Biological decay of 18O-labeled phosphate in soils

There is an increasing demand to develop a means to trace phosphorus (P) movement through the environment as excessive inputs of P have led to the eutrophication of many fresh water bodies. 18O labeled phosphate has been suggested as a potential tool for tracing P, and other researchers are using information from natural abundance 18O levels of phosphate to study phosphorus cycling. The objective of this research was to determine the rate of biological de-labeling of 18O in soils. This objective was achieved using a laboratory incubation study in which three silt-loam textured soils were incubated with 250 mg kg−1 P18O4-P for a period of 3, 10, 30, or 50 d. The incubations were conducted on both sterilized and unsterilized soils. Following incubation, phosphate from soils was extracted with a modified Bray extractant and analyzed using electrospray ionization mass spectrometry to determine the distribution of labeled phosphate species. The half-life of P18O4 in the non-sterile soils ranged from 15 to 22 d, while there was no observed P18O4 de-labeling in sterile soils after 50 d. A parameterized numerical model was developed which provided insight into the dynamics of the individual labeled phosphate species, including their half-lives and relative concentrations across the incubation period. The use of P18O4 may be useful in areas where use of radioisotopes of P is restricted, and P18O4 has potential to be useful to elucidate the dynamics of the P cycle in soils.