Modeling Phosphorus Retention and Release in Riparian Wetlands Restored on Historically Farmed Land

Abstract

Nature-based solutions are of interest in efforts to achieve reductions in phosphorus (P) loads to aquatic ecosystems. One potential solution of this kind is the restoration of riparian wetlands. Many candidate sites for riparian wetland restoration were formerly used for agriculture and therefore may contain legacy soil P from past fertilizer and/or manure applications. Here, we combined 2-year field studies of 3 restored riparian wetlands on formerly farmed land in the Lake Champlain Basin, Vermont, United States, with implementation of a novel wetlandP model to estimate net P retention. In the field, we measured variable inorganic P deposition ranging up to approximately 1 g P m -2 yr -1 and collected data on P stocks and fluxes required for modeling. At 2 sites, observed water quality dynamics during flood events were indicative of internal dissolved inorganic P (DIP) release from soils during low oxygen conditions. We calibrated and verified the wetlandP model using field data and used it to examine numerous scenarios. Our simulations indicated variable net total P (TP) retention, driven by a trade-off between particulate P trapping and DIP release, with most plausible scenarios (95 out of 108) indicating that the study wetlands serve as net TP sinks. Our net TP retention estimates (range = -0.06 to 0.45 g P m -2 yr -1 , mean P retention efficiency = 35%) are comparable to prior literature and help clarify key drivers. Our modeling results also show that release of legacy soil P as DIP can be sizable in some cases (range in net DIP retention = -0.11 to 0.02 g P m -2 yr -1 ), especially for wetlands receiving river/stream water with low DIP concentration. We present a conceptual framework to help guide prioritization of riparian wetland restoration by ecological engineers and designers when water quality improvement via P retention is a goal.