A pedogeomorphic strategy for tidal marsh carbon accounting

Tidal marsh soils have some of the largest carbon stocks of any ecosystem, yet uncertainty remains about the best approach to quantify these C stocks. We tested whether separating tidal marshes into pedogeomorphic units (PGUs) could improve regional‐scale blue C accounting in two separate regions on the Atlantic coast: southern New England (NE) and northern Southeast (SE). We identified four dominant PGUs in each region and measured soil C stocks in 105 pedons to 2 m. Average carbon stocks among PGUs ranged from 84 to 430 and 140 to 790 Mg C ha −1 for the 1‐ and 2‐m sampling depths, respectively. We found significant differences in C stocks among PGUs ( p < 0.0001) in each region with NE marshes having significantly higher per area C stocks compared to SE at both sampling depths. A common benchmark in blue C inventories is to sample to 1 m, but we found that on average 40% of the C within 2 m occurred in the lower meter, underscoring the need to sample to 2 m. Recent studies suggest that until a better approach is identified, a single value of 27 kg C m −3 be used for C accounting in tidal marshes. On average, this overestimated C stocks in the SE by 37% and underestimated in the NE by 35%. Our findings emphasize that C stocks in tidal marshes can vary greatly by geomorphic setting and suggest a pedogeomorphic framework with sampling to at least 2 m offers an effective approach for future tidal marsh C accounting.