Soil carbon dioxide levels control salt marsh alkalinity generation

Abstract

Recent studies have highlighted salt marshes as hotspots for alkalinity generation and export to coastal waters. Several studies emphasize the critical role of anaerobic microbial metabolism as a major source of total alkalinity (TA) generation in marsh soils. However, the contribution of mineral dissolution to salt marsh TA generation has yet to be extensively studied from a mechanistic viewpoint. Therefore, we conducted a lab‐based soil column experiment to investigate the influence of soil pCO2 levels on TA generation in organic‐poor minerogenic salt marsh soils, considering their significance in soil mineral dissolution. Results showed a maximum TA increase of up to twofold when the pCO2 level was increased from 440 to 150,000 μatm. The relationship between TA and net Ca2+ release was 1.73, close to the theoretical stoichiometric ratio of HCO3− to Ca2+ for calcium carbonate dissolution. In addition, the net release of dissolved silica responded significantly to the change in soil pCO2. Therefore, calcium carbonate dissolution was identified as the possible dominant driving force behind TA generation in the minerogenic salt marsh soil, with an additional contribution from silicate minerals. We suggest high pCO2 levels, such as the rhizosphere, might be the hotspot for alkalinity generation via mineral dissolution. Our findings advance the mechanistic understanding of TA generation in salt marshes and highlight the importance of the overlooked role of inorganic carbon. Evaluating the contribution of blue carbon ecosystems to alkalinity production is essential for integrating the roles of both organic and inorganic carbon into climate mitigation assessments.