Tea plantation age shapes trade-offs between SOC stabilization and accumulation via microbial necromass

Tea plantations can store substantial amounts of soil organic carbon (SOC), but the microbial mechanisms regulating SOC accumulation and stabilization during plantation development remain unclear. We sampled topsoil (0–20 cm) and subsoil (20–40 cm) across a chronosequence of tea plantations aged 4, 18, and 27 years and quantified SOC fractions and characterized microbial communities, microbial necromass carbon, and plant-derived carbon using phospholipid fatty acids (PLFAs), amino sugars, and lignin phenols, respectively. SOC stocks increased with plantation age, but accumulation rates declined significantly in both soil layers, while SOC composition shifted from labile to more recalcitrant fractions, indicating enhanced carbon stabilization. Microbial necromass carbon contributed 30.8–50.5 % of SOC, exceeding plant–derived carbon (11.2–27.8 %). The amino sugar–to–PLFA ratio (AS/PLFA) increased significantly only in the topsoil and showed no significant change in the subsoil. Depth–specific drivers differed: SOC dynamics in topsoil were primarily associated with fungal necromass accumulation per unit biomass (GluN/FPLFA), whereas subsoil SOC was more strongly related to bacterial necromass accumulation per unit biomass (MurA/BPLFA). These results highlight how microbial biomass turnover and necromass retention shape trade-offs between SOC accumulation and stabilization during plantation development.