Core microbial taxonomies that maintain high organic carbon content in upland soil

The accumulation of soil carbon (C) is crucial for the productivity and ecological function of farmland ecosystems. The balance between microbial carbon dioxide (CO₂) emission and fixation determines the sustained accumulation potential of C in soil. Microorganisms involved in this process are highly obscure, thus hindering identification and further application of microorganisms with fertile soil function. In this study, a series of typical upland farmland soils were collected from 29 regions and their microbial community structure and soil C fractions were analyzed. Additionally, the rates of CO₂ emission and fixation in each soil were measured. The results showed that the correlation between soil CO₂ emissions and the SOC concentration was logarithmic, while that between CO₂ fixation and SOC was linear. Bacterial and fungal diversity showed an upward trend with increasing soil C, and their α diversity was significantly correlated with CO₂ fixation, but not correlated with CO₂ emission. Fungi were more associated with soil C than bacteria, and the strength of linkage with soil C varied among the different phyla of microorganisms. Furthermore, the core microbial taxa in soils with low, medium and high SOC levels were identified by discarding redundant amplicon sequence variants, and their community differentiation was significantly driven by soil CO₂ emission and fixation based on Mantel analysis. The high abundance of Chloroflexi, Nitrospirota, Actinobacteria, and Mortierellomycota in core taxa might indicate a high level of SOC level. This study highlights that SOC fluctuations are mainly driven by the core microbial taxa, rather than all microbial taxa in the agricultural system. Our research sheds light on the targeted regulation of the soil microbial community structure in upland farmland for soil fertility enhancement.