Highly weathered tropical soils have low P plant availability and very high inorganic phosphorus (Pi) adsorption capacity. Therefore, maintenance of organic sources able to supply P by mineralization is essential for P availability under highly weathered conditions. Organic matter cycling and tolerance to low P soil concentration of the plant community may guarantee sustainability in natural ecosystems. The aim of this study was to evaluate organic phosphorus (Po) content and labile P fractions in the soils of three sites. The first site was a natural forest at 900 m asl, the second natural forest at 600 m asl and the third a Corymbia citriodora stand at 250 m asl. The same P fractions were evaluated in pasture areas adjacent to each site. The recovery rate of Pi + Po ranged from 50 to 82 % of the nitric-perchloric extraction. Mean total Po in the forest was 160 mg kg-1 and in pasture soil 69.8 mg kg-1. The greatest difference between forest and pasture was observed at site one (-74 %), followed by site three (-53 %) and site two (-25 %). Organic P accounted for 14.6 to 36.9 % of the total extracted P. Labile Po accounted for over 80 % of the total labile P in the natural forests and 65 % in the Corymbia citriodora plantation. Total and labile Po were positively correlated with organic C, and the labile Po was positively related to available P.
Atlantic forest; available P; labile P fraction
