Assessing intra-event phosphorus dynamics in drainage water using phosphate stable oxygen isotopes

Quantifying fluxes and pathways of dissolved reactive phosphorus (DRP) in tile-drained landscapes has been hampered by a lack of in situ measurements that are sensitive to P fate and transport processes. One potential tool to help understand these dynamics is the oxygen isotope signature of phosphate (Δ¹⁸O_PO4); however, its potential benefits and limitations are not well understood for intra-event dynamics at the field scale. The objectives of this study were to quantify intra-event variability of Δ¹⁸O_PO4 signatures in tile drainage water and assess the efficacy of Δ¹⁸O_PO4 to elucidate mechanisms and flow pathways controlling DRP transport to tile drains. We collected water samples during a summer storm event from a subsurface (tile)-drained field located in west-central Ohio and analyzed for Δ¹⁸O_PO4 of DRP. Supplementary water quality measurements, hydrologic modeling, and soil temperature data were used to help understand intra-event Δ¹⁸O_PO4 dynamics. Results of the soil extraction analysis from our study site highlight that the soil water-extractable P (WEP) pool was not in equilibrium with long-term, temperature-dependent water isotope values. This result suggests that P-rich soils may, at least partially, retain their original source signature, which has significant implications for identifying hotspots of P delivery in watershed-scale applications. Results of the storm event analysis highlight that equilibration of leached DRP in soil water creates a gradient between isotopic compositions of pre-event shallow subsurface sources, pre-event deep subsurface sources, and the WEP tied up in surface soils. The current study represents the first intra-event analysis of Δ¹⁸O_PO4 and highlights the potential for phosphate oxygen isotopes as a novel tool to improve understanding of P fate and transport in artificially drained agroecosystems.