Microbial community changes mediate the decline in soil organic carbon mineralization with depth in tea plantations

Background Tea plantation soils serve as vital carbon (C) sinks rich in soil organic carbon (SOC), yet existing research primarily focuses on the 0–20 cm topsoil, leaving a lack of systematic study of SOC mineralization characteristics and microbial regulatory mechanisms across the full 0–100 cm soil profile. Therefore, we conducted incubation experiments with typical tea plantation soils to clarify changes in SOC mineralization, the SOC pool, the microbial community, and the functional genes (such as GH48 and cbh I) and enzyme activities related to C decomposition at five soil depths (0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm), and to elucidate their relationships, thereby revealing the mechanisms affecting SOC mineralization in different soil layers. Results The results revealed significant declines in SOC mineralization rate (from 842 to 431 mg kg⁻¹), particulate organic carbon, water-soluble organic carbon, microbial biomass carbon, β-glucosidase (from 71.6 to 18.2 µg g − 1 h − 1 at the end of incubation), cellobiohydrolase (from 0.229 to 0.091 mg g − 1 3d − 1 at the end of incubation), GH48 and cbh I gene abundances (decreased from 9.2 × 10 7 to 1.8 × 10 7 and 7.6 × 10 7 to 1.4 × 10 6 copies g − 1 at the end of incubation, respectively), respectively with increasing soil depth. The decrease in the SOC mineralization rate with increasing soil depth was significantly associated with declines in the labile C fraction, C-decomposition-related extracellular enzyme activity, and functional gene abundance. Additionally, increasing soil depth significantly altered the microbial community structure and composition, particularly the relative abundances of dominant taxa such as Alphaproteobacteria, Bacilli, Sordariomycetes, Tremellomycetes, Mortierellomycetes, and Dothideomycetes, thereby further influencing SOC mineralization rate. Conclusions Our findings demonstrate that increasing soil depth significantly alters soil microbial community characteristics, particularly by reducing the abundance of C-degrading functional genes and enzyme activities, thereby lowering SOC mineralization rate and attenuating soil carbon emissions. These results underscore the importance of deep tillage during fertilization and the incorporation of pruning residue to enhance SOC sequestration in tea plantations.