Soil-water and root dynamics under hedgerow intercropping in semiarid Kenya

Competition for growth resources between woody and crop species is said to be the main reason for failure of hedgerow intercropping (alleycropping) in semiarid tropics, but the mechanisms of competition are not clearly understood. Tn this study, conducted in the semiarid highlands of Kenya, soil-water changes and root dynamics were monitored during two rainy seasons from a long-term, replicated, alleycropping experiment. The treatments were (i) maize (Zea mays L.) grown alone, without fertilizer; (ii) maize, without fertilizer, intercropped between hedgerows of leucaena [Leucaena leucocephala (Lam.) de Wit], with hedgerow prunings returned to the alleys; and (iii) maize grown alone, with fertilizer at 40 kg N and 18 kg P ha(-1). Available water in 1.25 m of soil depth under alleycropping was lower than with maize alone. Depletion of soil water by hedgerows continued after maize harvest and carried water deficits in alleycropping from one season to the next. Leucaena provided 1.45 Mg ha(-1) leaf biomass, which contained 41.6 kg N and 2.5 kg P ha(-1); moreover, it added 0.51 Mg ha(-1) season(-1) of root biomass to the soil, equivalent to 7 kg N and 0.2 kg P ha(-1). Therefore, N contributed through the alleycropping system was equivalent to the recommended fertilizer level; the system, however, did not meet the P requirements of the crop. Maize did not respond to fertilizer in both seasons, and the alleycropped maize yielded lower than the unfertilized maize. Our study indicates that under water-limiting conditions, alleycropping is detrimental to crop yields because competition of trees with crops for water outweighs the likely soil-fertility benefits.