Horizon 1 1 17 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES Plant and Soil 185-198 Canopy soil P fractions Sahel Semi-arid Shrubs Total C 2010 fdi:010049227 ENG Plant and Soil 0032-079X ISI:000273754800015 1-2 10.1007/s11104-009-0044-8 https://www.documentation.ird.fr/hor/fdi:010049227 https://www.documentation.ird.fr/intranet/publi/2010/02/010049227.pdf 327 Horizon (IRD) Two shrub species (Piliostigma reticulatum (D.C.) Hochst (Caesalpinioideae) and Guiera senegalensis J.F. Gmel (Combretaceae) are commonly found in farmers' fields at varying densities in semi-arid Senegal and throughout the Sahel where soils have chronically low phosphorus (P) availability. It seems plausible that shrub litter and the rhizospheres could influence P fractions and other chemical soil properties that affect crop productivity. Thus, a study was done at two sites, on the distribution of inorganic and organic soil P pools, organic C levels, and pH in soil beneath and outside the canopies of P. reticulatum and G. senegalensis (0-30 cm depth). Both sites had low total P ranging from 64 mg P kg(-1) to 135 mg P kg(-1), and low extractable PO4 (resin Pi) (1-6 mg P kg(-1)) with P fractions dominated by NaOH-P. Organic P (Po) made up about 50% of total P, and most of the organic P (> 60%) was found in the NaOH-P fractions. The labile P, particularly bicarb-Po was higher in soil beneath shrub canopies (8.4 mg P kg (-1)), than outside the canopy (6.2 mg P kg (-1)). Similarly, C, N and P to a lesser extent, were more concentrated beneath shrub canopies. P. reticulatum soil was dominated by the NaOH-Po fraction, whereas G. senegalensis had higher bicarb-Po at one of the study sites. An index of biologically available organic P (Bicarb-Po) / (Bicarb-Po + Bicar-Pi + Resin Pi) was > 60% and indicates that biological processes represent an important part of P cycling in these shrub ecosystems. The differential ability of shrubs in modifying soil chemical properties under their canopies has major implications for biogeochemical cycling of nutrients and C in sandy soils of semi arid Sahelian ecosystems. 068 UR179