Molecular insights into the warming-induced alterations of water-extractable organic matter (WEOM) in soil:...

Arctic warming is accelerating at a rate approximately four times faster than the global average, exerting profound effects on soil organic matter and microbial activity, particularly in permafrost regions rich in soil carbon stocks. This study investigates the molecular composition of water-extractable organic matter (WEOM) in response to a 7-year period of warming via open-top chambers across different soil layers in a dry Arctic tundra ecosystem. We focused on elucidating the depth-dependent responses of WEOM to warming, emphasizing compositional shifts and proportional changes in WEOM constituents using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Our results indicate that the organic layer exhibited minimal changes in soil properties and WEOM composition in response to warming. In contrast, the mineral layer demonstrated significant alterations with warming, including increased total dissolved nitrogen content, enhanced biological activity, and shifts in WEOM molecular composition. Particularly, the warming treatment led to an increase in the abundance of highly unsaturated and phenolic compounds (HUP) and peptide-like compounds in the mineral layer, reflecting enhanced microbial utilization of WEOM. This study underscores the critical importance of considering soil depth and layer when assessing the ecological impacts of climate warming, particularly in Arctic regions where microbial activities remain limited. These results suggest that warming-induced changes in the mineral layer may reflect stimulation of microbial communities by belowground processes, such as rhizosphere expansion or organic input from upper soil horizons, although these mechanisms remain to be directly confirmed. These results provide valuable insights into the mechanisms driving WEOM transformations under warming conditions, contributing to a more comprehensive understanding and prediction of biogeochemical processes occurring in a warming Arctic.