Increased Soluble Phosphorus Loads to Lake Erie: Unintended Consequences of Conservation Practices?

Abstract

Cumulative daily load time series show that the early 2000s marked a step-change increase in riverine soluble reactive phosphorus (SRP) loads entering the Western Lake Erie Basin from three major tributaries: the Maumee, Sandusky, and Raisin Rivers. These elevated SRP loads have been sustained over the last 12 yr. Empirical regression models were used to estimate the contributions from (i) increased runoff from changing weather and precipitation patterns and (ii) increased SRP delivery (the combined effects of increased source availability and/or increased transport efficiency of labile phosphorus [P] fractions). Approximately 65% of the SRP load increase after 2002 was attributable to increased SRP delivery, with higher runoff volumes accounting for the remaining 35%. Increased SRP delivery occurred concomitantly with declining watershed P budgets. However, within these watersheds, there have been long-term, largescale changes in land management: reduced tillage to minimize erosion and particulate P loss, and increased tile drainage to improve field operations and profitability. These practices can inadvertently increase labile P fractions at the soil surface and transmission of soluble P via subsurface drainage. Our findings suggest that changes in agricultural practices, including some conservation practices designed to reduce erosion and particulate P transport, may have had unintended, cumulative, and converging impacts contributing to the increased SRP loads, reaching a critical threshold around 2002.

Lake Erie, phosphorus, and microcystin: Is it really the farmer's fault?

“…These surveys correspond well to studies examining the ranges in soil test levels across the WLEB. In a study of soil test P (as Mehlich 3) in the Sandusky River Basin, as well as from a database of Ohio soil tests for the WLEB, most soils (70%) were within the "build-up" or "maintenance" ranges for corn and soybean (Baker et al 2017). This implies that only 30% of fields were bad actors, and further analysis suggests that only targeting reductions in this 30% would be insufficient to reach reductions required to reduce algal blooms in Lake Erie.…”

“…This implies that only 30% of fields were bad actors, and further analysis suggests that only targeting reductions in this 30% would be insufficient to reach reductions required to reduce algal blooms in Lake Erie. As Baker et al (2017) concluded, "Achieving a 40% reduction in dissolved reactive P loading will require adoption of BMPs in fields across the full range of agronomic soil-test levels. "…”

“…Indeed, Williams et al (2016) found that surface broadcast fertilizer on a no-till field significantly increased P concentrations in tile discharge compared to fertilizer that was incorporated into the soil. In addition, Baker et al (2017) found substantial stratification of P in soils, even though many were in rotational no-till and Table 3 Fertilizer formulation, phosphorus (P) content, and solubility. received chisel or disk plowing prior to corn (72%).…”

“…Based on these analyses, 24% of the fields had more than 20 kg P ha -1 applied when the Tri-State Fertility Guide recommended no additional P fertilizer. Over-application of P relative to current fertility guidance could be a source of P losses to Lake Erie, although there are many other potential contributors than over-fertilizing roughly one-quarter of fields (Baker et al 2017;Jarvie et al 2017;King et al 2015;Smith et al 2007Smith et al , 2015, so it is unlikely to be the only cause. Since the majority of fields have P applied near or below recommended levels, assigning blame for Lake Erie re-eutrophication to all farmers is unjustified, especially since those managing three-quarters of the fields are adhering to the current state-of-the-science fertility guidance.…”

Watershed‐scale phosphorus balances to establish reasonable water quality expectations

“…When conservation practices that are designed to trap sediment are implemented to reduce agricultural phosphorus water quality impacts, nutrients are retained on the landscape. If improperly managed, this accumulated phosphorus can eventually leak out of the system through erosion or desorption from the soil, which results in long-term losses of phosphorus to the stream that are referred to as legacy phosphorus (Jarvie et al, 2017).…”