The rapid growth of population and reduction in productive agricultural lands are forcing smallholder subsistence farmers of Ethiopia to adopt intensive farming systems, to which they respond by growing more and more cereals in place of traditional fallow practices, causing widespread soil degradation. In this regard, systematic soil incorporation of N2-fixing legume crops as a green manure could be an important agronomic approach in order to reduce the need for costly external inputs and improve internal resources for sustainable production. Two consecutive experiments were conducted in the cropping seasons of 2007 and 2008 at Assosa area, western Ethiopia, to determine the effects of stages at incorporation of four legume crops on biomass input, soil organic C content and the agro-economic performance of succeeding maize crop. Factorial combination of four legume crops and four residue managements were laid in randomized complete block design (RCBD). Among the legume crops, cowpea has recorded the highest number and fresh mass of total and effective nodules than the rest, particularly at mid-flowering stage of growth. Cowpea and pod-setting stage of growth gave the highest dry matter (DM) input that improved the organic C content of the soil. Total DM and grain yields of maize crop in the subsequent season were also significantly affected by main effects, legume species and residue management (P < 0.01). In this regard, cowpea and delayed incorporation increased the DM and grain yields of the subsequent maize crop, attributable to the differential green manure biomass yield and corresponding improvement of the soil-plant environment. This is corroborated in the present study, by highly significant positive relationships observed between soil organic C content and DM (r = 0.88**) and grain yield (r = 0.80**) of maize crop. The economic evaluation revealed that the highest net benefits were obtained from cowpea green manure and pod-setting stage of growth.
Keywords: dry matter, grain yield, organic C, partial budgeting, yield attributes.