Abstract: Drought is one of the major stresses that forest ecosystems are facing globally, directly affecting plant growth and soil microorganism activities and indirectly altering soil organic carbon cycling. Temperate forests play an important role in global carbon storage and climate regulation, but the mechanism of soil carbon dynamics in response to drought stress remains less understood, particularly mycorrhiza-mediated soil organic carbon process. In this study, in a warm temperate oak forest (dominated by Quercus aliena var. acuteserrat) that received long term manipulative drought, we investigated the respective effect of fine roots, mycorrhizal fungi and free-living microorganisms on soil enzyme activities and organic carbon physical fractions, i.e., particulate organic carbon (POC) and mineral associated organic carbon (MAOC), using in-situ incubation of mesocosms with different mesh sizes (0.001 mm, 0.053 mm, 1.45 mm). The results showed that plants cope with water stress by increasing underground carbon allocation, fine roots and mycorrhizal fungal exudates provided key carbon sources to support the enhanced activity of hydrolytic enzymes. In contrast, oxidative enzyme activity was primarily regulated by water availability and soil pH. Peroxidase (PER) activity significantly decreased under drought treatment, which promoted the accumulation of POC in the 0.001 mm and 0.053 mm microcosms by inhibiting the decomposition of complex compounds. Furthermore, carbon inputs from fine roots and mycorrhizal fungi also played a significant role in the formation of POC. Compared to the effects of biological components, the accumulation of MAOC was more influenced by microbial metabolic activity and changes in the soil environment under drought conditions. In this study, we elucidated for the first time the functional differentiation of fine roots, mycorrhizal fungi and non-symbiotic microorganisms and their synergistic roles under drought stress in a warm-temperate oak forest. The results indicate that drought significantly affects the stability of soil carbon pools by modifying the interaction mechanisms among biological components and regulating the dynamics of enzyme activities and carbon fractions. These findings provide a new theoretical basis for the prediction of forest soil carbon cycle under climate change, as well as scientific support for soil management and carbon pool optimization.

Key words: Drought, Extracellular enzyme activity, Soil organic carbon fractions, Ectomycorrhizal fungi, Temperate forests