δ 18 O as a tracer of PO 4 3- 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 PO ₄ ^3- (δ ¹⁸ O _P ) is a promising tracer of point source pollution, but its ability to track diffuse agricultural pollution is unclear. We tested the hypothesis that δ ¹⁸ O _P could distinguish between agricultural PO ₄ ^3- sources by measuring the integrated δ ¹⁸ O _P composition and P speciation of contrasting inorganic fertilisers (compound vs rock) and soil textures (sand, loam, clay) in southwestern Australia. δ ¹⁸ O _P composition differed between the three soil textures sampled across six livestock farms: sandy soils had lower overall δ ¹⁸ O _P values (21 ± 1‰) than the loams (23 ± 1‰), which corresponded with a smaller, but more readily leachable, PO ₄ ^3- pool. Fertilisers had greater δ ¹⁸ O _P variability (∼8‰), with fluctuations due to type and manufacturing year. Consequently, catchment 'agricultural soil leaching' δ ¹⁸ O _P 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 δ ¹⁸ O _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.
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