The Science and Policy of the Verified Carbon Standard Methodology for Tidal Wetland and Seagrass Restoration

The restoration of tidal wetland and seagrass systems has the potential for significant greenhouse gas benefits, but project-level accounting procedures have not been available at an international scale. In this paper, we describe the Verified Carbon Standard Methodology for Tidal Wetland and Seagrass Restoration, which provides greenhouse gas accounting procedures for marsh, mangrove, tidal forested wetland, and seagrasses systems across a diversity of geomorphic conditions and restoration techniques. We discuss and critique the essential science and policy elements of the methodology and underlying knowledge gaps. We developed a method for estimating mineral-protected (recalcitrant) allochthonous carbon in tidal wetland systems using field-collected soils data and literature-derived default values of the recalcitrant carbon that accompanies mineral deposition. We provided default values for methane emissions from polyhaline soils but did not provide default values for freshwater, oligohaline, and mesohaline soils due to high variability of emissions in these systems. Additional topics covered are soil carbon sequestration default values, soil carbon fate following erosion, avoided losses in organic and mineral soils, nitrous oxide emissions, soil profile sampling methods, sample size, prescribed fire, additionality, and leakage. Knowledge gaps that limit the application of the methodology include the estimation of CH₄ emissions from fresh and brackish tidal wetlands, lack of validation of our approach for the estimation of recalcitrant allochthonous carbon, understanding of carbon oxidation rates following drainage of mineral tidal wetland soils, estimation of the effects of prescribed fire on soil carbon stocks, and the analysis of additionality for projects outside of the USA.