Potential for soil legacy phosphorus release from restored riparian wetlands within an agricultural landscape

Authors: Wiegman, Adrian; MYERS, HARRISON - University Of Vermont; AUGUSTIN, ISABELLE - University Of Vermont; KUBOW, MARCOS - University Of Vermont; FEIN-COLE, MAYA - University Of Vermont; PERILLO, VANESA - Universidad Nacional Del Sur (UNS); ROSS, DONALD - University Of Vermont; DIEHL, REBECCA - University Of Vermont; UNDERWOOD, KRISTEN - University Of Vermont; BOWDEN, WILLIAM - University Of Vermont; ROY, ERIC - University Of Vermont

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary: Stakeholders across the U.S. and beyond are searching for ways to decrease phosphorus (P) loading to aquatic ecosystems, while creating additional ecological co-benefits. Wetlands have been degraded in agricultural regions across the world, and wetland restoration on formerly farmed land is currently being explored as a strategy to reduce P loads to freshwater ecosystems. However, the effectiveness of such projects for retaining P in watersheds remains unclear, necessitating research on dominant P cycling processes including both P deposition with sediment introduced by rivers and internal cycling of soluble reactive P during inundation. This study investigates the factors that influence soluble reactive phosphorus release from riparian soils to floodwaters in Vermont’s Lake Champlain Basin, and examines the relationships between land use and soil properties that are important for understanding the distribution and potential mobility of legacy P in riparian zones. Results will increase our scientific understanding of P fate and transport in wetland regions, and will inform stakeholders involved in wetland restoration and watershed P management.

Technical Abstract: Wetlands are valuable ecosystems because they are highly productive, support a wide range of wildlife, and serve as hotspots for biogeochemical cycling. Historically, vast areas of wetlands in the United States (US) were drained and converted to agriculture. Efforts are currently underway to restore wetland and floodplain functioning across the US and elsewhere. Re-wetting historically drained and farmed soils can potentially liberate legacy P to surface waters as soluble reactive P (SRP), offsetting P retained by sedimentation during floods. A better understanding of the controls on SRP release is needed to estimate net P retention in these settings. Soil P Saturation Ratio (PSR) and Soil P Storage Capacity (SPSC) are two proxies for SRP runoff risk that have shown promise for characterizing restored wetlands but require further testing. In this study, we examined riparian soils at 42 riparian sites ranging from active farms to mature wetlands in the Vermont portion of the Lake Champlain Basin (USA), where phosphorus load reduction is a critical goal to achieve in-lake water quality targets. We additionally quantified potential SRP release to overlying water using intact soil cores from 20 plots spanning 14 sites. Final SRP concentrations in intact cores spanned two orders of magnitude and were predicted well by SPSC and PSR. SRP release was greatest at more recently and frequently farmed sites. Several soil properties, including PSR and SPSC, were correlated to farming frequency and time since farming, indicating that SRP release could be mapped using existing geodata for soils, hydrology and land use. Our findings confirm that soil SRP release during flooding needs to be considered in estimates of net P balance for restored riparian wetlands in agricultural landscapes.