Below-ground interactions in dryland agroforestry

Abstract

This paper discusses the effects of intercropping and tree pruning on root distribution and soil water depletion in an alley cropping system with Acacia saligna and Sorghum bicolor in northern Kenya. Root distribution was determined by destructive sampling, and the soil water suction was measured with tensiometers and gypsum blocks, both up to 150 cm depth. The root systems of the intercropped trees and crops were distinguished using the natural 13 C discrimination between C 3 and C 4 plants. The root carbohydrate contents were used to estimate plant water stress integrated over time. The highest root length density was always measured in the topsoil, regardless of season 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; the same was found for the proportion of roots in the subsoil compared to the topsoil. Pruning decreased the total root length density of sole cropped trees by 47%. The highest root length density was found when the pruned trees were intercropped with Sorghum. If the trees were not pruned, combining trees and crops did not increase root length density. Intercropping resulted in a spatial separation of the root systems of trees and crops between the 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 cropped trees and crops. Soil water depletion was higher under the tree row than 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 monoculture. The agroforestry combination used the soil water between the hedgerows 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 Acacia and Sorghum exploited a larger soil volume utilizing soil resources more efficiently than the respective monocultures.