Mangroves represent an integral component of coastal blue carbon ecosystems, contributing to climate change mitigation through high-capacity natural carbon sequestration. However, the dominant mechanisms and rates of alkalinity production in mangrove systems remain poorly understood. Based on global data review and analysis, we quantified that sulfate reduction is the primary driver of alkalinity production in mangrove sediments, accounting for over 80% of the total, substantially exceeding the contributions from denitrification, iron reduction, and manganese reduction. To complement this global analysis, we conducted a field investigation in a tropical mangrove ecosystem in Wenchang Estuary, Hainan, China. Ex-Situ sediment incubation experiments revealed that sulfate reduction contributed more than 60% of total alkalinity production, with both the alkalinity production rate and the proportional contribution of sulfate reduction increasing progressively from landward to seaward zones. The high relative abundance of sulfate reducing bacteria in mangrove community further corroborated the predominance of this pathway. Using radium and radon isotopes as tracers, we estimated porewater exchange-derived alkalinity fluxes, which were comparable in magnitude to the organic carbon burial. Our findings underscore the necessity of incorporating inorganic carbon sequestration into blue carbon accounting frameworks to more accurately constrain the capacity of mangrove ecosystems as nature-based solutions for climate change mitigation.