The effect of sample treatments on the oxygen isotopic composition of phosphate pools in soils

The oxygen isotopic composition of phosphate (δ18OP) has been increasingly used as an effective tracer for the biogeochemical cycling of phosphorus (P) in soils and other environments. However, diverse pretreatment methods (e.g. storage, preparation and extraction) are being used for soil samples. For the uniformity of methods as well as for the comparison of results, it is important to understand if specific treatment methods can compromise original δ18OP values. Here, Ag3PO4 and KH2PO4 were used to test whether a modified Hedley sequential extraction and purification procedure can alter the δ18OP values of phosphate standards. Additionally, to test the effect of sample storage and drying conditions, two types of soils were first processed by using eight different pretreatment methods including sterilizing, storing, drying, and sieving and then the δ18OP values of each soil P pool were measured. Results indicate that the extraction and purification procedure, drying temperature (< 0 °C to 80 °C) and sieving mesh (20 to 100) had no significant effect on the δ18OP values of Pi (inorganic P) pools, but storage at room temperature (without microbial growth inhibitor-HgCl2 added) can lead to significant changes in δ18OP values of almost all P pools. For the two soils studied, the δ18OP values of Pi pools decrease from H2O (H2O-Pi) to NaHCO3 (NaHCO3-Pi), NaOH (NaOH-Pi) and HCl (HCl-Pi), and organic P was also found in the extraction solution of HCl. Furthermore, the δ18OP values calculated from isotope mass balance were different from the measured values suggesting variable extraction of different P pools during single and sequential extraction methods. Collectively these results highlight the need for a unified and standard processing and extraction methods for soil samples to allow meaningful intercomparison of results.