N uptake, biomass production and grain yield were compared among the phase 1 treatments. To determine N-fertiliser equivalence values for subsequent maize crop yields, different rates of fertiliser (0–150 kg urea-N/ha) were applied in phase 3. Retained biomass of butterfly pea, centro and lablab increased phase 3 unfertilised maize grain yield by 6–8 t/ha and N uptake by 95–200 kg N/ha compared with a previous cereal crop, contributing the equivalent of 100–150 kg urea-N/ha. When legume biomass was cut and removed, grain yield in the phase 3 maize crop did not increase significantly. When butterfly pea, centro and lablab biomass was retained rather than removed, the maize accumulated an additional 80–132 kg N/ha. After fodder removal, centro was the only legume that provided N benefits to the phase 3 maize crop (equivalent of 33 kg urea-N/ha). Burgundy bean did not increase subsequent crop production when biomass was either retained or removed. The study found that a range of tropical forage legumes could contribute large amounts of N to subsequent crops, potentially tripling maize grain yield. However, when these legumes were cut and removed, the benefits were greatly diminished and the legumes provided little residual N benefit to a subsequent crop. Given the large N trade-offs between retaining and removing legume biomass, quantification of N inputs under livestock grazing or when greater residual biomass is retained may provide an alternative to achieving dual soil N–fodder benefits.