Phosphorus dynamics in restored riparian wetlands within an agricultural basin

Adrian Wiegman

University of Vermont

Co-Authors: W. B. Bowden, K. L. Underwood, and E. D. Roy

The contribution of riparian wetland restoration to meeting phosphorus (P) load reductions in agricultural watersheds is uncertain, as many potential restoration sites overlay former agricultural soils that can contain legacy P. Hydrologic changes associated with restoration can potentially mobilize legacy soil P as soluble reactive P (SRP), decreasing net P retention efficiency. However, understanding of the general magnitude of SRP release versus retention of sediment-bound P during flooding remains very limited, as do the spatial and temporal dimensions of SRP release. We are using a multi-scale approach to develop indices of potential SRP loss risk. This is part of a broader research project focused on P dynamics in restored riparian wetlands in the Vermont portion of the Lake Champlain Basin. To date, we have conducted laboratory experiments using intact cores from 14 sites, from which we observed SRP release rates that span two orders of magnitude, with corresponding equilibrium SRP concentrations ranging from 0.01 to 1 mg P L-1. We are still evaluating possible predictors for intact core SRP flux, but preliminary results indicate that soil characteristics and landscape metrics are both important. Ongoing work includes additional intact core experiments and complementary field monitoring of P dynamics, as well as model development and simulations to determine the time horizon of legacy P impacts. Our ultimate goal is to inform site selection and design for riparian wetland restoration projects in Vermont so that P retention benefits are increased.

Please post comments and questions for the author below.

5 thoughts on “Phosphorus dynamics in restored riparian wetlands within an agricultural basin

    1. Sure, Mike Ament has a presentation on the P-retention effects of drinking water treatment residuals in green storm water infrastructure. In practice this might be difficult to achieve if fields have already been in fallow for a long time. But there should be a way to use conventional harvesting equipment to spread sorption media. The questions i have about that are: (1) what unintended impacts might this have on wildlife and ecosystem development, (2) is a centralized approached such as p removal at damns and in rivers More efficient. What has been applied successfully in other impaired ecosystems (everglades and great lakes) is Topsoil removal. It would be interesting to compare the pros and cons of topsoil removal vs sorption media.

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  1. Hi there! This is a really interesting facet of wetland restoration that I hadn’t considered, so thank you so much for talking about it! I work on studying edge-of-field best management practices and how they relate to capturing legacy phosphorus. This seems particularly interesting because the Federal CRP particularly encourages converting unproductive agricultural land into wetlands– based on these findings, would you advise against this incentivization program? Or, as Colin asked, are there management practices that could be used to prevent this? Also, I was wondering whether there’s a reason that you used SRP rather than DRP for measuring P levels? Thank you!

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  2. Hi Samantha! For the purposes of this poster dissolved reactive P and soluble reactive P are effectively the same measurement. Ideally the edge of field strategy for P load reduction should be selected based on the change net P balance resulting from restoration activities at a given site. We have only estimated SRP release risk. A full balance would include some estimation of particulate P trapping (see figure 2). Return to natural conditions increases particle trapping efficiency because vegetation volume increases. So all restoration should increase particle trapping. The big question is how does that increase in particle trapping compare the change in dissolved P flux. Since we don’t quantify both i can’t directly answer your question. SRP flux may increase for some period after restoration but then decrease over time (figure 5), but the nature of that trend and how it compares to particle flux is context dependent. I can say with some confidence that sites receiving higher loads of sediments and soils that have better drainage are likely good candidates for return to natural. Its the sites receiving low sediment input and having pore drainage that we need to be more careful with what mitigation strategies are used.

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