Millennial rice cultivation shapes depth‑dependent organic carbon mineralization and microbial life strategies in saline paddy soils

Reclaimed tidal flats are crucial for expanding agricultural land, yet the effects of long-term rice cultivation on organic carbon (OC) mineralization and microbial life strategies in saline soils remain insufficiently understood. This study investigated OC dynamics and microbial responses in saline paddy soils across a millennium-long chronosequence (50, 200, 500, and 1000 years). Using ¹³C-labeled glucose (2 % of SOC) as a tracer, we evaluated exogenous labile C and SOC mineralization in saline soils sampled at four depths (0–20, 20–40, 40–80, and 80–100 cm). Surface soils exhibited higher mineralization rates for both glucose (46 %–73 %) and SOC (1.6 %–3.5 %) than deeper layers (4 %–25 % and 0.6–1.6 %). Prolonged cultivation enhanced glucose mineralization but stabilized SOC. Higher levels of nutrients, microbial biomass, and enzyme activity enhanced OC mineralization, whereas increased sand content reduced physical protection of SOC, consequently accelerating its mineralization. Microbial community succession was strongly linked to glucose mineralization. The opportunistic K-strategists were dominated and actively mineralized glucose at early cultivation stages. After long-term cultivation, resource-conserving r-strategists became predominant, which benefited to SOC mineralization. This strategic shift corresponded with reduced salinity and improved soil aggregation, which restricted microbial access to glucose. Our findings highlight the dynamic interplay between microbial community adaptations and soil properties in regulating mineralization of exogenous OC and SOC. This study underscores the critical role of long-term cultivation in shaping carbon turnover and sequestration in coastal saline paddy soils.