Responses of Riverine Dissolved Organic Carbon to Global Warming and Permafrost Thaw on the Tibetan Plateau

Abstract

Riverine dissolved organic carbon (DOC) plays a vital role in the global carbon cycle, but its spatiotemporal dynamics and controlling mechanisms in alpine permafrost regions remain unclear. Here, by integrating a comprehensive in situ DOC data set, multi-source environmental variables, and machine learning approaches, we reconstruct the riverine DOC concentration (C_DOC) and flux (F_DOC) at five headwater rivers on the eastern Tibetan Plateau (TP) from 2000 to 2024 and investigate their spatiotemporal patterns. Results show that the mean C_DOC across the five rivers is 3.27 ± 0.96 mg/L but remains highly heterogeneous, with higher values observed in the permafrost-dominated headwaters of the Yellow (4.31 ± 0.78 mg/L) and Yangtze Rivers (3.39 ± 0.86 mg/L). The permafrost coverage, soil organic carbon content, and vegetation type shape this spatial pattern. The southeastern rivers (Mekong and Salween Rivers) exhibit declining C_DOC over the past 25 years, primarily driven by soil moisture reduction under climate warming. In contrast, significant C_DOC increases are detected in Yellow (+0.073 mg L⁻¹ decade⁻¹) and Yangtze headwaters (+0.029 mg L⁻¹ decade⁻¹), due to the enhanced vegetation conditions associated with climate warming and permafrost thaw. Total F_DOC at the outlets of major rivers increases markedly, reaching approximately 1.37 Tg yr⁻¹ in response to the rising river discharge. This study presents the spatiotemporal dynamics of DOC across TP rivers, elucidates the underlying mechanisms, and provides a basis for land-river carbon transfer and regional carbon budget assessments in high-elevation permafrost environments.