Abstract
RationaleThe isotopic composition of oxygen bound to phosphorus (δ18OP 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 δ18OP 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 δ18OP values of available P in tropical forest soils.MethodsWe compared P concentrations and δ18OP 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.ResultsDepending 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 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.ConclusionsThe 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 δ18OP values of available P in tropical soils.
Highlights
Tropical forest soils sustain a large net primary production despite low phosphorus (P) availability.[1]
The anion-exchange membrane (AEM) method takes longer than the Bray-1 method, so the possibility cannot be excluded that some microbial P is released during the AEM extraction of unfumigated samples
Release of inorganic P from microbial cells would not be detected using18O-labelled and unlabelled solutions as no O exchange occurs, but it would reduce the δ18OP values in our soils because the δ18OP values of microbial P are probably lower than the values for available P based on the results for AEM Punf and Pfum
Summary
Tropical forest soils sustain a large net primary production despite low phosphorus (P) availability.[1]. Soils from Plantation Road and Madden Dam were used for the determination of the δ18OP values of Bray-1 Punf and Pfum using a solution-to-soil ratio of 10, extraction time of 5 min and an extraction temperature of 22C. Those two soils were chosen for their contrasting properties, including P concentrations, organic carbon content and soil taxonomic class (Table 1). Based on the result of the t-test, the δ18OP values obtained with18O-labelled and unlabelled Bray-1 solutions were considered as replicates for the other treatments (solution-to-soil ratio, extraction temperature and fumigation time). All statistical analyses were performed with the program R.36
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