Progressively greater temperature sensitivity of organic carbon decomposition in subsoil relative to topsoil along a millennial chronosequence of paddy soils

The effects of the temperature sensitivity (Q₁₀) of soil organic carbon (SOC) decomposition in subsoil versus topsoil on soil development and SOC accumulation over centuries of agricultural cultivation remain unclear. This study investigated Q₁₀ variations across soil developmental stages and depths and the key influencing factors based on a millennial soil chronosequence from the coastal region of Cixi, China. The Q₁₀ of paddy soil samples from different developmental periods and depths was measured through short-term incubation experiments with sequential temperature changes. Q₁₀ values increased with soil development time and depth. High Q₁₀ values in the vertical profiles of older soils were primarily attributed to increased substrate availability due to SOC accumulation and soil matrix-based physicochemical protection of SOC. We found that the higher level of Q₁₀ was in deeper soil, reflecting the distinct SOC formation mechanisms at different depths. Mineral-bound organic compounds derived from plant material, which have higher energy contents, higher C:N ratios, and greater activation energies, resulted in higher Q₁₀ values in deeper soils. Moreover, microbial communities in deeper soils appeared less tolerant to warming, as indicated by the substantially higher qCO₂ in deeper soils at temperatures above 20 ℃. These deep microbial communities also exhibited lower diversity, simpler structures, and higher proportions of r-strategists, potentially contributing to their warming vulnerability. Overall, this study suggests that the reduced dominance of necromass carbon in SOC and limited thermal tolerance of microbial communities jointly contributed to the enhanced temperature sensitivity of SOC decomposition in deeper soils over the millennial timescale.