Nitrogen fixation and biomass productivity of indigenous legumes for fertility restoration of abandoned soils in smallholder farming systems

Abstract

Most legume-based soil fertility technologies often fail to make the desired impact on nutrient-depleted soils partly due to low N₂-fixation rates and poor biomass productivity. A study was conducted in the 2004/05 and 2005/06 rainfall seasons to evaluate biomass productivity and N₂-fixation of indigenous legumes on nutrient-depleted fields under low (450- 650 mm yr⁻¹) to high (> 800 mm yr⁻¹) rainfall areas of Zimbabwe. Legume species, mostly of Crotalaria, Indigofera and Tephrosia genera, were sown in mixtures on disturbed soil. Indigenous legume fallows (indifallows) produced 2–5 t biomass ha⁻¹ under low and 5–15 t biomass ha⁻¹ under high rainfall. They significantly (P<0.05) out-yielded natural fallows by 84% and a sunnhemp (Crotalaria juncea) green manure fallow by 32% over a growth period of six months. Cumulatively, indifallows produced more biomass (~29 t ha⁻¹) than natural fallows (~ 8 t ha⁻¹) over two seasons. Indigenous legumes derived 61–90% of their N from the atmosphere with amounts fixed ranging from 2–57 kg N ha⁻¹ under medium rainfall conditions to 1–173 kg N ha⁻¹ under high rainfall. Application of P increased indifallow biomass productivity by 15% under low rainfall, and N₂-fixation by 32% and 18% under low and high rainfall, respectively. These results demonstrated that indigenous legumes generate sufficiently high biomass and fix nitrogen on nutrient-depleted fields where most conventional green manure and grain legume cultivars often fail to establish.