Abstract: Soil dissolved organic matter (DOM) is vital in terrestrial ecosystem carbon (C) cycling; however, the regulatory effects of forest types and elevations on soil DOM dynamics in mountain ecosystems remain incompletely understood. Here, we investigated DOM content, spectroscopic characteristics, molecular traits and their potential drivers along an elevational gradient (2600-3500 m) in the Hengduan Mountains. Our results showed that soil dissolved organic C (DOC) content was higher in broad-leaf forest soil (at 2900 m and 3500 m) than in coniferous forest soil (at 2600 m and 3200 m) irrespective of elevation, with a greater amount in wet season than in dry season. Humification index (HIX) trends aligned with the DOC content, while the aromaticity index (AI) showed an inverse relationship. These patterns were linked to the quality of litter carbon sources. Molecular-level analysis of DOM suggested that lignins/CRAM-like structure compounds and tannins predominated in soil DOM, indicating that the molecular composition of soil DOM was typical of plant-derived sources in our study region. Additionally, the relative abundance of lignin compounds decreased gradually with increasing elevation during the dry season. We detected that soil properties (especially, NH₄⁺-N content) predominantly mediated DOM dynamics in dry season, whereas litter traits (i.e., leaf-DOC content) were the key factors across elevations in wet season. Overall, our results revealed litter traits and soil properties predominantly regulated soil DOM mechanism along elevational gradient, indicating that soil DOM dynamics associated with tree species in alpine mountain ecosystems may differentially influence soil C sequestration under future climate change scenarios.

Key words: alpine forest, climates, vegetation types, litter traits, soil carbon dynamics