Improved fallows: effects of species interaction on growth and productivity in monoculture and mixed stands

To derive optimal benefits from short-term planted legume fallows, farmers need to make important initial decisions on the establishment and growth performance of specific species in monoculture stands and in mixtures. We hypothesized that species with contrasting growth characteristics could be mixed to optimize aboveground resource capture due to their complementary or compensatory gains in resource acquisition. A selection of tree/shrub/herbaceous legumes planted in monoculture and mixed stands were evaluated for growth, biomass and nitrogen productivity on a Kandiudalfic Eutrudox in western Kenya. Species evaluated included: sesbania (Sesbania sesban (L.) Merr.), crotalaria (Crotalaria grahamiana Wight and Arn.), pigeonpea (Cajanus cajan (L.) Millsp.), and siratro (Macroptilium atropurpureum (DC.) Urb). Total aboveground biomass ranged from 5 to 12 Mg ha^-1 (monocultures) and 3.4–8 Mg ha^-1 (mixtures). N yield per plant was linearly correlated with total plant biomass (R^2=0.95,slope=0.018) across all of the fallows, despite different amounts of leaf and wood with widely differing N concentrations being present with the different species. Biomass and N yield of sesbania was negatively affected by mixing with other species. For instance, sesbania N yield decreased when mixed with pigeonpea (54%) or crotalaria (67%). Crotalaria and pigeonpea established best under relay cropping with maize and emerged to be better competitors in mixtures than sesbania and siratro. Results of this study suggest that mixing species in fallows provide a better risk management strategy through compensatory biomass and nutrient production gains obtained from the strongly competing species.