Calcium and microbial diversity jointly regulate organic carbon stabilization in evaporative lakes

Mineral carbon pump (MnCP), predominantly mediated by iron and aluminum, is the dominant mechanism for organic carbon (OC) accumulation in acidic environments. However, the roles of calcium and microbial communities for regulating OC stabilization in evaporative lakes are still poorly understood. To address the different mechanisms by which dithionite-extractable calcium (Ca d ) and microbial communities influence OC stabilization, sediment samples were collected from ten inland evaporative lakes on the Inner Mongolian Plateau. These samples were subjected to systematic analysis of geochemical variables, physical fraction and molecular composition of OC and microbial 16S rRNA gene amplicon sequencing. The results indicated that OC concentrations varied from 0.20 to 6.16 mg/g, and Ca d from 1.30 to 32.24 mg/g. Mineral associated organic carbon (MAOC) showed a significant positive correlation with Ca d (R 2 = 0.83, p < 0.001). The molecular components of OC mainly consisted of polysaccharide-C, alcoholic and phenolic-C and aromatic-C as indicated by Fourier transform infrared spectroscopy (FTIR), which together accounted for approximately 92.90%. Ca d is positively linked with alcoholic and phenolic-C ( r = 0.85, p < 0.01), and aromatic-C ( r = 0.67, p < 0.05), whereas a negatively correlated with polysaccharide-C ( r = −0.90, p < 0.001). Furthermore, bacterial diversity showed a moderately strong positive correlation with particulate organic carbon (POC) ( r = 0.42, p < 0.05). Our findings suggest two critical mechanisms for OC stabilization in evaporative lake sediments: (i) Ca d stabilized OC through the adsorption of MAOC, and (ii) Under salt stress microbial communities may influence POC accumulation via extracellular polymeric substance (EPS) secretion. These results highlight the role of Ca d and microbial mediation in OC stabilization, offering new insights into OC cycling and implications for assessing the regional carbon balance in arid and semi-arid ecosystems.