Soil Phosphorus Forms along a Strong Nutrient Gradient in a Tropical Ombrotrophic Wetland

Phosphorus cycling influences productivity and diversity in tropical wetlands, yet little is known about the forms of P found in the accreting organic matter of these ecosystems. We used alkaline (NaOH–ethylenediamine tetraacetic acid [EDTA]) extraction and solution 31P nuclear magnetic resonance (NMR) spectroscopy to characterize P in surface soils across a strong nutrient gradient within a tropical ombrotrophic peat dome. From the interior bog plain to the marginal Raphia taedigera swamp, total soil P increased from 14.6 to 70.9 g m−3 and resin-extractable P from 0.1 to 30 mg kg−1. Phosphatase activity declined across the same transect (364–46 μmol methylumbelliferone kg−1 min−1), indicating an increase in P availability toward the periphery of the wetland. Organic P identified by solution 31P NMR spectroscopy included phosphomonoesters (12–17%), phosphodiesters (10–14%), and phosphonates (up to 3.3% of total P). Inositol phosphates were not detected in these acidic peats. Inorganic P forms included orthophosphate (9–25% of total P), pyrophosphate (up to 3%), and long-chain polyphosphates; the latter occurred in concentrations (up to 24% of total soil P) considerably higher than previously found in wetland soils. The concentration of residual (unextractable) P was similar among sites (mean 280 mg kg−1), resulting in an increase in its proportion of the total soil P from 29% at the P-rich margins to 55% at the P-poor interior. This is the first information on the P composition of tropical wetland soils and provides a basis for further study of the cycling and contribution of P forms to the nutrition of plants and microorganisms.