Soil organic phosphorus and the nutrition of tropical forests

Organic phosphorus is abundant in tropical forest soils, but is rarely considered to have ecological significance. However, plants can use soil organic phosphorus through adaptations such as the secretion of organic anions, synthesis of phosphatase enzymes, and association with mycorrhizal fungi. Of particular relevance to tropical forests are the arbuscular mycorrhizae, at least some of which can use organic phosphorus as a sole phosphorus source. Organic phosphorus occurs in range of compounds that react differently in soil, which influences their potential bioavailability. For example, the inositol phosphates are strongly stabilized in soil and are considered to be relatively unavailable. It is therefore notable that the inositol phosphates are largely absent from the tropical forest soils assessed so far, indicating their rapid degradation. Rates of organic phosphorus turnover are hard to assess, although a large-scale litter removal experiment in lowland tropical forest in Panama revealed that soil organic phosphorus declined by 30% in the first three years, despite no detectable change in forest productivity. There are also marked seasonal changes in soil organic phosphorus, which reflect expected patterns of nutrient availability. Finally, the use of soil organic phosphorus in tropical forests suggests the possibility of resource partitioning for soil phosphorus, as occurs for nitrogen in Arctic ecosystems.