Large porewater exchange reshapes saltmarsh carbon and greenhouse gas budgets on local and global scales

Saltmarshes are carbon-rich ecosystems that play a role in climate regulation by efficiently sequestrating atmospheric CO₂. Climate change is a major threat to saltmarshes and will impact saltmarsh carbon sequestration. Notably, porewater-derived carbon outwelling emerges as a crucial pathway for carbon sequestration, but it has been largely overlooked in saltmarshes. Here, we assessed the drivers of dissolved carbon exports and greenhouse gas emissions in a saltmarsh and placed those observations into a global context. In our study site, porewater exchange (PEX) was the major process controlling dissolved carbon and greenhouse gas dynamics. Combining our results with 33 other studies in saltmarshes worldwide, porewater-derived carbon fluxes were equivalent to ∼48% of average global saltmarsh primary productivity and 2.2 times greater than global organic carbon burial. Hence, our local observations and global upscaling demonstrate the major role played by PEX in saltmarsh carbon budgets. While part of dissolved inorganic carbon returns to the atmosphere as CO₂, the remaining bicarbonate fraction is stored in the ocean over long time scales. Our findings strengthen global blue carbon assessments and build arguments for the use of saltmarshes as nature-based solutions for global climate change.