d18O as a tracer of PO43- losses from agricultural landscapes

Accurately tracing the sources and fate of excess PO(4)(3- )in waterways is necessary for sustainable catchment management. The natural abundance isotopic composition of O in PO43-(delta O-18(P)) is a promising tracer of point source pollution, but its ability to track diffuse agricultural pollution is unclear. We tested the hypothesis that delta O-18(P) could distinguish between agricultural PO(4)(3- )sources by measuring the integrated delta O-18(P) composition and P speciation of contrasting inorganic fertilisers (compound vs rock) and soil textures (sand, loam, clay) in south -western Australia. delta O-18(P) composition differed between the three soil textures sampled across six livestock farms: sandy soils had lower overall delta O-18(P) values (21 +/- 1%o) than the loams (23 +/- 1%o), which corresponded with a smaller, but more readily leachable, PO(4)(3- )pool. Fertilisers had greater delta O-18(P) variability (~8%o), with fluctuations due to type and manufacturing year. Consequently, catchment 'agricultural soil leaching' delta O-18(P) signatures could span from 18 to 25%o depending on both fertiliser type and timing (lag between application and leaching). These findings emphasise the potential of delta O-18(P) to untangle soil-fertiliser P dynamics under controlled conditions, but that its use to trace catchment-scale agricultural PO(4)(3-)losses is limited by uncertainties in soil biological P cycling and its associated isotopic fractionation.

Automated summary

Accurately tracing the sources and fate of excess PO43- in waterways is crucial for sustainable catchment management. The natural abundance isotopic composition of O in PO43- (δO-18(P)) shows promise as a tracer for point source pollution, but its ability to track diffuse agricultural pollution is still uncertain.