Forage legumes for improved fallows in agropastoral systems of subhumid West Africa. II. Green manure production and decomposition after incorporation in the soil

A short-term improved fallow system based on forage legumes for enhancing c rop and livestock components of mixed farming systems was tested in the sub humid zone of West Africa. As part of the evaluation, the ability of 11 leg ume species (Centrosema macrocarpum, C. pubescens, Stylosanthes guianensis, Pueraria phaseoloides, Mucuna pruriens, Zornia glabra, Dioclea guian- ensi s, Arachis pintoi, Aeschynomene histrix, Calopogonium caeruleum, Flemingia macrophylla) to accumulate biomass and nitrogen after dry season harvest of herbage was assessed at 2 sites in south-west Nigeria. Litter bags were su bsequently used to study the potential nutrient contribution to maize from decomposing green manure for 6 of the 11 species in comparison with natural fallow vegetation. Accumulation of green manure biomass and nitrogen was related to the regene ration potential of the legumes in the absence of rainfall and their appare nt ability to fix atmospheric nitrogen. Following 4-month regrowth after a dry season harvest, F. macrophylla yielded the highest amounts of green man ure dry matter (4.0-5.7 t/ha) and nitrogen (102-144 kg/ha N) at the 2 sites , followed by P. phaseoloides, C. pubescens and the other species. Decompos ition of green manure was governed by initial concentrations of cellulose, hemicellulose, lignin, lignin:nitrogen ratio and amounts of green manure in corporated, with nitrogen disappearing more slowly than dry matter, Highest loss rates were observed for potassium followed by phosphorus and nitrogen . Waif-life values for undecomposed residue dry matter were in the range of 2-8 weeks. Nitrogen release after 3 months ranged between 26-88 kg/ha N an d 19-52 kg/ha N at the 2 sites. The rested green manures can contribute sig nificantly to subsequent crops as well as to the mineralisable nitrogen poo l of the soil. Rapid decay rates of dry matter and nutrients indicate the n eed to synchronise nutrient release from green manure residues with crop re quirements.