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 (H₂) 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 H₂ flux dynamics and their climatic and biotic drivers. Net H₂ uptake dominated both forests, with annual sink strengths of −1.28 ± 0.13 and −1.23 ± 0.05 kg H₂ 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 H₂ emissions that partially offset microbial consumption. Litter layers reduced H₂ uptake by 15%–31% seasonally, acting as both a diffusion barrier and a minor photolytic H₂ 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 H₂ sinks to hydroclimatic variability and indicate the need to integrate litter-mediated processes into global H₂ budgets.