Sugarcane/soybean intercropping with reduced nitrogen addition enhances residue-derived labile soil organic carbon and microbial network complexity in the soil during straw decomposition

Sugarcane/soybean intercropping with reduced nitrogen addition is an important sustainable agricultural pattern that can alter soil ecological functions, thereby affecting straw decomposition in the soil. However, the mechanisms underlying changes in soil organic carbon (SOC) composition and microbial communities during straw decomposition under long-term intercropping with reduced nitrogen addition remain unclear. In this study, we conducted an in-situ microplot incubation experiment with C-labeled soybean straw residue addition in a two-factor (cropping pattern: sugarcane monoculture (MS) and sugarcane/soybean intercropping (SB); nitrogen addition levels: reduced nitrogen addition (N1) and conventional nitrogen addition (N2)) long-term experimental field plot. The results showed that the SBN1 treatment significantly increased the residual particulate organic carbon (POC) and residual microbial biomass carbon (MBC) contents during straw decomposition, and the straw carbon in soil was mainly conserved as POC. Straw addition changed the structure and reduced the diversity of the soil microbial community, but microbial diversity gradually recovered with decomposition time. During straw decomposition, the intercropping pattern significantly increased the relative abundances of Firmicutes and Ascomycota. In addition, straw addition reduced microbial network complexity in the sugarcane/soybean intercropping pattern but increased it in the sugarcane monoculture pattern. Nevertheless, microbial network complexity remained higher in the SBN1 treatment than in the MSN1 treatment. In general, the SBN1 treatment significantly increased the diversity of microbial communities and the relative abundance of microorganisms associated with organic matter decomposition, and the changes in microbial communities were mainly driven by the residual labile SOC fractions. These findings suggest that more straw carbon can be sequestered in the soil under sugarcane/soybean intercropping with reduced nitrogen addition to maintain microbial diversity and contribute to the development of sustainable agriculture.