A rapid ammonium fluoride method to determine the oxygen isotope ratio of available phosphorus in tropical soils

Rationale The isotopic composition of oxygen bound to phosphorus (δ18OP) offers an opportunity to gain insight into P cycling mechanisms. However, there is little information for tropical forest soils, which present a challenge for δ18OP measurements due to low available P concentrations. Here we report the use of a rapid ammonium fluoride extraction (Bray-1) as an alternative to the widely used anion-exchange membrane (AEM) method for quantification of δ18OP values of available P in tropical forest soils.
Methods We compared P concentrations and δ18OP of available and microbial P determined by AEM and Bray-1 extraction for a series of tropical forest soils from Panama spanning a steep P gradient. This involved an assessment of the influence of extraction conditions, including temperature, extraction time, fumigation time, and solution-to-soil ratio, on P concentrations and isotope ratios.
Results Depending on the extraction conditions, Bray-1 P concentrations ranged from 0.2 to 66.3 mg P kg-1 across the soils. Extraction time and temperature had only minor effects on Bray-1 P, but concentrations increased markedly as the solution-to-soil ratio increased. In contrast, extraction conditions did not affect Bray-1 δ18OP values, indicating that Bray-1 provides a robust measure of the isotopic composition of available soil P. For a relatively high P soil, available and fumigation-released (microbial) δ18OP values determined by Bray-1 extraction (20‰ and 16‰, respectively) were higher than values determined by AEM (18‰ and 12‰, respectively), which we attribute to slightly different P pools extracted by the two methods and/or differences resulting from the longer extraction time for the AEM method.
Conclusions The short extraction time, insensitivity to extraction conditions, and smaller mass of soil required to extract sufficient P for isotopic analysis, make Bray-1 extraction a suitable alternative to the AEM method for the determination of δ18OP values of available P in tropical soils.