δO as a tracer of PO losses from agricultural landscapes.

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