Phosphorus Sorption Dynamics of Anion Exchange Resin Membranes in Tropical Rain Forest Soils

Bioavailable P is recognized as a major constraint on productivity in many tropical rain forests. Nevertheless, insufficient knowledge about short-term temporal patterns in soil labile P limits our understanding of the mechanisms controlling soil P supply in tropical forest soils. The use of field-deployed anion exchange resin membranes (AEMs) to determine soil nutrient supply rates is increasingly common, however, the lack of information regarding their behavior over time in tropical wet forest soils interferes with interpretation of results. This study quantifies in situ labile P as measured by AEMs over various incubation periods (4, 8, 12, 14, 20, and 24 d) in the surface soil of two neighboring Costa Rican tropical rain forest Oxisols of differing P content (0.46 vs. 1.18 Mg ha(-1), 0-10 cm). Using a nested design, we evaluated the importance of incubation length, environmental conditions (i.e., rainfall, temperature, and solar radiation), vegetation characteristics (i.e., basal area, stem density, and species composition), and total P status in driving spatial and temporal variability in AEM P. Spatially averaged AEM P in the site with higher P content ranged from 0.26 to 1.38 mu g membrane(-1) across all incubation periods; approximately two fold higher than AEM P in the more P-deficient site (0.11-0.77 mu g membrane(-1)). Temporal variability in AEM P was best explained by rainfall. Despite consistently high volumetric soil moisture (50-66%), increasing rainfall was associated with greater P availability in both sites. Mean P availability increased with plant basal area and basal area of legumes.