Effects of application of biochar or earthworm on the microbial community succession in an orchard soil of southern China

Soil degradation threatens global agricultural sustainability. While earthworm inoculation and biochar application are popular strategies to improve the quality of degraded soils, how they would regulate the soil microbial community succession remain unclear. This study investigated how endogenous (the initial physiochemical and microbial properties shaped by sterilization treatment) and exogenous (earthworm or biochar application) factors influence the succession of bacterial and fungal communities and associated functions (acid buffering capacity and CO₂ efflux) in a degraded lateritic red soil from a subtropical orchard over a 90-day incubation period, with soil samples destructively collected on days 15, 30, 60, and 90 for analyses. The results showed that sterilization treatment shaped significantly different bacterial and fungal community composition and diversity, while the addition treatments significantly altered the soil microbial community composition rather than the diversity. Compared with sterilized soils, non-sterilized ones contained significantly higher bacterial and fungal richness within the investigation period by 3.33%~86.33% and 3.21%~24.02%, respectively. At the phylum level, sterilization significantly increased the relative abundance of Pseudomonadota (10.34%~28.88%) but decreased that of Actinobacteriota (18.45%~86.21%). Soil bacterial and fungal community succession obviously occurred in the two soils of different microbial diversity established by sterilization treatment, and experimental treatments of additions of earthworm or biochar and sterilization did not substantially shift the succession process of soil microbial communities. At each sampling point, the addition treatment of earthworm or biochar resulted in relatively greater shifts in the bacterial communities of the soil with low microbial diversity (sterilized soil) than that with high diversity (non-sterilized soil), supporting the diversity and stability relationship of soil microbial communities. Experimental treatments significantly altered the soil functions. Specifically, sterilization treatment decreased soil acid buffering capacity by 11.11%~29.68%, which could decline by 21.34%~26.26% over time in this study, and sterilization decreased soil CO₂ efflux by 37.89%~51.22% and earthworm inoculation increased it by 28.80%~133.50%, indicating a higher sensitivity of soil functions than microbial community composition. These observations suggest that the initial soil biotic–abiotic state created by sterilization (including microbial diversity) exerts a stronger influence on soil microbial communities than the tested exogenous agricultural practices (addition of earthworm and biochar at the current dose), and support the diversity–stability relationship in soil microbial communities.