Soil phosphate stable oxygen isotopes across rainfall and bedrock gradients

The stable oxygen isotope compositions of soil phosphate (δ ¹⁸O _p) were suggested recently to be a tracer of phosphorus cycling in soils and plants. Here we present a survey of bioavailable (resin-extractable or resin-P) inorganic phosphate δ ¹⁸O _p across natural and experimental rainfall gradients, and across soil formed on sedimentary and igneous bedrock. In addition, we analyzed the soil HCl-extractable inorganic δ ¹⁸O _p, which mainly represents calcium-bound inorganic phosphate. The resin-P values were in the range 14.5-21.2%. A similar range, 15.6-21.3%, was found for the HCl-extractable inorganic δ ¹⁸O _p, with the exception of samples from a soil of igneous origin that show lower values, 8.2-10.9%, which indicate that a large fraction of the inorganic phosphate in this soil is still in the form of a primary mineral. The available-P δ ¹⁸O _p values are considerably higher than the values we calculated for extracellular hydrolysis of organic phosphate, based on the known fractionation from lab experiments. However, these values are close to the values expected for enzymatic-mediated phosphate equilibration with soil-water. The possible processes that can explain this observation are (1) extracellular equilibration of the inorganic phosphate in the soil; (2) fractionations in the soil are different than the ones measured at the lab; (3) effect of fractionation during uptake; and (4) a flux of intercellular-equilibrated inorganic phosphate from the soil microbiota, which is considerably larger than the flux of hydrolyzed organic-P.