QUANTIFYING THE SOURCE AND PATHWAY OF DISSOLVED REACTIVE PHOSPHATE IN KARST DRAINAGE OF THE INNER-BLUEGRASS

In the Midwestern U.S. seasonal hypoxia experienced in the Gulf of Mexico and harmful algal blooms in inland freshwater ponds, lakes, and rivers are partly fueled by dissolved orthophosphate loadings from disturbed landscapes. Efforts to reduce dissolved reactive phosphate (DRP) loadings have had varying levels of success and have led to insufficient water quality improvements. Inefficiencies in conservation strategies can stem from poor understanding of phosphate source and flow pathway dynamics. This study focused on monitoring sources and flow pathways of dissolved reactive P in a karst agroecosystem with phosphatic limestone. We collected event water samples at the Camden Creek watershed outlet in Woodford County, Kentucky and characterized P sources by sampling spring water and soils across the watershed. Oxygen isotope results for orthophosphate at springs suggested significant differences in isotope signatures at high and low flows, despite similar concentrations, likely reflecting differences in connectivity to anthropogenic and ambient P sources. Multiple linear regression models to predict DRP concentrations revealed that a mass-balance unmixing approach may help distinguish between DRP pathways in a heterogeneous karst system better than commonly used hydrograph recession methods. Soils from our study site had high extractable P concentrations at both the surface and deeper soil zones, with high heterogeneity reflecting soil composition and spatial variability in management. Overall, this work provides insight into phosphate source and transport in a karst agroecosystem and provides broader implications for implementing best management strategies to reduce DRP loading in such systems

Impact of water source dynamics on dissolved reactive phosphorus loadings in heterogenous karst agroecosystems with phosphatic limestones

Karst landscapes underlain with phosphatic limestones are now recognized to be an important contributor of fluvial phosphorus (P) to coastal waters. Specifically, karst agroecosystems may be a hotspot for dissolved reactive P (DRP) due to chronic over-application of organic and inorganic fertilizers that create legacy P accumulation in surface soils. Nevertheless, few studies have assessed the hydrologic controls on DRP transport in these systems at the watershed scale, which is the focus of this study. We analysed soil moisture, soil water extractable P, and storm event hydrologic and water quality data from a small heterogenous karst watershed (10.7 km2) in the Inner-Bluegrass Region of Central Kentucky, USA. Four storm events were sampled in winter, 2020 and were analysed for flow pathways using hydrograph recession analysis and water source connectivity using a tracer-based unmixing model. Based on hydrograph separation results, multiple linear regression analysis was performed to assess drivers of DRP concentrations and loadings. Soil water extractable P results showed stark vertical gradients with greater concentrations at both the surface and deeper soil zones, and minimum concentrations in the root zone. Results for the storm event analysis showed that water source connectivity provided superior prediction of DRP concentrations over the flow pathway analysis, which reflected the heterogeneity of karst maturity masking intermediate flow pathways. Findings from the MLR and loading analysis suggest waters sourced from the soil/epikarst produced significantly higher loadings compared with phreatic and precipitation water source in the three largest events, although concentrations fell between the phreatic (low) and precipitation (high) sources. Findings highlight variable activation of matrix-macropore exchange at different depths throughout the event. Collectively these results suggest existing models and approaches to assess karst hydrology need revision to improve management strategies in this critical landscape