The efficiency of sequential extraction of phosphorus in soil and sediment: insights from the oxygen isotope ratio of phosphate

Purpose Phosphorus (P) occurs in different chemical forms in soil and sediment, and they play different ecological and environmental roles. Thus, separation and quantification of different P pools are essential to the understanding of P dynamics in the environment. Over the past decades, different sequential extraction methods have been used concurrently. However, systematic evaluation of different extraction methods is lacking, making choice of methods and comparisons between results difficult. Materials and methods We compared the oxygen isotope compositions (delta O-18(P)) of different phosphate pools that were extracted by two major extraction methods, Hedley (Hedley et al., Soil Sci Soc Am J 46:970-976, 1982) and SEDEX methods (Ruttenberg, Limnol Oceanogr 37:1460-1482, 1992), for both a soil and an estuarine sediment sample. Additionally, Ag3PO4 and KH2PO4 standards were used to test the fidelity of the extraction and purification procedure. The purity of the precipitated Ag3PO4 was also examined. Results and discussion The delta O-18(P) values have large differences among phosphate pools for the soil extracted by the Hedley method and for the sediment extracted by the SEDEX method. The delta O-18(P) values of Ag3PO4 and KH2PO4 standards after extraction, purification, and/or re-precipitation are similar to their original delta O-18(P) values. The XRD patterns of the precipitated Ag3PO4 are identical to that of Ag3PO4 standard. The residual carbon and nitrogen contents in the precipitated Ag3PO4 are low. Conclusions The Hedley method has good efficiency for separating isotopically distinct P-i pools (different sources/bioavailability) in the soil samples, while the SEDEX method has good efficiency for separating isotopically distinct P-i pools (different sources/bioavailability) in sediments that are more marine in character such as for the estuarine sample. The extraction and purification processes do not alter delta O-18(P), and the precipitated Ag3PO4 has high purity.