Modeling phosphorus retention and release in riparian wetlands restored on historically farmed land

Authors: Wiegman, Adrian; UNDERWOOD, KRISTEN - University Of Vermont; BOWDEN, WILLIAM - University Of Vermont; CHIN, TIFFANY - University Of Vermont; ROY, ERIC - University Of Vermont

Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Tools that quantify the environmental benefits of conservation practices are needed to adequately incentivize farmers to participate in habitat restoration programs. In this study, developed and tested a novel process model (wetlandP) to estimate net phosphorus (P) retention rates for wetlands restored upon former farm fields. We were able to calibrate and verify the wetlandP model using field data and used it to examine a range of scenarios for each field site. Our simulations indicated variable net P retention, driven by a trade-off between particulate P trapping and soluble P release, with the majority of scenarios indicating that the study wetlands serve as net P sinks on the landscape. We leveraged these results to develop a conceptual framework that can help guide prioritization of riparian wetland restoration by ecological engineers and designers when water quality improvement via P capture and storage is a priority.

Technical Abstract: State and non-profit organizations are interested in nature-based solutions to help meet reductions in phosphorus (P) loads to aquatic ecosystems required by the U.S. EPA’s total maximum daily load (TMDL) program. Similar efforts are underway in other nations as well. One potential solution of this kind is the restoration of riparian wetland ecosystems. Many candidate sites for such restoration were formerly used for agriculture. While the capacity for wetlands to serve as nutrient sinks has been established generally, very few studies have examined net P retention for the specific case of restored riparian wetlands on historically drained and farmed land, with no such studies in the Northeast U.S. Here, we address this important knowledge gap using a combination of field studies for three restored riparian wetlands in the Vermont portion of the Lake Champlain Basin, laboratory measurements, and a novel process-based model (wetlandP). Accounting for both particulate and dissolved P dynamics, simulated net P retention was 0.09±0.1 g P m-2 yr-1 across scenarios that represent a range of plausible conditions for our study wetlands. For these simulations, the modeled wetland retention efficiencies for total P (TP) and soluble reactive P (SRP) were on average (±1 standard deviation) equal to +35±29% (net gain) and -43±69% (net loss), respectively. Very few simulations resulted in net total P export (8%), suggesting that the wetlands investigated in this study are generally sinks of total P on the landscape. Our results illustrate the importance of soils, hydrology, and influent water quality, and highlight the potential TP load reduction benefits associated with the restoration of riparian wetlands on formerly farmed land. However, our results also show that release of legacy soil P in soluble forms can be sizable in some cases, especially for riparian wetlands receiving river water with relatively low P concentration. Future research should include monitoring and modeling of additional sites and larger flood pulses than those observed here to inform estimates of P retention in restored riparian wetlands more broadly across space and time.