Enhancing Soil Carbon Sequestration in Soybean-Maize Intercropping Systems of Northeast China's Black Soil Region: A Density-Dependent Root-Microbe Interaction Study

Aims: Soybean-maize intercropping has demonstrated potential for enhancing soil carbon sequestration in Northeast China's black soil region. However, the mechanisms—particularly those involving density-dependent root-microbe interactions—remain unclear. This study aims to elucidate how planting density modulates soil carbon sequestration via plant–microbe interactions. Methods: We conducted a three-year field experiment with five cropping systems, including varying intercropping densities and monocultures. Soil organic carbon, root biomass, nodule biomass, root exudates, and microbial communities were analyzed using standard biochemical assays, high-performance liquid chromatography (HPLC), and 16S rRNA gene sequencing. Structural equation modeling (SEM) was applied to explore mechanistic pathways. Results: Moderate planting density (45,000 plants ha⁻¹) achieved the highest soil organic carbon sequestration rate (0.85 t C ha⁻¹ yr⁻¹), correlating with increased root and nodule biomass, phenolic exudation, and microbial diversity—especially Bradyrhizobium abundance. SEM revealed that 68% of the SOC variance could be explained by plant–microbe interactions. Conclusions: Our findings demonstrate that optimizing intercropping density strengthens root–microbe interactions and significantly enhances carbon sequestration. These insights provide a foundation for sustainable agricultural practices aimed at restoring black soils and mitigating climate change.