Temperature-dependent soil CH4 fluxes and their drivers across a forest–wetland ecotone in degrading permafrost, Northeast China

Methane (CH 4 ) fluxes in permafrost regions are commonly represented as monotonic functions of temperature, despite pronounced seasonal hydrological variability and habitat heterogeneity. Here, using two years of in situ high-frequency measurements, we examined whether CH 4 fluxes exhibit trajectory-dependent temperature responses across a forest–wetland ecotone in a degrading permafrost landscape, and how hydrological and microbial processes modulated. We found that: (1) Wetland patches consistently acted as CH 4 sources, with an average flux of 5.14 nmol m −2 s −1 , whereas forests exhibited seasonal source–sink shifts and a much lower mean flux of 0.08 nmol m −2 s −1 . Importantly, phase-stratified segmented regressions revealed distinct, nonlinear temperature thresholds and hysteresis in wetlands, with contrasting CH 4 –temperature relationships during warming and cooling phases, while forests showed weaker trajectory dependence. (2) Across habitats and seasons, soil methanogenic communities varied more strongly than methanotrophic communities and explained a larger (35%) of CH 4 flux variability than methanotrophic attributes (21%) and soil properties (24%). (3) Soil moisture and inorganic nitrogen emerged as key factors linking hydrological conditions, microbial communities, and CH 4 dynamics. These findings demonstrate that CH 4 fluxes in permafrost ecotones are governed by the interaction of phase-dependent temperature responses, hydrological constraints, and microbial processes. Incorporating warming–cooling asymmetry and hydrological–microbial coupling may improve the prediction of CH 4 dynamics in heterogeneous permafrost landscapes undergoing thaw.