Although autotrophic or chemotrophic microorganisms can assimilate CO2 or carbonate, it is still unclear how microorganisms convert soil inorganic carbon (SIC) to organic carbon (SOC), owing to the lack of a microbial indicator between SIC and SOC. Herein, we hypothesized that carbonate-rich saline soils are a potential source that contribute to the SOC pool through the transformation of microbial necromass. SIC levels linearly decreased with an increase in salinity, while SOC and microbial residues exponentially declined. A structural equation model verified the causality of SIC-microbial residues-SOC, suggesting that microbial residues can serve as an indicator of SIC transition to SOC. This study highlights the regulation of microbial necromass in SIC cycling, thus enhancing the application of SIC for C biogeochemical cycles and enriching organic C reservoirs in global saline or dry lands.