δ¹⁸O as a tracer of PO₄³¯ losses from agricultural landscapes

Abstract

Accurately tracing the sources and fate of excess PO₄³¯ in waterways is necessary for sustainable catchment management. The natural abundance isotopic composition of O in PO₄³¯ (δ¹⁸OP) is a promising tracer of point source pollution, but its ability to track diffuse agricultural pollution is unclear. We tested the hypothesis that δ¹⁸OP could distinguish between agricultural PO₄³¯ sources by measuring the integrated δ¹⁸OP composition and P speciation of contrasting inorganic fertilisers (compound vs rock) and soil textures (sand, loam, clay) in southwestern Australia.δ¹⁸OP composition differed between the three soil textures sampled across six livestock farms: sandy soils had lower overall δ¹⁸OP values (21 ± 1‰) than the loams (23 ± 1‰), which corresponded with a smaller, but more readily leachable, PO₄³¯pool. Fertilisers had greater δ¹⁸OP variability (∼8‰), with fluctuations due to type and manufacturing year. Consequently, catchment ‘agricultural soil leaching’ δ¹⁸OP 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 δ¹⁸OP to untangle soil-fertiliser P dynamics under controlled conditions, but that its use to trace catchment-scale agricultural PO₄³¯ losses is limited by uncertainties in soil biological P cycling and its associated isotopic fractionation.