Microbial-mediated soil carbon accrual under 12-year reduced nitrogen fertilisation and intercropping

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Knowledge-based agricultural practices, such as reduced nitrogen (N) fertilisation and intercropping, enhance soil organic carbon (SOC) sequestration. However the microbial mechanisms remain unclear. In order to investigate the mechanisms, we launched a 270-day in situ 13 C-labelled residue return experiment under a full factorial design. The design included three N fertilization levels (no N, reduced N (600 kg N ha −1 yr −1 ), and traditional N (720 kg N ha −1 yr −1 )) and two cultivation models (maize monoculture and maize–soybean intercropping) after 12 years of continuous application. Residue-derived organic C was quantified by tracing the decomposition of 13 C-labelled maize and maize–soybean residues at days 10, 30, 120, and 270. At day 270, reduced N fertilisation increased residue-derived SOC, particulate OC (POC), and recalcitrant C by 64.3%, 52.4%, and 22.1%, respectively, compared to traditional N fertilisation, Meanwhile, intercropping increased them by 90.2%, 33.2%, and 124.4%, respectively. The total concentrations of SOC, POC, and recalcitrant C increased by comparable proportions to the residue-derived fractions under reduced N fertilisation and intercropping. The positive effects of reduced N fertilisation and intercropping on SOC sequestration were associated with their enhancement of soil microbial biomass C concentration and bacterial community network complexity. Additionally, intercropping enhanced the network complexity of the fungal community. Furthermore, both reduced N fertilisation and intercropping supported the growth of keystone taxa that contribute to residue-C sequestration and N fixation. Overall, our study suggests that reduced N fertilisation and intercropping promote SOC sequestration by creating a more favorable environment for soil microbes, providing a scientific basis for these knowledge-based agricultural practices.