Hydroclimatic and Abiotic Factors Modulate Soil‐Atmosphere Hydrogen Exchange in Subtropical Forests of Southeast China

Subtropical forests play a critical but poorly constrained role in atmospheric hydrogen (H 2 ) cycling. Here, we conducted a 2‐year field study in two subtropical forests (Evergreen Deciduous Broadleaf Mixed Forest, EDBF; Moso Bamboo Forest, MBF) to quantify soil H 2 flux dynamics and their climatic and biotic drivers. Net H 2 uptake dominated both forests, with annual sink strengths of −1.28 ± 0.13 and −1.23 ± 0.05 kg H 2 ha −1 yr −1 for EDBF and MBF, respectively. Seasonal uptake peaked in summer and autumn, driven by temperature‐dependent microbial activity, but declined obviously during extreme precipitation events due to soil moisture‐induced diffusion limitations. Additionally, dry‐wet cycles enhanced nitrogen fixation, generating sporadic H 2 emissions that partially offset microbial consumption. Litter layers reduced H 2 uptake by 15%–31% seasonally, acting as both a diffusion barrier and a minor photolytic H 2 source (transparent vs. opaque chambers: 8.05 ± 5.19 vs. 3.37 ± 3.36 μg m −2 h −1 ). This inhibitory is significant during dry days but not significant during wet days. Diurnal patterns showed 15% overestimation of daily uptake by daytime‐only sampling. Machine learning (XGBoost‐SHAP) identified precipitation, soil temperature, moisture, and total nitrogen as dominant flux drivers, with nonlinear or linear thresholds governing uptake level. These findings highlight the vulnerability of subtropical H 2 sinks to hydroclimatic variability and indicate the need to integrate litter‐mediated processes into global H 2 budgets.