Influence of shrubs on soil characteristics and their function in Sahelianagro-ecosystems in semi-arid Niger

Soil beneath shrubs form 'fertile islands' in fallow sites and millet field s in semi-arid Niger. To gain more information about this phenomena differe nt shrub species in fallow sites following a gradient from 350-650 mm preci pitation were examined. For each shrub two different areas were distinguish ed: an area under the canopy of the shrubs and an area in the nearby open l and. Soil samples were taken from a depth of 0-10 cm and analysed for C-org , N-total, P-Bray, pH(H2O), exchangeable cations, effective cation exchange capacity (ECEC) and soil texture. Significantly higher concentrations betw een 38-51% for C, N, P and 22% on ECEC for K+ were found in the soil under the shrubs. The pH showed only slight but significant differences, whereas Al3+ and H+ rates on ECEC under the shrubs were increased by 44-55%. For Gu iera senegalensis, the most common shrub of the studied area, enrichment ra tios of most soil properties increased relatively more with increasing arid ity. In general, enrichment ratios decreased with the age of the fallows, w hereas concentrations showed no clear evolution. The chemical composition o f the shrub litter seems to influence the degree of soil enrichment. The ma in step of fertile island formation takes place during the cultivation peri od by trapping wind-blown sediment. This work shows that shrubs are of vita l importance for the accumulation of nutrients and maintenance of soil fert ility within agro-ecosystems of Niger. (C) 2000 Academic Press.