The phosphorus status of tropical forest soils - what can oxygen isotopes tell us?

It is vital to understand how tropical forests react to future environmental change, however the role of soil phosphorus (P) in regulating these responses is still not well understood. The isotopic composition of oxygen bound to phosphorus (δ¹⁸O_P) offers an opportunity to provide insight into P cycling mechanisms. Available phosphate, the form taken up by plants from the soil solution, is influenced by different biotic and abiotic processes like enzymatic hydrolysis of organic P and desorption of P from soil particles. Each of those processes could influence the δ¹⁸O_P of available phosphate, yet there is little information on these processes in tropical forest soils, which present a challenge for δ¹⁸O_P measurements due to the low concentrations of available phosphate. We successfully tested the use of a rapid acidic ammonium fluoride extraction (Bray-1) as an alternative to the widely used anion-exchange resin membrane (AEM) method for determination of δ¹⁸O_P values of available phosphate in tropical forest soils. The Bray-1 method has the advantage of being faster and insensitive to the extraction conditions, requiring less soil, and allowing a rapid sample throughput. We now assess whether this method (δ¹⁸O_P values of Bray-1 extracts) can provide insight into the P status of tropical forest soils. We analyse soil samples, taken from 30 sites across a P-gradient along the isthmus of Panama, extract them with Bray-1 solution and determine the corresponding δ¹⁸O_P values. We then compare these data with other soil data, including soil phosphatase activities and nutrient concentrations, to identify patterns in the δ¹⁸O_P data. First results indicate that δ¹⁸O_P values of available phosphate correlate positively with dissolved organic carbon concentrations in soils and negatively with concentrations of inorganic P extracted with sodium hydroxide, a soil P pool which includes oxide-bound P. Our study furthers the development of the δ¹⁸O_P into a reliable test method for the P status of tropical forest soils, which ultimately increases our understanding of soil P cycling.