Minerogenic salt marshes can function as important inorganic carbon stores

Abstract

Stocks and fluxes of soil inorganic carbon have long been ignored in the context of coastal carbon sequestration, and their implications for the climate cooling effect of blue carbon ecosystems are complex. Here, we investigate the role of soil inorganic carbon in five salt marshes along the northern coast of the European Wadden Sea, one of the world's largest intertidal areas, harboring ~ 20% of European salt‐marsh area. We demonstrate a substantial contribution of inorganic carbon (average: 29%; range: 7–57%) to the total soil carbon stock of the top 1 m. Notably, inorganic exceeded organic carbon stocks in one of the studied sites; a finding that we ascribe to site geomorphic features, such as proximity to marine calcium carbonate sources and hydrodynamic exposure. Contrary to our hypothesis that inorganic carbon stocks would decline along the successional gradient from tidal flat to high marsh, as carbonate deposits would progressively dissolve in increasingly organic‐rich rooted sediments, our findings demonstrate the opposite pattern—an increase in inorganic carbon stocks along the successional gradient. This suggests that the dissolution of calcium carbonates in the root zone is counterbalanced by other processes, such as trapping of sedimentary carbonates by marsh vegetation and calcium carbonate precipitation in anaerobic subsoils. In the context of blue carbon, it will be critical to develop an improved understanding of these plant‐ and microbiota‐mediated processes in calcium carbonate cycling.

Blue Carbon in a Sub-Antarctic Marine Protected Area: Current and Future Perspectives

“…In seagrasses, values of carbonate in sediments could suggest even a net production of CO 2 (from ~30% to ~50% of dry mass) 32,[53][54][55] . Moreover, the mean carbonate content in mangrove forests and tidal marshes was also high (from ~13% to ~38%), and some of them even grew over carbonate banks 54,56,57 .…”

Plant‐sediment interactions decouple inorganic from organic carbon stock development in salt marsh soils

“…Given the limited scope of the present study to surface soils (0-5 cm), further evaluation is needed to determine the extent to which the observed relationship between SIC density and SIC accumulation rate can be applied to the entire soil profile. The few available studies investigating depth profiles of salt marsh SIC (e.g., Kennedy et al 2018;Lu et al 2019;Mueller et al 2023) suggest that downcore changes in salt marsh SIC follow a non-linear pattern, influenced by dissolution processes of calcium carbonates in the rhizosphere and re-precipitation in deeper soil layers (Vranken et al 1990;Mueller et al 2023). Contrasting to the positive relationship between SIC density and SIC accumulation rate, SOC density exhibited a negative correlation with SOC accumulation rate (Spearman rank correlation, ρ = À0.47, p < 0.001; linear regression, R 2 = 0.31; Supporting Information Fig.…”

“…This finding implies that the climate cooling effect of these blue carbon ecosystems can be amplified by the dissolution of allochthonous carbonates and the resulting alkalinity production (Santos et al 2021;but see Van Dam et al 2023). It is, however, unclear whether this finding is transferable to salt marshes, despite recent evidence that SIC can represent a large fraction of the total soil carbon stock of these ecosystems (Mueller et al 2023). SIC assessments are so far restricted to few SIC content or stock observations (e.g., Vranken et al 1990;Spohn et al 2013;Gallagher et al 2021;Voltz et al 2021;Zhang et al 2021), while the drivers of SIC accumulation rates remain widely unexplored in salt marshes.…”

“…SIC-based greenhouse-gas removal requires SIC dissolution, driven by soil acidification, which leads to alkalinity release. Rhizosphere processes linked to plant belowground traits are instrumental in soil acidification, emerging as key drivers of SIC dissolution (Vicca et al 2022;Mueller et al 2023). SIC-based greenhouse-gas removal in coastal wetlands thus depends on the interplay between of plant aboveground traits, governing calcium carbonate trapping, and belowground traits, controlling calcium carbonate dissolution.…”