Assessing Intra-Event Phosphorus Dynamics in Drainage Water Using Phosphate Stable Oxygen Isotopes

Abstract

Abstract. Quantifying fluxes and pathways of dissolved reactive phosphorus (DRP) in tile-drained landscapes has been hampered by a lack of 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 (d18OPO4); 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 d18OPO4 signatures in tile drainage water and assess the efficacy of d18OPO4 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 d18OPO4 of DRP. Supplementary water quality measurements, hydrologic modeling, and soil temperature data were used to help understand intra-event d18OPO4 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 d18OPO4 and highlights the potential for phosphate oxygen isotopes as a novel tool to improve understanding of P fate and transport in artificially drained agroecosystems. Keywords: Agriculture, Edge-of-field, Macropores, Phosphate oxygen isotopes, Tile-drainage.