Below-ground interactions in dryland agroforestry

This paper discusses the effects of intercropping and tree pruning on root distribution and soil water depletion in an alley cropping system with Acac ia saligna and Sorghum bicolor in northern Kenya. Root distribution was det ermined by destructive sampling, and the soil water suction was measured wi th tensiometers and gypsum blocks, both up to 150 cm depth. The root system s of the intercropped trees and crops were distinguished using the natural C-13 discrimination between C-3 and C-4 plants. The root carbohydrate conte nts were used to estimate plant water stress integrated over time. The high est root length density was always measured in the topsoil, regardless of s eason or cropping system. In the dry season, the proportion of roots under the tree row compared to the alley was higher than during the wet season; t he same was found for the proportion of roots in the subsoil compared to th e topsoil. Pruning decreased the total root length density of sole cropped trees by 47%. The highest root length density was found when the pruned tre es were intercropped with Sorghum. If the trees were not pruned, combining trees and crops did not increase root length density. Intercropping resulte d in a spatial separation of the root systems of trees and crops between th e hedgerows, Sorghum having more roots in the topsoil and the trees having more roots in the subsoil under alley cropping than in monoculture. At the hedgerow of the agroforestry system, however, the root systems of trees and crop overlapped and more roots were found than the sum of roots of sole cr opped trees and crops. Soil water depletion was higher under the tree row t han in the alley and higher in alley cropping than in monocultural systems. Water competition between tree and crop was confirmed by the carbohydrate analyses showing lower sugar contents of roots in agroforestry than in mono culture. The agroforestry combination used the soil water between the hedge rows more efficiently than the sole cropped trees or crops, as water uptake of the trees reached deeper and started earlier after the flood irrigation than of the Sorghum, whereas the crop could better utilize topsoil water. Under the experimental conditions, the root system of the alley cropped Aca cia and Sorghum exploited a larger soil volume utilizing soil resources mor e efficiently than the respective monocultures. (C) 1998 Elsevier Science B .V.