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Tropical forage legumes provide large N benefits to maize except when fodder is removed
Integrating tropical forage legumes into cropping systems has the potential to improve subsequent crop N supply, but removal of legume biomass for forage is likely to diminish these benefits. This study aimed to determine 1) under irrigated conditions the potential N inputs that can be provided by different tropical forage legumes to a subsequent cereal crop and 2) 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) were compared with maize (Zea mays) when legume biomass was retained or cut and removed. After these legumes, a subsequent bioassay of an oat (Avena sativa) cover crop and a maize grain crop were grown and N uptake, biomass production and grain yield were compared among tropical legumes and the maize control. To determine N fertiliser equivalence values for subsequent maize crop yields different rates of fertiliser (0, 50, 100 and 150 kg urea-N/ha) were applied. Butterfly pea, centro and lablab with biomass retained increased subsequent unfertilised maize grain yield by 6-8 t/ha and N uptake by 95-200 kg N/ha compared with following a previous cereal crop, contributing equivalent to 100-150 kg urea-N/ha. When legume biomass was cut and removed, grain yield for the following maize crop did not increase significantly. Thus, when butterfly pea, centro and lablab biomass was retained rather than removed, maize accumulated an additional 80-132 kg N/ha. After fodder removal, centro was the only legume that still provided N benefits to the subsequent maize crop (equivalent of 33 kg urea-N/ha). Burgundy bean did not increase subsequent crop production when biomass was retained or removed – the reasons for this are unclear. Overall this study found a range of tropical forage legumes can 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 N trade-offs between retaining and removing legume biomass are large, quantification of the N inputs under livestock grazing or when greater residual biomass is retained may provide an alternative to achieving dual soil N-fodder benefits.
CP17081 Accepted 14 November 2017
© CSIRO 2017