Synthetic Microbial Community Promotes Straw Humification and Improves Soil Organic Carbon in Cropland

Straw return is considered an effective approach to increase soil organic carbon (SOC) and improve soil multifunctionality (SMF), yet the relatively low humification rate under field conditions limits these benefits. The application of synthetic microbial communities (SynComs) represents a promising strategy for promoting straw humification. Nevertheless, challenges remain in designing functionally stable SynComs for straw humification. Here, by combining progressive dilution-microcosm cultivation with co-occurrence network and functional analyses, we identified keystone taxa that promoted maize straw humification in the microcosm. From these taxa, we constructed a two-membered SynCom (<i>Bacillus siamensis</i> B and <i>Bradyrhizobium japonicum</i> G). By cross-feeding, SynCom members enable each other to grow, increase the activities of key enzymes (carboxymethyl cellulase, xylanase, lignin peroxidase, and nitrogenase), and thereby promote maize straw humification. Field experiments further demonstrated that SynCom inoculation enhanced the in vivo turnover pathway of the microbial carbon pump, thereby promoting straw humification and improving SOC. Overall, we provide a systematic approach for identifying and applying keystone taxa to construct SynComs that promote maize straw humification and improve SOC.