Soil phosphorus forms under leguminous tree species, secondary forest and pasture in northern Rio de Janeiro state, Brazil

In highly weathered tropical soils phosphorus (P) bioavailability to plants is closely associated with soil organic P forms, which in turn are influenced by the vegetation cover. The aim of this study was to evaluate total organic P (Po), labile P fractions and microbial biomass P in soils under Acacia auriculiformis and Mimosa caesalpiniifolia stands, secondary forest and pasture. The clay-adjusted average content of total Po and labile Po were 375 mg kg(-1) P and 40.2 mg kg(-1), respectively. The Po contents (total and labile) in soils under Acacia auriculiformis and Mimosa caesalpiniifolia were higher than under pasture and secondary forest soils. The proportion of Po ranged from 22.6 to 39.6 % of total extracted P in soils under the vegetation cover types studied. The labile Po represented about 67.1 % of total labile P. The greatest amount of soil microbial biomass P (MBP) was observed under Mimosa caesalpiniifolia, followed by secondary forest, Acacia auriculiformis and pasture. However, the soil microbial biomass of the secondary forest was the most efficient regarding P immobilization (MBP/total Po ratio). Available P was positively correlated with soil microbial biomass C and microbial P, and microbial P was positively related to microbial C.