Tree species selection and soil tillage in alley cropping systems with Phaseolus vulgaris L. in a humid premontane climate:: biomass production, nutrient cycling and crop responses

The development of alley cropping systems is based on the assumption that leguminous trees planted in hedgerows influence the yield of associated crops favourably by means of the additional nutrient pool applied to the soil through tree prunings. An on-station field study (split-plot design in a randomised block design) was conducted on an Eutric Cambisol under humid premontane climate conditions in Costa Rica in order to evaluate the ability of Erythrina poeppigiana, Calliandra calothyrsus and Gliricidia sepium to increase bean (Phaseolus vulgaris) yields compared to sole cropping. Soil tillage was applied as a sub-treatment in order to evaluate if soil preparation would additionally alter soil fertility and bean yield. After seven years with pruning twice per year, the size of both the total N and P pool in the pruned tree material was about three times higher for Erythrina prunings than for Calliandra and Gliricidia prunings. Two and five weeks after mulch application 50-150% higher inorganic N pools were measured in the soil from Erythrina plots, the bean shoot biomass at harvest was increased by 65-100% and the bean yield was 15-50% higher than in plots with beans alone. Hence, of the three tree species, Erythrina was the best choice for alley cropping systems in the pedoclimatic environment studied. Soil tillage reduced bean yield, soil organic matter, total soil N content and soil microbial biomass N in the top soil and is not recommended for similar soils in humid premontane climates.