<p>Climate change mitigation strategies focus primarily on reducing greenhouse gas emissions. An alternative strategy, carbon (C) sequestration through land and ocean ecosystems, is gaining momentum in climate policy. This study explores C sequestration by microbial mats in two semiarid coastal tidal flats in Buenos Aires Province, Argentina: Puerto Cuatreros (PC) and Paso Seco (PS). Interannual CO₂ fluxes and C storage were measured using an optical CO₂ sensor and sediment core analyses, respectively. Microbial mats in PS exhibited larger CO₂ fluxes, releasing more CO₂ into the atmosphere compared to PC. Seasonal trends showed higher CO₂ release during spring and summer at both sites. Sediment core analyses indicate that PC had higher organic C content, whereas PS had significant inorganic C accumulation, attributed to periodic CaCO₃ precipitation in the evaporitic environment. Microbial mats in these semiarid regions contribute to C sequestration, but their net effect varies with environmental conditions and seasonal changes. These results underscore the need for further research on microbial mats in semiarid regions to fully understand their role in global C cycling and their potential as natural climate solutions. Finally, the presence of buried CaCO₃ in PS highlights the importance of considering both organic and inorganic C in C stock calculations for an accurate global C budgeting.</p>