『Abstract
　Phosphorus availability is commonly assumed to limit productivity in lowland tropical rainforests, yet there is relatively little information on the chemical forms of soil phosphorus in such ecosystems. We used the Hedley sequential fractionation scheme to assess phosphorus chemistry in five soils supporting tropical rainforest on Barro Colorado Island, Republic of Panama. The soils represented a range of orders (Inceptisols, Alfisols, and Oxisols) formed on contrasting geological substrates and topography, but under uniform climate and vegetation. Total phosphorus in surface horizons ranged between 315 and 1114 mg P kg^-1, being lowest on a soil derived from marine sediments and highest on soils derived from andesite. The majority of the phosphorus occurred in recalcitrant forms, although between 14％ and 39％ occurred as organic phosphorus. Readily-available phosphate, as extracted by anion-exchange membranes, occurred in small concentrations (4-13 mg P kg^-1), although labile phosphorus, defined as phosphate extracted by anion-exchange membrane plus inorganic and organic phosphorus extracted by 0.5 M NaHCO3, accounted for between 4.7％ and 11.4％ of the total soil phosphorus. Our results indicate a strong control of geology and topography on soil phosphorus in tropical rainforests, which may have important implications for understanding the diversity and distribution of plant species in such ecosystems. Further, some of the most common soils on Barro Colorado Island, including those on the 50 ha forest dynamics plot, are rich in phosphorus despite their relatively advanced stage of pedogenesis.

Keywords: Barro Colorado Island; Hedley sequential fractionation; Inorganic phosphorus; Organic phosphorus; Panama; Phosphorus cycle』

1. Introduction
2. Materials and methods
　2.1. Site description and soil sampling
　2.2. Phosphorus fractionation
　2.3. Determination of soil properties
　2.4. Data analysis
3 Results
　3.1. Soil properties
　3.2. Concentrations of phosphorus fractions
　3.3. Sequential fractions as a proportion of the total soil phosphorus
　3.4. Total inorganic and organic phosphorus
　3.5. Carbon to organic phosphorus ratios
　3.6. Labile phosphorus
　3.7. Correlations between phosphorus fractions and soil properties
4. Discussion
　4.1. Forms and distribution of soil phosphorus
　4.2. Labile phosphorus and the contribution of bicarbonate extractable organic phosphorus
　4.3. Phosphorus status and the significance of organic phosphorus
5. Conclusions
Acknowledgments
References