ASSESSING THE IMPACTS OF LEGACY PHOSPHORUS IN TILE DRAINED AGRICULTURAL LANDSCAPES

Phosphorus (P) is vital for agriculture but can be transported to surface waters via runoff and tile drainage promoting the growth and development of harmful and toxic algal blooms. Excess anthropogenic P inputs have led to the accumulation of “legacy P” in agricultural landscapes. Legacy P can persist for decades, with its loss to surface waters influenced by the complex interactions among hydrologic, edaphic, and field management factors, along with its spatial and temporal variability. While conservation efforts have reduced edge-of-field P losses, downstream water quality remains largely unchanged. Research suggests that the chronic release of P from legacy stores in agricultural soils and water bodies sustains high P levels in surface waters. Addressing these sources is a major challenge for meeting long-term water quality goals, as their impacts can persist for decades or longer. This dissertation consists of four research studies assessing the impact of legacy P and the effect of hydrological processes on P loss in tile-drained agricultural watersheds, with a focus on the role of management practices and extreme weather events. The first study (Chapter 2) examines spatial patterns of soil P within closed depressions and hillslopes. Findings show that depressions accumulate significantly higher levels of P, making them hotspots for P loss. Soil P accumulation was influenced by long-term management practices such as fertilizer application history and tillage. Flow-through experiments revealed that the risk of P loss was closely related to Mehlich-3 P concentration for both hillslope and depression soils. The study suggests that incorporating depressions into soil sampling and utilizing variable rate P application can help reduce P loading to surface waters. The second study (Chapter 3) investigated the effect of hydrologic variables on dissolved P (DRP) leaching during laboratory rainfall simulations. Results showed that leachate flow rates, preferential flow, soil-water contact time, and DRP concentration varied substantially among soil columns and rainfall simulations, with both connectivity and soil adsorption/desorption kinetics controlling DRP transport. Laboratory simulations revealed a parabolic relationship between water source, soil-water contact time, and DRP concentration. The results highlight the importance of understanding the interaction between soil P kinetics and hydrology to improve predictions of DRP leaching and to inform management practices. Differentiating between newly applied and legacy P sources is essential for identifying and implementing conservation strategies. The third study (Chapter 4) aimed to quantify old and new P losses in surface runoff and subsurface leachate during storm events following fertilizer application using the oxygen-18 signature of phosphate (δ18OPO4) as a tracer. Findings showed that DRP concentration in runoff and leachate substantially increased during the rainfall simulation immediately after fertilizer application, with runoff and leachate δ18OPO4 similar to fertilizer δ18OPO4 signatures. Beyond the first rainfall event after fertilizer application, DRP concentration decreased and leachate δ18OPO4 values differed from the fertilizer values. While the δ18OPO4 method successfully identified the contribution of fertilizer P to DRP losses immediately after application, post-application samples showed complex interactions between abiotic and biotic processes, complicating source differentiation. Results highlight both potential opportunities and challenges of using δ18OPO4 to trace sources of P through the landscape. Finally, the last study (Chapter 5) explored the impact of extreme weather events, particularly drought followed by heavy precipitation on P export dynamics from a drained headwater watershed. Analysis of a 12-year dataset revealed that seasonal precipitation extremes coupled with nutrient management practices resulted in significantly higher P losses in 2021 compared to previous years. A period of excess wetness followed prolonged drought reactivating hydrological pathways and mobilizing accumulated soil P and newly applied fertilizer, with 43% and 29% of cumulative 12-yr DRP and TP loading, respectively, exported within a 9-mo period. A single extreme precipitation event following manure application accounted for a substantial portion of the annual DRP load, emphasizing the critical role of extreme events in exacerbating P loss. The findings underscore the importance of adaptive nutrient management strategies, including optimized fertilizer application timing and placement, and legacy P mitigation, to reduce both short- and long-term eutrophication risks, especially as precipitation patterns become more variable with climate change.