Diverse protective mechanisms drive changes in functional carbon pools of paddy soil under continuous biochar and inorganic fertilizer application

Biochar amendment is believed to be an effective practice for organic carbon sequestration in paddy soil. Nevertheless, the influences of biochar on soil functional carbon pools and the underlying mechanisms remain poorly understood. To address this, a six-year (from 2017 to 2022) field trial was implemented involving four treatments: no application of inorganic fertilizers or biochar (Control), inorganic fertilizers alone (NPK); biochar alone (Biochar); and joint use of inorganic fertilizers and biochar (NPKB). The distribution of functional carbon pools, soil organic carbon (SOC) structure, and microbial community were investigated in two soil depths (0–15 cm and 15–30 cm). Results showed that biochar application led to obviously higher organic carbon in unprotected and physically protected pools in the topsoil (92–763 %) and subsoil (34–290 %), with over 90 % of the additional SOC concentrated in particulate organic matter (POM) pools. By contrast, inorganic fertilization did not affect topsoil carbon pools but enhanced various subsoil carbon pools (except fPOM) by 0.3–5.7 g kg⁻¹. The additive effect of inorganic fertilization and biochar addition was primarily recorded in the subsoil, especially in the POM pools. Furthermore, biochar addition significantly increased SOC aromaticity in the topsoil (373 %) and subsoil (203 %), while NPKB resulted in the highest total phospholipid fatty acids (PLFA) abundance, 42–118 % and 209–258 % greater than other treatments in the topsoil and subsoil, respectively. Redundancy analysis and Mantel test indicated that SOC structure, soil nutrients and total PLFA were the primary factors influencing the distribution of functional C pools. Partial Least Squares Path Modelling further demonstrated that biochar addition regulated topsoil carbon pool distribution by influencing SOC structure, while inorganic fertilization affected it by shaping microbial community structure in the subsoil. Overall, the joint application of biochar and inorganic fertilizers holds potential for enhancing subsoil carbon sequestration in paddy soils.