Tidal Wetland Carbon Calculator

Soil carbon in a tidal wetland is reconstructed from three quantities that a sediment core yields directly: dry bulk density in grams per cubic centimeter, organic matter as a percentage of dry weight, and the fraction of that organic matter which is carbon. The last is taken as 0.58, the Van Bemmelen conversion, and the product of the three gives carbon per unit volume, which is then multiplied by depth and area. Bulk density and organic content move in opposite directions, which is why the model derives density from organic content rather than asking for it twice: peaty mangrove and marsh soils at 30 percent organic matter fall near 0.4 g per cubic centimeter, while mineral-dominated tidal flats approach 1.1. Above-ground biomass is added at ecosystem-specific rates, largest for mangroves whose woody stems and prop roots hold substantial standing carbon and negligible for tidal flats. Salinity enters as a methane control rather than a growth control. Sulfate in seawater lets sulfate-reducing bacteria outcompete methanogens, so above roughly 18 parts per thousand methane emission is largely suppressed and the site is close to a clean carbon sink. Below about 10 ppt, methanogenesis proceeds and the emitted methane, with a global warming potential near 28 over a century, can cancel a large share of the carbon buried. That is why a freshwater tidal wetland with impressive soil carbon may deliver a poor net climate benefit. Three caveats matter for real projects: the 0.58 factor is a generic average and varies with organic matter source, a 1 m core often truncates deposits that continue deeper, and stock is not the same as additionality, since only accumulation attributable to the intervention can be credited.

The 0.58 organic-matter-to-carbon conversion at the core of this calculation traces to Jacob Maarten van Bemmelen, a Dutch soil chemist working in the late nineteenth century, and it entered wetland science through general soil survey practice long before anyone was counting carbon for climate purposes. Coastal wetlands were studied for most of the twentieth century as fisheries habitat, storm buffers and nutrient filters. Salt marsh ecology in particular was shaped by work at institutions such as the University of Georgia Sapelo Island research site, where energy and nutrient flow rather than carbon burial was the organizing question. The carbon reframing is recent. A 2009 United Nations Environment Programme report with FAO and IOC-UNESCO introduced blue carbon as a policy category. Daniel Donato and coauthors reported in Nature Geoscience in 2011 that Indo-Pacific mangroves are among the most carbon-rich forests measured, averaging on the order of 1000 tonnes of carbon per hectare with most of it below ground, and a companion 2011 paper by Elizabeth Mcleod and colleagues titled A Blueprint for Blue Carbon set out comparative sequestration rates for mangroves, marshes and seagrass. Accounting infrastructure followed quickly: the IPCC published its 2013 Wetlands Supplement extending national inventory guidance to coastal wetland drainage and rewetting, and Verra approved methodology VM0033 for tidal wetland and seagrass restoration in 2015, which is the instrument most blue carbon credit projects now use.