Microbial residues as the nexus transforming inorganic carbon to organic carbon in coastal saline soils

Soil inorganic carbon (SIC), including mainly carbonate, is a key component of terrestrial soil C pool. Autotrophic microorganisms can assimilate carbonate as the main or unique C source, how microorganisms convert SIC to soil organic carbon (SOC) remains unclear. A systematic field survey (n = 94) was performed to evaluate the shift in soil C components (i.e., SIC, SOC, and microbial residues) along a natural salinity gradient (ranging from 0.5 ‰ to 19 ‰), and further to explore how microbial necromass as an indicator converting SIC into SOC in the Yellow River delta. We observed that SIC levels linearly decreased with increasing salinity, ranging from ∼12 g kg−1 (salinity < 6 ‰) to ∼10 g kg−1 (salinity >6 ‰). Additionally, the concentrations of SOC and microbial residues exponentially decreased from salinity < 6 ‰ to salinity >6 ‰, with the decline of 39% and 70%, respectively. Microbial residues and SOC was tightly related to the variations in SIC. The structural equation model showed the causality on explanation of SOC variations with SIC through microbial residues, which can contribute 89% of the variance in SOC storage combined with SIC. Taken together, these two statistical analyses can support that microbial residues can serve as an indicator of SIC transition to SOC. This study highlights the regulation of microbial residues in SIC cycling, enhancing the role of SIC playing in C biogeochemical cycles and enriching organic C reservoirs in coastal saline soils.