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

Authors: FORD, WILLIAM - University Of Kentucky; Williams, Mark; YOUNG, MEGAN - Us Geological Survey (USGS); King, Kevin; Fischer, Eric

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2018
Publication Date: 7/1/2018
Citation: Ford, W.I., Williams, M.R., Young, M.B., King, K.W., Fischer, E.N. 2018. Assessing intra-event phosphorus dynamics in drainage water using phosphate stable oxygen isotopes. Transactions of the ASABE. 61(4):1379-1392. https://doi.org/10.13031/trans.12804.
DOI: https://doi.org/10.13031/trans.12804

Interpretive Summary: Identifying sources of phosphorus in tile-drained landscapes is critical for developing and implementing field management practices to decrease phosphorus losses. Intensive agricultural management has resulted in the accumulation of phosphorus in many fields, especially in surface soils. In this study, we trace dissolved phosphorus using isotopes to determine the source and flow pathways of phosphorus delivery in an agricultural field with high soil test phosphorus levels. Results showed that isotope tracing approach used in the study was able to identify dissolved phosphorus fate and transport in tile-drained agricultural fields and potentially for quantifying the ability of the soil to resupply the soil solution with available phosphorus that could be used for crop uptake. This research may lead to a reduced phosphorus application that will further reduce phosphorus runoff and water quality problems.

Technical Abstract: Quantifying fluxes and pathways of dissolved reactive phosphorus (DRP) in tile drained landscapes has been hampered by a lack of ambient indicators of P. One potential tool to help understand these dynamics is the oxygen isotope signature of phosphate (d18OPO4); however, it’s potential benefits and limitations are not well understood for within-event applications at the field-scale. We collected water samples during a large summer storm event from a tile-drained field located in west-central Ohio, USA and analyzed for d18OPO4. 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 pool was not in equilibrium with long-term temperature dependent water isotope values which suggest that high soil test P soils may, at least partially, retain their parent isotope signature which has significant implications for identifying hot-spots of P delivery in watershed-scale applications. Results of the storm event analysis highlight that equilibration of leached DRP, during previous events, creates a gradient that allows partitioning of DRP that originates from pre-event shallow subsurface sources, pre-event deep subsurface sources, and the water extractable P tied up in surface soils. The current study represents the first intra-event analysis of d18OPO4 and highlights the potential for phosphate isotopes as a novel tool for improved agroecosystem management.