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.

Abstract Aims Phosphorus (P) is an essential nutrient for plant growth and a critical factor of determining forest productivity. Most terrestrial plants can form symbiotic associations with either arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi to enhance their phosphorus uptake. AM mycorrhiza and ECM mycorrhiza vary significantly in their P-absorption strategies, which impacts the P cycling in ecosystems. Understanding how soil P availability varies across the forests dominated by AM trees and ECM trees can be conductive to elucidating the mechanisms of productivity maintenance in subtropical forests and guiding forest nutrient management. Methods We established 35 forest plots across a natural gradient of ECM tree dominance in the Badagongshan Nature Reserve, Hunan Province. We measured the contents of four forms of soil bioavailable P (CaCl2-P, Enzyme-P, Citrate-P, HCl-P), and analyzed their relationships with ECM tree dominance. Correlation analyses were further employed to identify the key factors influencing soil bioavailable P contents. Important findings The results revealed that Enzyme-P content increased significantly with ECM tree dominance, while CaCl2-P, Citrate-P and HCl-P contents showed no significant correlations with ECM tree dominance. CaCl2-P content was positively correlated with leaf litter P content. Citrate-P and HCl-P contents were positively correlated with the contents of soil organic carbon, total nitrogen, and microbial biomass carbon. Enzyme-P content was positively correlated with the contents of soil organic carbon and dissolved organic carbon, and negatively correlated with soil pH. Additionally, soil acid phosphatase activity increased significantly with ECM tree dominance. In conclusion, ECM-dominated forests exhibit higher levels of enzyme-hydrolysable P and acid phosphatase activity, which can facilitate P solubilization through organic P mineralization, thereby promoting the rapid growth of ECM trees in subtropical forests.

Aims This study examines the growth responses of major afforestation tree species in Northeast China, specifically Fraxinus mandshurica (a broadleaf species associated with arbuscular mycorrhizae) and Larix gmelinii (a coniferous species associated with ectomycorrhizae), to varying levels of dry and wet reduced nitrogen deposition. By investigating the impact of increased reduced nitrogen on these seedlings, this research aims to enhance our understanding of the relationship between different seedling growth patterns and atmospheric nitrogen deposition forms, thereby providing a theoretical foundation for precise seedling cultivation. Methods The research utilized Fraxinus mandshurica with arbuscular mycorrhizal (AM) seedlings and Larix gmelinii with ectomycorrhizal (ECM) seedlings as test subjects to simulate varying concentrations of dry and wet reduced nitrogen deposition: 0 (CK), 35 (ND-35), 70 (ND-70), 35 (NW-35), and 70 kg N ha-1 yr-1 (NW-70). We assessed changes in seedling growth, photosynthetic capacity, root development, and mycorrhizal infection rates of two types of seedlings. Important findings Under conditions of atmospheric dry and wet nitrogen deposition, seedlings of both mycorrhizal types gradually reduce their dependence on mycorrhizal fungi. The primary response forms are the enhancement of their own photosynthetic performance and an increase in root absorption capacity. (1) During dry sedimentation of Fraxinus mandshurica with arbuscular mycorrhizal, a significant enhancement in leaf accumulation and total biomass was observed, attributed to changes in photosynthetic capacity. Under the ND-70 treatment, the net photosynthetic rate, total chlorophyll, and leaf biomass increased by 49.61% compared to the control, with additional increases of 76.29% and 53.84%, respectively. In contrast, during wet deposition, nitrogen use efficiency improved primarily due to an increased contact area between absorbing roots and soil. Under the NW-35 and NW-70 treatments, the surface area of absorbing roots increased by 14.96% and 16.17%, respectively, compared to the control. (2) Larix gmelinii, characterized by ectomycorrhizal roots, exhibited a more pronounced response to wet sedimentation, primarily enhancing the absorptive capacity of the root system through the elongation and thinning of the absorptive roots. Under the NW-70 treatment, the absorptive root length increased by 20.70% compared to the control, while the average absorptive root diameter and cortical thickness decreased by 10.14% and 27.25%, respectively. This research provides an in-depth analysis of the relationship between mycorrhizal types and atmospheric reduced deposition, serving as a reference for the precise nutrient management of seedlings with varying mycorrhizal types.