Higher CH4 production in permafrost while oxidation prevails in the active layer: Insights from a forest‒wetland ecotone in Northeast China

Continuous climate warming results in a deeper active layer and elevated CH₄ emissions. However, our knowledge about the CH₄ production and oxidation potentials in active layers and permafrost remains limited. This study explored the CH₄ production and oxidation potentials in these layers and their responses to temperature variations across two habitats in the Da Xing'an Mountains in northeast China. The results showed that the CH₄ production potential was higher in permafrost than in the active layer across forest and wetland habitats (p < 0.05). The temperature sensitivity was greater in the active layer (1.98 ± 0.20 in forests, 3.42 ± 0.37 in wetlands) than in permafrost (1.59 ± 0.13 in forests, 2.08 ± 0.34 in wetlands). CH₄ oxidation potential was higher in the active layer than in permafrost (p < 0.05). However, its temperature sensitivity was higher in permafrost (3.65 ± 0.76 in forests, 2.62 ± 0.50 in wetlands) than in the active layer (3.46 ± 0.66 in forests, 1.90 ± 0.32 in wetlands). The key abiotic driving these processes were pH and soil water content. Amongst microbial properties, CH₄ production was enhanced by the abundance of mcrA genes and methanogenic diversity rather than by community composition. On the contrary, CH₄ oxidation was influenced by pmoA gene abundance, methanotrophic diversity and community composition. Soil parameters and microbial factors accounted for 78% of the variance in CH₄ production and 60% in CH₄ oxidation potential. Overall, this study highlights the different production and oxidation potentials of CH₄ in the active layer and permafrost within a forest‒wetland ecotone, as well as their controlling factors. These findings emphasise the importance of considering changes in the active layer when modelling greenhouse gas emissions from permafrost regions under a warming climate.