Climate Warming-Driven Changes in the Molecular Composition of Soil Dissolved Organic Matter Across Depth: A Case Study on the Tibetan Plateau

Dissolved organic matter (DOM) is critical for soil carbon sequestration in terrestrial ecosystems. DOM molecular composition varies with soil depth. However, the spatial heterogeneity of depth-dependent DOM in response to climate warming remains unclear, especially in alpine ecosystems. In this study, the DOM of alpine meadow soil samples was characterized comprehensively by using spectroscopy and mass spectrometry, and open-top chambers (OTCs) were employed to simulate warming. It was found that climate warming had the greatest impact on the upper layer (0–30 cm), followed by the lower layer (60–80 cm), while the middle layer (30–60 cm) was the most stable among the three soil layers. The reasons for the obvious changes in DOM in the upper and lower layers of soil were further explained based on biotic and abiotic factors. Specifically, soil nutrients (NH4+–N, NO3––N, TC, and TP) affected the molecular composition of DOM in layer L1 (0–15 cm), while pH affected layer L5 (60–80 cm). Gemmatimonadetes, Proteobacteria, and Actinobacteria played important roles in the composition of DOM in the L5 layer (60–80 cm), while the dominant fungal groups affecting the DOM composition increased in the L1 layer (0–15 cm) under warming. In summary, this research has contributed to a deeper understanding of depth-dependent changes in DOM molecular composition in alpine ecosystems.