Do Species Mixtures Increase Above‐ and Belowground Resource Capture in Woody and Herbaceous Tropical Legumes?

Abstract

The rotation of crops with planted, N2‐fixing legumes is a promising agroforestry innovation for replenishing soil fertility in the tropics. We postulated that woody and herbaceous legumes with different growth and rooting patterns could be mixed to optimize above‐ and belowground resource capture. The objective of this study was to evaluate the effect of species interactions on resource utilization by legumes grown in mixtures on a Kandiudalfic Eutrudox in western Kenya. Four woody legume shrubs—pigeonpea [Cajanus cajan (L.) Millsp.], sesbania [Sesbania sesban (L.) Merr.], crotalaria (Crotalaria grahamiana Wight and Arn.), and tephrosia (Tephrosia vogelii Hook F.)—grown in monoculture and mixed stands were evaluated for light interception, soil N and water uptake, and biomass production. Siratro [Macroptilium atropurpureum (DC.) Urb.] and groundnut (Arachis hypogaea L.) were undersown in woody legume stands. Total aboveground biomass production ranged from 9 to 13 Mg ha−1 for monoculture and 8 to 15 Mg ha−1 for mixtures of woody legumes. Total N in woody‐legume stands ranged from 100 to 178 kg N ha−1. Biomass and plant N were not significantly different among woody‐legume treatments. However, undersowing siratro as a supplement increased stand productivity and recycled biomass N. Species complementarity in topsoil and subsoil utilization of mineral N was observed in crotalaria + sesbania and pigeonpea + tephrosia mixed stands. Dense soil cover created by siratro led to better conservation of soil water. Results indicated that the tested mixtures provide a better risk management strategy through compensatory growth potential. Greatest opportunities for intensifying resource utilization appear to exist through undersowing a creeping legume with an open‐canopy woody legume.