Identification of phosphorus sources using oxygen isotope signatures of the recalcitrant and residual phosphorus pools in soils and sediments

Excess phosphorus (P) loading has led to eutrophication and deteriorated the water quality globally. The existing limitation of analytical methods for tracing residual and recalcitrant P pools has posed a challenge to discern specific sources, specifically legacy sources, which are considered the missing piece of the global P puzzle. In this study, residual P pools of soils and sediments from the East Creek micro-watershed in the Chesapeake Bay watershed were sequentially extracted using 10 M HNO₃, 10 M NaOH, and aqua regia, and phosphate oxygen isotopes (δ¹⁸O_P) of these soil P pools were then measured. Additionally, the isotope integrity of each reagent was assessed and found that 10 M HNO₃ and 10 M NaOH do not significantly compromise the original isotope values over the extraction time. The isotopes of inorganic residual soil P_i pools revealed a distinct range of isotope values among different land uses as well as specific landforms within each land use, suggesting the usefulness of isotopes of residual P_i pools to discriminate different P sources in sub-land uses. Furthermore, isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) of residual soil pools showed distinct signatures among sources and pointed to potential signals imprinted from the past cropping systems. Contributions of soils from wetland, streambank, agricultural land, and forest sources varied systematically along East Creek. The Bayesian elemental fingerprinting model provided the effect of the dynamic nature of sources, as well as potential creek meanders. In conclusion, the integration of multi-isotope and elemental fingerprinting methods of residual soil P_i pools presents a promising and complementary tool for tracking P sources in watersheds.