The Accurate Measurement And Control Factors Of The Phosphate Oxygen Isotope Ratios Of Different Phosphate Forms In Farmland Soil

The oxygen-isotope ratios in phosphate(?18OP) have been increasingly used as an effective tracer for quantifying bio-available phosphorus(P), organic and inorganic transformation as well as phase conversion of different P pools in soils. However, methods and procedures being used to process samples and analyze isotope compositions have varied significantly. Therefore, for the uniformity of method as well as for the comparison of results, it is important to understand if specific sample treatment method can cause any changes in the original ?18OP values. Here we applied 18 O spiking method where KH2PO4 reagent of known ?18OP values was spiked in a soil with known P pool concentrations. Furthermore, P pools in two types of soils were extracted using eight different methods including variants of sequentially extraction that included deionized H2 O, 0.5 M NaHCO3, 1.0 N NaOH and1.0 N HCl. Based on the methods tested in this study, sample storage and inclusion of washing steps after certain reagent are most critical factors that may impact isotope compositions. Variation in sample storage conditions may impact microbial activities and thus reset new isotope compositions. Similarly, incomplete separation of P pool causes to carry over of the particular P pool to next pool. While the effect of soil sieving and freeze vs air drying was found to be insignificant in this study, it maynot be always the case for other soils. For sequential extraction can get specific ?18OP values of different forms of phosphate in soil, it is more effective than single-step extraction to trace the cycling of phosphate in soil.