Predicting Post-Restoration Risk of Soil Legacy Phosphorus Release in Historically Drained and Farmed Riparian Wetlands

The contribution of riparian wetland restoration to phosphorus (P) load reductions in agricultural watersheds is uncertain, as many potential restoration sites overlay former agricultural soils that have accumulated legacy P. Hydrologic changes associated with restoration can promote P retention through particle capture and accumulation of organic soil but may also promote P export through mobilization of legacy soil P as soluble reactive P (SRP). In theory, SRP release from soils should decline over time with ecosystem development. However, understanding of the general magnitude of SRP release versus retention of sediment-bound P during flooding remains limited across both space and time. In this study, we sampled soils and examined soil P dynamics at sites in Vermont (USA) along a chrono-sequence of post farming succession, ranging from active farms to mature wetlands. To estimate soil legacy P release risk, we monitored SRP flux during simulated floods inside intact soil cores that were incubated in the lab for two weeks under aerobic and anaerobic conditions. To predict risk, we fit models of SRP flux to soil properties, as well as widely accessible geospatial data (such as soil drainage class and estimates of soil properties from SoilGrids.org). Preliminary results indicate that soil characteristics (such as aluminum content, organic matter, and soil test P) and landscape metrics (such as time since farming) are important predictors of SRP flux. Ongoing work includes additional intact core experiments on recent fallow fields and active farms in summer 2020. Ultimately, this project will generate a framework that can inform approaches to identify and mitigate areas with high risk of soil legacy phosphorus release, and improve P retention in riparian wetland restoration projects as a result.