Calcium forms influence soil organic carbon by mediating labile organic carbon fractions, carbon pool management indices and microbial communities in calcareous alkaline soils

Soil carbon (C) pools influence the global C balance of terrestrial ecosystems. Calcium (Ca) affects soil organic carbon (SOC) by mediating organic-mineral interactions. In this study, we aimed to elucidate the relationships among Ca forms, labile organic carbon (OC) fractions, carbon pool management indices and microbial communities.

We selected three soils with different calcium carbonate (CaCO₃) contents: black soil (4.29 g.kg⁻¹), fluvo-aquic soil (94.86 g.kg⁻¹) and loessial soil (133.87 g.kg⁻¹). We measured different Ca forms, microbial communities and labile OC fractions while calculating the soil carbon pool management indices.

The results revealed a significant increase in major Ca forms and a significant decrease (P < 0.05) in labile OC fractions as well as carbon pool management indices in fluvo-aquic soil and loessial soil. The bacterial community diversity decreased, and the dominant bacteria and fungi shifted from oligotrophs to copiotrophs. Ca forms were significantly negatively (P < 0.05) correlated with the labile OC fraction and carbon pool management index (CPMI), and the bacterial community structure was closely related to bound to organic-Ca. Bacteria played a dominant role, and the symbiotic network robustness and complexity of the network modules were greater for bacteria than for fungi in calcareous alkaline soils.

The results suggested the existence of interactions between Ca forms, labile OC fractions, the CPMI and microbial communities in calcareous alkaline soils. Ca affects the labile OC fractions, which act on the CPMI and change the community composition of microbial (bacterial communities), influencing the SOC content.