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

The isotopic composition of oxygen bound to phosphorus (δ¹⁸ O_P value) offers an opportunity to gain insight into P cycling mechanisms. However, there is little information for tropical forest soils, which presents a challenge for δ¹⁸ O_P measurements due to low available P concentrations. Here we report the use of a rapid ammonium fluoride extraction method (Bray-1) as an alternative to the widely used anion-exchange membrane (AEM) method for quantification of δ¹⁸ O_P values of available P in tropical forest soils.

We compared P concentrations and δ¹⁸ O_P values 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.

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 δ¹⁸ O_P 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) δ¹⁸ O_P values determined by Bray-1 extraction (20‰ and 16‰, respectively) were higher than those determined by the AEM method (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 needed for the AEM method.

The short extraction time, insensitivity to extraction conditions and smaller mass of soil required to extract sufficient P for isotopic analysis make Bray-1extraction a suitable alternative to the AEM method for the determination of δ¹⁸ O_P values of available P in tropical soils.