Cereal-legume intercropping stimulates straw decomposition and promotes soil organic carbon stability

Increasing carbon (C) sequestration and stability in agricultural soils is a key strategy to mitigate climate change towards C neutrality. Crop diversification is an initiative to increase C sequestration in fields, but it is unclear how legume-based crop diversification impacts the functional components of soil organic carbon (SOC) in dryland, including the formation and transformation of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). We investigated the decomposition of straw residues, the fate of photosynthesized C, as well as the formation of MAOC and POC fractions using an in situ¹³C labeling technique in the soybean-wheat intercropping, soybean-maize intercropping and their respective monocropping systems, with and without cover crops. After 4-year treatments, the total SOC content in bulk soil remained unchanged, while MAOC content increased significantly by 5.6% with intercropping. Moreover, the in situ¹³C labeling results confirmed that more photosynthesized C was transferred to MAOC, and less was retained in the POC fraction. Intercropping significantly increased total soil N and mineral N content by 15.3% and 13.4%, respectively, and decreased soil and microbial C/N ratio by 11.3% and 17.4%, respectively. This outcome, therefore, relieved microbial N limitation and accelerated straw residue decomposition. Accordingly, the potential of MAOC formation was strengthened for better SOC persistence. Our study suggests that legume-based crop diversification can effectively enrich N and support POC transformation to MAOC, accordingly contributing to the persistent SOC pool and thus potentially achieving C neutrality under climate change in dryland agroecosystems.