Tropical forest phosphorus cycling under a changing precipitation regime

Tropical forests account for approximately one third of the World's soil carbon (C) pool, and have the highest soil respiration rates globally. These highly productive systems are typically considered phosphorous (P) limited, and P availability has been linked with various processes related to C cycling in tropical forests, such as forest productivity and soil respiration. It follows that the long-term stability of C stocks in tropical forests could depend, in part on the availability of soil P. Here I review our current understanding of moisture controls on soil P availability across a range of tropical forested ecosystems. Precipitation patterns in tropical regions are expected to change and thus understanding moisture controls on soil P availability could provide key information on the potential for nutrient feedbacks to C cycling. Research from multiple tropical forest sites suggest that the available soil P pool is highly dynamic, even in relatively a-seasonal tropical forests. Soil P availability and soil moisture availability tend to be correlated; however, the direction of the relationship is not consistent across tropical forest sites. In the iron (Fe) rich tropical soils, microbial demand, in addition to soil sorption capacity, can determine the partitioning of available P into biological vs. geochemical sinks. There is evidence that soil P availability and soil moisture are positively related in wet tropical forests, and this is hypothesized to be due in part to low redox conditions and the release of Fe-bound P. However, the reverse has been observed in dry tropical forest sites, with higher soil P during dry season months when P sorption to mineral soils is high and plant demand is low. High soil P during dry season months is hypothesized to be due to microbial biomass serving as a reserve of plant available soil P. Interestingly, findings from wet tropical forests support this hypothesis, with higher microbial biomass observed during wet season months during times of higher available soil P. Research that evaluates the importance of Fe-P dynamics as well as plant/microbial competition across a range of tropical forest sites may improve our ability to predict how changes in precipitation regime will affect soil P availability and ultimately feedback to C cycling in these important ecosystems.