Soil organic phosphorus in lowland tropical rain forests

Phosphorus is widely considered to constrain primary productivity in tropical rain forests, yet the chemistry of soil organic phosphorus in such ecosystems remains poorly understood. We assessed the composition of soil organic phosphorus in 19 contrasting soils under lowland tropical forest in the Republic of Panama using NaOH-EDTA extraction and solution ³¹P nuclear magnetic resonance spectroscopy. The soils spanned a strong rainfall gradient (1730-3404 mm y⁻¹) and contained a wide range of chemical properties (pH 3.3-7.0; total carbon 2.8-10.4%; total phosphorus 74-1650 mg P kg⁻¹). Soil organic phosphorus concentrations ranged between 22 and 494 mg P kg⁻¹ and were correlated positively with total soil phosphorus, pH, and total carbon, but not with annual rainfall. Organic phosphorus constituted 26 ± 1% (mean ± STD error, n = 19) of the total phosphorus, suggesting that this represents a broad emergent property of tropical forest soils. Organic phosphorus occurred mainly as phosphate monoesters (68-96% of total organic phosphorus) with smaller concentrations of phosphate diesters in the form of DNA (4-32% of total organic phosphorus). Phosphonates, which contain a direct carbon-phosphorus bond, were detected in only two soils (3% of the organic phosphorus), while pyrophosphate, an inorganic polyphosphate with a chain length of two, was detected in all soils at concentrations up to 13 mg P kg⁻¹ (3-13% of extracted inorganic phosphorus). Phosphate monoesters were a greater proportion of the total organic phosphorus in neutral soils with high concentrations of phosphorus and organic matter, whereas the proportion of phosphate diesters was greater in very acidic soils low in phosphorus and organic matter. Most soils did not contain detectable concentrations of either myo- or scyllo-inositol hexakisphosphate, which is in marked contrast to many temperate mineral soils that contain abundant inositol phosphates. We conclude that soil properties exert a strong control on the amounts and forms of soil organic phosphorus in tropical rain forests, but that the proportion of the total phosphorus in organic forms is relatively insensitive to variation in climate and soil properties. Further work is now required to assess the contribution of soil organic phosphorus to the nutrition and diversity of plants in these species-rich ecosystems.