Quantifying multiple ecosystem services and their underlying ecosystem functions in North Carolina's largest wetlands mitigation bank

Background/Question/Methods

More than 50% of the wetland area of the United States has been destroyed, primarily through conversion to agriculture. Removing agricultural fields from production and restoring wetland hydrology and vegetation to them is thus an appealing idea. By reversing the long-term trend of draining wetlands, some portion of the services once provided by wetlands (habitat, carbon sequestration, nutrient retention, and flood mitigation) may be recovered. Yet newly re-flooded former agricultural fields are not equivalent to mature wetland ecosystems, and ecosystem services provided by wetlands do not recover along the same trajectories or at the same rates. Legally, wetland restoration success is evaluated according to two criteria: the survival of planted vegetation and water tables rising to within 30cm of the soil surface for one month each year. Water quality improvement—a primary objective motivating wetland restoration—and underlying ecosystem functions (biomass production, soil carbon accretion, denitrification) are rarely assessed. Our study quantified multiple ecosystem functional changes within a large (440 ha) wetland mitigation bank in coastal North Carolina after wetland hydrology was restored in 2007. We aimed to understand how restoration affected dissolved and gaseous exports of C, N and P from the ecosystem.  

Results/Conclusions

Although restoration did not reduce total nitrogen exports, nitrate exports were substantially lower, with over 90% of NO₃-N inputs retained. Dissolved organic N and NH₄-N dominated N exports following inundation. Export of formerly Fe-bound P that had accumulated during decades of fertilizer application was substantial (0.6 kg TP ha^-1 y^-1) in the first year, but later declined as this pool of P was depleted. While fertilizer legacies contributed to nutrient exports following restoration, they were not associated with higher greenhouse gas (GHG) emissions. Soil GHG fluxes from the restored wetland were lower than in nearby reference wetlands or an adjacent active agricultural field (278, 430, and 426 mg CO₂-equivalents m^-2 h^-1 respectively). Hydrologic reconnection of the formerly drained wetland allowed brackish waters from Albemarle Sound to enter the site during severe droughts. Saltwater intrusion substantially enhanced NH₄-N exports, and, in laboratory experiments, led to dramatic increases in methane production. Our results illustrate that land use legacies can constrain the ability of a newly restored wetland to provide expected ecosystem services; that ecosystem functions linked to water quality improvement are affected differentially by restoration; and that future altered hydrologic regimes and sea level rise will complicate predictions of recovery rates of individual ecosystem functions and multiple ecosystem services.