Bacterial functions are main driving factors on paddy soil organic carbon in both surface soil and subsoil

Subsoil contains a significant amount of soil organic carbon (SOC) in rice paddies, playing a crucial role in global carbon (C) cycling. However, the mechanisms driving SOC dynamics in topsoil and subsoil remain elusive, as the vertical distribution of soil microbes and their associated functions varies. In our study, we explored the linkages between SOC dynamics and microbial functions across different soil depths (0-10 cm for topsoil, and 10-20 and 20-30 cm for subsoil) within a framework of contrasting tillage managements, including conventional tillage (CT), reduced tillage (RT), and no-tillage (NT). In the topsoil, bacterial biosynthesis and degradation functions negatively contributed (P < 0.05) to SOC accumulation, whereas in the subsoil layers, these functions exhibited a positive effect. Notably, both partial least squares path modeling (PLSPM) and stepwise regression showed that bacterial biosynthesis and degradation functions exert a stronger influence on SOC accumulation than fungal functions. Metagenomic sequencing revealed that the cellulose degradation (K01179) and reductive tricarboxylic acid cycle (rTCA cycle) (K00174 and K00175) were main factors driving SOC variation in topsoil. Starch degradation (K00705 and K01187) and rTCA cycle (K00175 and K00031) contributed to 93 % of the SOC variation in subsoil. Therefore, the rTCA cycle contributed to SOC variation in paddy soil across 0-30 cm soil profile. Moreover, tillage management could influence the abundances of enzymes associated with starch degradation and rTCA cycle. The contrasting driving factors influencing SOC in topsoil and subsoil could be associated with soil microbial functions, which would response differently to tillage managements.