The Greater and Lesser Khingan Mountains ecotone exhibits transitional characteristics between northern coniferous forests and cold-temperate mixed coniferous-broadleaf forests. However, it remains unknown how vegetation characteristics and soil properties jointly influence the structural patterns of soil microbial communities within the forest systems of this ecological transition zone. Therefore, we used high-throughput sequencing and soil physicochemical analysis to investigate the response mechanisms of soil microbial communities during forest succession (five representative natural secondary forests) and their environmental driving factors. The results showed that the bacterial communities in the Greater and Lesser Khingan Mountains ecotone were primarily dominated by Proteobacteria, Acidobacteriota, Actinobacteriota, Chloroflexi, and Verrucomicrobiota, while the fungal communities were primarily dominated by Basidiomycota and Ascomycota. Their relative abundances exhibited significant differences dependent on vegetation types. Different vegetation types significantly influenced the α-diversity of soil microbial communities at the study site; however, there were no significant differences in microbial α-diversity except for the bacterial Chao 1 index. The pH, NO₃⁻-N, NH₄⁺-N, MAOC, TN, TP, and DOC significantly influenced bacterial communities, while DOC and pH were the key environmental factors shaping soil fungal communities. This study reveals the synergistic succession patterns among vegetation, soil, and microorganisms, providing a microbiological basis for high-latitude cold-region forest restoration under climate change.

Distinct paradigms, such as the “enzymic latch” and “iron gate” theories, have been proposed to elucidate SOC loss or accumulation, but their relative significance and whether they are mutually exclusive in permafrost peatlands remain unclear. To address this, we evaluated their relative importance and identified the dominant factors controlling SOC stability. Therefore, we employed a space-for-time substitution approach across a permafrost gradient (continuous, discontinuous, and isolated) by systematically quantifying extracellular enzyme activities, iron (Fe) phases, and iron-bound soil organic carbon (Fe-SOC) at various depths (0–10, 10–30, and 30–50 cm) in peatlands. Our results did not support the “enzymic latch” theory, with hydrolytic enzyme activities (β-glucosidase (BG), cellobiohydrolase (CBH), and β-N-acetylglucosaminidase (NAG)) showing positive correlations with phenolics but negative correlations with phenol oxidase (PHO) activity. However, ferrous iron (Fe(II)) was significantly positively correlated with PHO activity, and ferric iron (Fe(III)) stabilized SOC through co-precipitation with it to form Fe-SOC, supporting the “iron gate” theory. Moreover, Fe-SOC decreased from the continuous to the isolated permafrost zone, and with soil depth from 0–10 cm to 30–50 cm. Partial least squares path modeling (PLS-PM) analysis indicated that Fe(III) directly and indirectly (via Fe-SOC and phenolics) affected SOC. Our study demonstrated the primacy of the “iron gate” mechanism in controlling carbon stability in the Great Hing’an Mountains permafrost peatlands, providing new insights for projecting carbon-climate feedback.

Global warming is profoundly altering the structure and function of permafrost peatland ecosystems, but how soil microorganisms as core regulators of biogeochemical cycles respond to the process remains unclear. We investigated peatlands of the continuous, the discontinuous, and the isolated permafrost zones in the climatically sensitive high-latitude Greater Khingan Mountains using 16S rRNA gene high-throughput sequencing and soil physicochemical analysis to systematically reveal the spatial differentiation patterns, community assembly processes, and primary environmental factors of bacterial communities. The results indicated that bacterial alpha diversity was highest in the discontinuous permafrost zone, and both permafrost type and soil depth exerted significant effects on bacterial community composition. From the continuous to the isolated permafrost zones, the relative abundance of the dominant phylum Proteobacteria decreased, while phylum Chloroflexota showed a gradual increasing trend. Co-occurrence network analysis suggested that bacterial network complexity was highest in the continuous permafrost zone, and network stability decreased along the permafrost gradient. From the continuous to the isolated permafrost zone, the relative contribution of stochastic processes declined, whereas that of deterministic processes increased. Partial least squares path modeling (PLS-PM) further demonstrated that soil pH, total organic carbon (TOC), total nitrogen (TN), and soil water content (SWC) were major drivers of bacterial communities, with their effects differing among permafrost zones. Our study elucidated the synergistic evolutionary patterns of bacterial community composition, assembly mechanisms, and environmental drivers under permafrost degradation, providing key scientific evidence for predicting the feedback of high-latitude peatlands to climate warming.