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

Abstract

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 ( 18 O 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  18 O PO4 signatures in tile drainage water and assess the efficacy of  18 O 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  18 O PO4 of DRP. Supplementary water quality measurements, hydrologic modeling, and soil temperature data were used to help understand intra-event  18 O 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  18 O 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.

Evaluating Hydrologic Response in Tile‐Drained Landscapes: Implications for Phosphorus Transport

“…Lastly, the lack of differences in P dynamics with response type may be the result of rapid connectivity between the surface and tiles that may in fact be found under the various response types. Indeed, Ford et al (2018) observed greater macropore flow contributions from clay soils in winter, when soils were wet. Kung et al (2000) also observed increased preferential flow into tile drains under wetter antecedent conditions and attributed this to the activation of mesopores.…”

“…The lack of relationship between event response type and ratios of P concentrations between surface and tile may reflect the sampling frequency used (i.e., 2‐ to 8‐h‐interval samples for the Ontario sites; daily composites from 6‐h‐interval samples for the Ohio sites). Previous studies linking preferential flow to P loss in tiles used finer sampling resolutions (i.e., 5 min to 1 h; Williams et al, 2016, 2018; Ford et al, 2018) to capture rapid connectivity; however, such an intensive sampling frequency on the order of minutes throughout a 3‐yr study was not feasible. A second potential explanation for the lack of relationship between event response type and P mobilization in tile drainage may be related to the importance of P supply.…”

Impacts of preferential flow and agroecosystem management on subsurface particulate phosphorus loadings in tile‐drained landscapes

“…Although numerous studies have quantified dissolved reactive P (DRP) loadings and have examined the mechanisms of DRP transport to subsurface drainage systems, less emphasis has been placed on other forms transported through tile drains, such as particulate P (PP) and dissolved unreactive P (Chen et al., 2018; Christianson, Harmel, Smith, Williams, & King, 2016; King et al., 2015; Radcliffe et al., 2015). In particular, as PP is lost from the landscape and delivered to downstream waterbodies, bioavailable P may be mobilized, promoting eutrophication and degradation of freshwater and marine sources or fueling in‐stream primary productivity (Brennan et al., 2017; Ford, Williams, Young, King, & Fischer, 2018; Zhu, Li, & Whelan, 2018).…”

Long‐term assessment of nutrient flow pathway dynamics and in‐stream fate in a temperate karst agroecosystem watershed

“…Nitrate concentration and subsequent leaching from the soil and epikarst zone have been found to control spring concentrations and loadings, whereas quickflow following storm events has been found to dilute concentrations relative to groundwater aquifer sources (Baran, Lepiller, & Mouvet, ; Husic, Fox, Ford, et al, and references within). Conversely, dissolved reactive phosphorus (DRP) has been found to be more concentrated in quickflow than in intermediate and slow flow pathways, stemming from agricultural management practices in which high soil P levels at the soil surface become connected to the stream network through macropores and overland flow during quickflow (Baker, Johnson, & Confesor, ; Ford, Williams, Young, King, & Fischer, ; Jarvie et al, ; King, Williams, & Fausey, ). Despite this, karst conduits have recently been recognized to be highly retentive of DRP, even during quickflow, because of transient sediment deposits within preferential pathways that adsorb or settle a portion of reactive P (Jarvie et al, ; Mellander et al, ).…”