Identifying dissolved reactive phosphorus sources in agricultural runoff and leachate using phosphate oxygen isotopes

Authors: MUMBI, ROSE - Purdue University; Williams, Mark; FORD, WILLIAM - University Of Kentucky; CAMBERATO, JAMES - Purdue University; Penn, Chad

Submitted to: Journal of Contaminant Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2025
Publication Date: 1/11/2024
Citation: Mumbi, R., Williams, M.R., Ford, W.I., Camberato, J.J., Penn, C.J. 2024. Identifying dissolved reactive phosphorus sources in agricultural runoff and leachate using phosphate oxygen isotopes. Journal of Contaminant Hydrology. https://doi.org/10.1016/j.jconhyd.2025.104501.
DOI: https://doi.org/10.1016/j.jconhyd.2025.104501

Interpretive Summary: Agricultural phosphorus (P) losses may result from either recently applied fertilizers or from P accumulated in soil and sediment. While both P sources pose a risk to freshwater water quality, differentiating between sources is crucial for identifying and implementing management practices to decrease loss. The objective of this study was to use a novel tracer (the oxygen-18 signature of phosphate) to separate new (recently applied) and old (soil P) sources of P in runoff and leachate. Rainfall simulations were conducted in the laboratory and the tracer was measured in the fertilizer, runoff, leachate, and soil, along with dissolved P concentration and load. Results showed that dissolved P concentration in runoff and leachate increased immediately after fertilizer application, with greater than 90% of the dissolved P load attributed to direct loss of P from fertilizer. Beyond the first rainfall event after fertilizer application, interpreting the tracer results was challenging due to several competing factors. Findings of this research highlight both potential opportunities and challenges associated with using the tracer to separate different P sources.

Technical Abstract: Agricultural phosphorus (P) losses may result from either recently applied fertilizers or from P accumulated in soil and sediment. While both P sources pose an environmental risk to freshwater systems, differentiating between sources is crucial for identifying and implementing management practices to decrease loss. In this study, laboratory rainfall simulations were completed on runoff boxes and undisturbed soil columns before and after fertilizer application. The oxygen-18 signature of phosphate (d18OPO4) in fertilizer, surface runoff, subsurface leachate, and soil were analyzed (n=107 samples) to quantify new (recently applied) and old (soil) P losses in runoff and leachate. Results showed that dissolved reactive P (DRP) concentration in runoff and leachate substantially increased during the rainfall simulation immediately after fertilizer application, with runoff and leachate d18OPO4 similar to fertilizer d18OPO4 signatures. Greater than 90% of the DRP load during this event could be attributed to direct loss of P from fertilizer using d18OPO4. Beyond the first rainfall event after fertilizer application, DRP concentration decreased and leachate d18OPO4 values differed from the fertilizer values. Interpretation of isotope results was challenging because both abiotic (isotope fractionation during transport) and biotic (P cycling) processes may have influenced d18OPO4 signatures during these subsequent events. While abiotic effects on d18OPO4 appear more probable given the experimental conditions in the current study (high soil test P concentration, short duration between rainfall simulations, and strong relationship between event water and d18OPO4 signature), tracing or separating P sources remains highly uncertain during these events post-fertilizer application. Findings highlight both potential opportunities and challenges of using d18OPO4 to trace sources of P through the landscape.