Less Agricultural Phosphorus Applied in 2019 Led to Less Dissolved Phosphorus Transported to Lake Erie

Abstract

Extreme precipitation events affect water quantity and quality in various regions of the world. Heavy precipitation in 2019 resulted in a record high area of unplanted agricultural fields in the U.S. and especially in the Maumee River Watershed (MRW). March−July phosphorus (P) loads from the MRW drive harmful algal bloom (HAB) severity in Lake Erie; hence changes in management that influence P export can ultimately affect HAB severity. In this study, we found that the 2019 dissolved reactive P (DRP) load from March−July was 29% lower than predicted, while the particulate P (PP) load was similar to the predicted value. Furthermore, the reduced DRP load resulted in a less severe HAB than predicted based on discharge volume. The 29% reduction in DRP loss in the MRW occurred with a 62% reduction in applied P, emphasizing the strong influence of recently applied P and subsequent incidental P losses on watershed P loading. Other possible contributing factors to this reduced load include lower precipitation intensity, altered tillage practices, and effects of fallow soils, but more data is needed to assess their importance. We recommend conservation practices focusing on P application techniques and timing and improving resiliency against extreme precipitation events.

One size does not fit all: Toward regional conservation practice guidance to reduce phosphorus loss risk in the Lake Erie watershed

“…The supply and availability of P in soils is a universal driver of P losses in runoff. Greater runoff losses of P have repeatedly been observed from fields with greater soil test P (STP) (Daniel et al, 1993;Duncan et al, 2017;Kleinman et al, 2011;Plach, Macrae, Ali, et al, 2018;Sharpley & Smith, 1989;Vadas et al, 2005), and legacy P build up has been identified as a significant P source (Guo et al, 2020;Osterholz et al, 2020). A drawdown in STP can considerably reduce P loads from fields and will likely be effective across all regions in the Lake Erie watershed, irrespective of their geography.…”

Nutrient Stewardship: Taking 4R Further

“…Opportunity remains, however, to better integrate subsurface placement into conservation tillage programs. Additional evidence of a connection of fertilizer to P load emerged in 2019 when an incredibly wet spring limited not only fertilizer application but even crop planting—followed by a sharp reduction in dissolved P in the Maumee river (Guo et al., 2020). Would we have seen this effect if in the years prior to and after 2019, more of the P fertilizer had been applied subsurface rather than broadcast?…”

Tile Drainage Increases Total Runoff and Phosphorus Export During Wet Years in the Western Lake Erie Basin

“…The most severe HABs occurring in the WLEB since 2002, indicated by the Lake Erie Harmful Algal Bloom severity index, have occurred in wetter than average years. Previous studies have identified very strong correlations between March-July stream discharge and March-July SRP and particulate phosphorus loads (R 2 = 0.83 and 0.90, respectively) from the major tributaries of the WLEB from 2002 to 2019 (Guo et al, 2020), implying phosphorus export is transport limited as indicated by chemostatic behavior with stream discharge (i.e., lower variability in concentration compared to stream discharge) (Williams et al, 2016a). From 1975 to 2017 annual precipitation in the Maumee River Basin, the largest tributary of the WLEB, increased by 102 mm, with increases in heavy (25.4-76.2 mm) and very heavy (>76.2 mm) precipitation events accounting for a majority of the increase, resulting in increased stream discharge and runoff ratio (defined as the ratio of area-normalized stream discharge to precipitation) in a majority of monitored streams (Williams and King, 2020).…”