Objective This study aims were to analyze the molecular composition, activity dynamics, and assembly mechanisms of dissolved organic matter (DOM) in peatlands during the freeze-thaw period, in order to deepen the understanding of DOM's response to freeze-thaw processes and to provide a theoretical basis for identifying and protecting the carbon sink function of peatlands in the context of climate change.

Method Surface soil samples from the Dajiuhu peatland were collected during the pre-freeze-thaw (October), mid-freeze-thaw (January), and late freeze-thaw (April) periods. The molecular composition of DOM was analyzed using Fourier transform ion cyclotron resonance mass spectrometry, while microbial communities were characterized through 16S rRNA high-throughput sequencing. A null model was employed to elucidate the assembly mechanisms of DOM and to reveal the transformation dynamics and driving mechanisms of DOM during the freeze-thaw period.

Result The DOM in the peatland was primarily composed of lignin-like compounds with CHO elemental compositions, and their contents remained relatively stable throughout the freeze-thaw period, with values of 64.6% ± 2.4%, 63.4% ± 3.5% and 63.6% ± 2% during the pre-, mid-, and late freeze-thaw periods, respectively. Aromatic compounds were significantly higher in the late freeze-thaw period (9.28% ± 0.46%) compared to the mid-freeze-thaw period (6.56% ± 1.01%), and their relative abundance showed a significant negative correlation with Chloroflexota. Additionally, the aromaticity (AImod: 2.29 ± 2.45) of DOM in the late freeze-thaw period was significantly higher than in the mid-freeze-thaw period (0.71 ± 0.66), and AImod was negatively correlated with the relative abundance of Chloroflexota, suggesting that Chloroflexota may be a key microbial group involved in the metabolism of aromatic compounds in peatlands. Furthermore, active DOM molecules exhibited higher numbers and relative abundances during the mid-freeze-thaw period (88.0% ± 2.8%). The number of active molecules was significantly negatively correlated with temperature and the relative abundance of Myxococcota, while their relative abundance was significantly negatively correlated with the relative abundance of Planctomycetota. These results indicated that these two microbial phyla could be important contributors to the metabolism of active DOM molecules in peatlands. The DOM assembly process also evolved during the freeze-thaw period, with stochastic processes dominating during the pre- and mid-freeze-thaw periods, while heterogeneous selection dominated during the late freeze-thaw period.

Conclusion During the freeze-thaw period in peatlands, the freeze-thaw cycle, through physical disturbances, environmental changes, and the synergistic effects of microbial metabolism, drives the dynamic succession of DOM. This, in turn, regulates its molecular activity and aromatic component structure. The DOM assembly process transitions from stochastic processes in the pre- and mid-freeze-thaw periods to deterministic processes in the late freeze-thaw period. This promotes the accumulation of a large number of active molecules during the mid-freeze-thaw period and drives the succession of DOM toward aromatization in the late freeze-thaw period, ultimately affecting the carbon sink capacity of peatlands and thereby influencing their carbon storage and cycling functions. The results enhances the understanding of DOM dynamics and their multiple driving factors during the freeze-thaw period in peatlands, providing theoretical support for assessing and protecting the carbon sink function of peatlands under climate change.