Compartment-Specific Responses of Soil Bacteria and Metabolites to Biochar in Rhizosphere and Bulk Soils Under Continuous Cassava Cropping

Continuous monocropping of cassava (Manihot esculenta Crantz) often leads to soil degradation and yield decline, commonly referred to as continuous cropping obstacles (CCOs), which are closely linked to changes in soil physicochemical properties and microbial communities. Biochar has been widely used as a soil amendment to improve soil quality and microbial activity and is considered a potential strategy for alleviating CCOs. Understanding the effects of biochar on soil bacteria and metabolites under field conditions is essential, as it provides insights into its practical effectiveness in reducing CCOs and improving soil health in cassava cultivation systems. In this study, a field experiment was conducted in a continuous cassava system to investigate the effects of a single biochar application rate on soil bacterial diversity, community composition, and metabolite profiles in both rhizosphere and bulk soils. High-throughput 16S rRNA gene sequencing and UHPLC–MS/MS-based non-targeted metabolomics were employed to analyze soil bacterial and metabolic patterns. Biochar was associated with increased α-diversity in rhizosphere soil and distinct shifts in β-diversity. Biochar increased the relative abundance of Chloroflexi and Actinobacteriota in the bulk soil, while Cyanobacteria and Nitrospirota were more abundant in the rhizosphere. Network analysis revealed the compartment-specific differences after biochar application, with higher network complexity in the rhizosphere and lower complexity in the bulk soil relative to the control. Metabolomic profiling identified 402 metabolites in positive ion mode and 357 in negative ion mode. In the rhizosphere, biochar-treated soil exhibited higher relative abundances of alkaloids (e.g., trigonelline, berberine, vincristine) and flavonoids (e.g., catechin, naringin, rutin, and taxifolin), which are commonly linked to plant stress responses. In the bulk soil, biochar application resulted in lower levels of several anthropogenic organic compounds (e.g., monobutyl phthalate, terephthalic acid, and p–toluenesulfonic acid). These findings provide preliminary field evidence that biochar application can lead to compartment-specific changes in soil bacterial communities and metabolite profiles. Such changes are closely related to soil quality and nutrient cycling, pointing to a possible role of biochar in mitigating soil degradation under continuous cassava cultivation.