The Water Quality Consequences of Restoring Wetland Hydrology to a Large Agricultural Watershed in the Southeastern Coastal Plain

To ameliorate local and coastal eutrophication, management agencies are increasingly turning to wetland restoration. A large portion of restoration is occurring in areas that were drained for agriculture. To recover wetland function these areas must be reflooded and disturbances to soils, including high nutrient content due to past fertilizer use, loss of organic matter and soil compaction, must be reversed. Here, we quantified nitrogen (N) and phosphorus (P) retention and transformation in a unique large-scale (440 ha) restored wetland in the North Carolina coastal plain, the Timberlake Restoration Project (TLRP). For 2 years following restoration, we quantified water and nutrient budgets for this former agricultural field. We anticipated that TLRP would export high concentrations of inorganic P immediately following reflooding, while retaining or transforming inorganic N. In the first 2 years after a return to the precipitation and wind-driven hydrology, TLRP retained or transformed 97% of NO₃–N, 32% of TDN, 25% of NH₄–N, and 53% of soluble reactive phosphorus (SRP) delivered from inflows and precipitation, while exporting 20% more dissolved organic nitrogen (DON), and 13% more total P (inorganic, organic, and particulate P) than inputs. Areal mass retention rates of N and P at TLRP were low compared to other restored wetlands; however, the site efficiently retained pulses of fertilizer NO₃–N derived from an upstream farm. This capacity for retaining N pulses indicates that the potential nutrient removal capacity of TLRP is much higher than measured annual rates. Our results illustrate the importance of considering both organic and inorganic forms of N and P when assessing the benefits of wetland restoration. We suggest that for wetland restoration to be an efficient tool in the amelioration of coastal eutrophication a better understanding of the coupled movement of the various forms of N and P is necessary.