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

A - Papers appearing in refereed journals

Pfahler, V., Bielnicka, A., Smith, A. C., Granger, S. J., Blackwell, M. S. A. and Turner, B. L. 2019. A rapid ammonium fluoride method to determine the oxygen isotope ratio of available phosphorus in tropical soils. Rapid Communications in Mass Spectrometry.

AuthorsPfahler, V., Bielnicka, A., Smith, A. C., Granger, S. J., Blackwell, M. S. A. and Turner, B. L.
Abstract

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.

KeywordsLow P availability; Oxygen isotope ratio; Panama; Tropical forest soils
Year of Publication2019
JournalRapid Communications in Mass Spectrometry
Digital Object Identifier (DOI)doi:10.1002/rcm.8647
Open accessPublished as non-open access
FunderBiotechnology and Biological Sciences Research Council
Natural Environment Research Council
Funder project or codeBuilding Phosphorus Research Potential: Developing existing methods and exploring the potential of emerging techniques
Optimisation of nutrients in soil-plant systems: Determining how phosphorus availability is regulated in soils
S2N - Soil to Nutrition - Work package 3 (WP3) - Sustainable intensification - optimisation at multiple scales
Output statusPublished
Publication dates
Online31 Oct 2019
PublisherWiley
ISSN0951-4198

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