Organic amendments induce crop-specific soil biotic homogenization associated with SOC sequestration: Evidence from a 35-year intercropping experiment

Organic amendments increase soil organic carbon (SOC) but may reduce belowground β-diversity or increase compositional similarity among soil communities in agroecosystems, a phenomenon known as biotic homogenization. However, it remains unclear whether and how biotic homogenization in turn mediates SOC accumulation in response to organic amendments, particularly in wheat/maize intercropping systems prevalent in Northwest China. To address this, we used amplicon sequencing to investigate the role of soil biota in SOC accumulation under wheat and maize strips in a long-term intercropping experiment (since 1988) with three organic amendments (wheat straw, mixed straw & cow manure, and cow manure) and one chemical fertilizer (Chemical). Results showed that the cow manure (Manure) led to significantly higher SOC than the straw and Chemical treatments for both crops. Notably, the β-diversity of soil biota was the primary driver of SOC variation. Compared with Chemical treatment, Manure significantly reduced β-diversity, reflecting biotic homogenization, but the mechanisms differed by crop: in maize soils, fungi significantly reduced the turnover component, whereas in wheat soils, protists significantly reduced the nestedness component. Core species driving biotic homogenization were positively correlated with SOC and significantly enriched under Manure treatment. Network analysis showed that core species in maize soils were linked to bacteria exhibiting functionally distinct β-diversity across treatments, whereas core species in wheat soils were associated with bacteria lacking such functional differentiation. Overall, our study demonstrates positive yet crop-specific effects of biotic homogenization on SOC accumulation in wheat/maize intercropping under organic amendments. These findings highlight the potential for targeted use of organic amendments to shape soil community composition and optimize SOC in intercropping systems.