Biogeochemical tradeoffs in wetland restoration: Storage and export of C, N and P in a hydrologically reconnected agricultural landscape

Background/Question/Methods

Beginning in the 1970’s, 750,000 acres of NC’s coastal plain wetlands were converted to agricultural fields.  This conversion required drainage infrastructure to maintain arable fields, and led to considerable soil organic matter loss.  Now, low profit margins, rising sea levels, saltwater intrusion and emerging mitigation markets are all forces encouraging the abandonment of agriculture and the intentional or inadvertent cessation of active drainage in these marginal farmlands.  While an increase in the spatial extent of wetland habitat is often considered a universal benefit to the environment, flooding formerly cultivated and fertilized fields may have some negative consequences.  The release of soil bound phosphorus and increased rates of production of N₂O and CH₄ from flooded soils may offset the anticipated benefits of reduced exports of inorganic nitrogen and carbon sequestration.  We have been estimating dissolved and trace gas fluxes and soil and biomass pools of C, N and P in a 400 ha restoration project in NC’s coastal plain (Timberlake) since this former agricultural field was reflooded in February 2007. Results/Conclusions

In the first full year following flooding, our solute mass balance study found that the Timberlake project was an annual net sink for dissolved inorganic nitrogen (retaining 8.3 kg ha^-1) but a net source of soluble reactive phosphorus (releasing 0.23 kg ha^-1).  No measurable changes in soil carbon or nitrogen were detected in the first year, but some sites lost measurable quantities of phosphorus.  Biomass accumulation of C, N and P was a major sink as the 750,000 trees planted as live stakes on the site in 2004 nearly doubled in basal area (from 7 m² ha^-1 to 13 m² ha^-1) over the 2007 growing season.  Nitrous oxide and methane emissions from the restored wetland in the first year were nearly always higher than from a nearby actively cultivated agricultural field (“pre-restoration condition) but were nearly always lower than emissions measured in two forested wetlands within nearby preservation areas (“post restoration goal”).  Results from this large-scale restoration project illustrate that there is unlikely to be a single restoration approach for maximizing all wetland ecosystem services within any individual project.