A method for the analysis of the <i>δ</i> ¹⁸ O of inorganic phosphate extracted from soils with HCl

Abstract

The oxygen isotope composition of phosphate ( δ 18 O‐PO 4 ) has successfully been used to study the biological cycling of phosphorus (P) in seawater and marine sediments. However, only a few studies have used this approach in soils. In order to analyse δ 18 O‐PO 4 , phosphate must be extracted from the soil, purified and converted to silver phosphate (Ag 3 PO 4 ). The published extraction methods, successfully applied to marine waters and sediments, lead to the precipitation of impure Ag 3 PO 4 when used with soils or organic‐rich samples. Here we present an improved purification protocol, designed for soils and other organic‐rich samples. After extraction with HCl, phosphate is purified with multiple mineral precipitations that do not require extreme pH adjustments of the solutions. We show that contaminant‐free Ag 3 PO 4 can be produced from fertilizers and various soils with different chemical and physical characteristics. Our first isotopic results confirm that differences in P status and availability in soils are expressed in the δ 18 O‐PO 4 signal, indicating the potential of this isotopic tracer to understand P dynamics in soil systems.