Soil organic carbon (SOC) is concentrated in coastal wetlands, and its permanence maintains a livable climate, yet dynamics that govern microbial activity and SOC persistence are not fully characterized in coastal wetlands. Though microbial activity is conventionally thought to facilitate SOC loss, soil microbes simultaneously direct SOC formation. In fact, microbially-processed materials constitute up to half of the terrestrial soil organic carbon pool. Environmental conditions can affect whether microbes yield a net gain or loss of SOC, yet there is little consensus on microbial drivers of soil carbon longevity in coastal ecosystems. I sought to identify which drivers of microbial activity have the greatest impact on SOC in salt marsh soils. To address this question, I conducted a meta-analysis using the PRISMA method. Based on an initial survey of 2,835 studies, numeric data on soil and ecosystem characteristics were collected across 50 studies on over 60 salt marshes located around the world. Integrative data analyses, including structural equation modeling (SEM), were applied to synthesized data to identify environmental drivers of SOC in salt marsh ecosystems. Across a wide range of study sites, analysis of over 20 variables shows that soil characteristics are tightly linked. Salinity, pH, nitrogen, and phosphorus are associated with increased microbial biomass and soil organic carbon. Correlations between microbial biomass carbon and SOC are strengthened by soil salinity and nitrogen, and they are weakened by moisture. Correlations were dependent on the means by which variables were measured, yet findings were consistent across study sites. These results suggest that soil carbon content is affected by drivers of microbial activity. Observational findings set the stage for experimental strategies that parcel causal effects of microbial activity on SOC from confounding effects of covariant environmental conditions. I identified that nitrogen, phosphorus, salinity, pH, and moisture influence microbial contributions to SOC. These environmental drivers, as well as microbial biomass and greenhouse gas flux, should be considered key indicators of soil health when assessing the sustainability of coastal SOC. Identification of environmental drivers of microbial function enables design of land management strategies that promote conditions conducive to coastal soil carbon longevity.