The Chemistry and Dynamics of Soil Organic Phosphorus in Tropical Rain Forests.

Phosphorus is widely considered to constrain primary productivity in tropical rain forests, yet the chemistry and dynamics of soil organic phosphorus in such ecosystems remain poorly understood. We assessed the composition and seasonal dynamics of soil organic phosphorus in a wide range of soils under lowland tropical forest spanning a strong rainfall gradient in the Republic of Panama. Soil organic phosphorus concentrations ranged between 22 and 494 mg P/kg and were correlated strongly and positively with soil pH. The dominant groups of organic phosphorus compounds determined by solution phosphorus-31 nuclear magnetic resonance spectroscopy were phosphate monoesters and phosphate diesters, with the latter being proportionally greater in acidic, infertile soils. Remarkably, inositol phosphates did not constitute a significant component of the phosphate monoesters, despite being abundant in most mineral soils in temperate ecosystems. Soil organic phosphorus concentrations varied seasonally, being greatest in the middle of the rainy season and least in the late dry season, with corresponding seasonal changes in the activity of phosphatase enzymes. A decade of phosphorus fertilization at one site of intermediate rainfall reduced phosphatase activity and caused soil organic phosphorus to accumulate at approximately 5% per year compared to controls. Similarly, the experimental addition or removal of leaf litter at the same site caused organic phosphorus to accumulate or decline at a comparable rate. Organic phosphorus therefore represents a dynamic pool of phosphorus in tropical forest soils that varies seasonally and is sensitive to perturbations in nutrient cycling and primary productivity.