Forest types predominantly regulate soil-dissolved organic matter dynamics along an elevational gradient in the Hengduan Mountains

Abstract Soil dissolved organic matter (DOM) is vital in terrestrial ecosystem carbon (C) cycling; yet how forest types and elevation regulate soil DOM dynamics in mountain ecosystems remains poorly 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 and 3500 m) than in coniferous forest soil (at 2600 and 3200 m), independent of elevation. DOC was higher 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 that litter traits and soil properties are the predominant regulators of soil DOM mechanism along elevational gradient. This suggests that soil DOM dynamics associated with tree species in alpine mountain ecosystems may differentially influence soil C sequestration under future climate change scenarios.