Seasonal reversal of river-groundwater exchange controls dissolved organic matter dynamics across the arid margin of the Tibetan Plateau.

Dissolved organic matter (DOM) is a key regulator of carbon cycling and water quality within surface water-groundwater interaction (SGI) zones. However, how the seasonal reorganization of SGI governs DOM dynamics in high-altitude arid basins remains poorly understood. Focusing on the Golmud River Basin along the arid Tibetan Plateau margin, this study systematically examines the spatiotemporal evolution of river-groundwater exchange and its influence on DOM sources, composition, and transport along a continuous transect from mountainous headwaters through alluvial fans to salinized plains. Results showed that hydrological connectivity exhibited distinct seasonal reversal. During high-flow periods, meltwater-driven river infiltration enhanced surface water-groundwater coupling, whereas channel desiccation in low-flow periods shifted recharge to lateral flow, substantially weakening SGI. This hydrological pathway switching profoundly reshaped the DOM biogeochemistry. Riverine DOM was dominated by runoff-sensitive humic substances, whereas groundwater DOM was enriched in stable microbial components under weakened SGI. Storm-driven flushing triggered rapid vertical transport of fresh protein-like components (C3) in headwaters and increased terrestrial humic substances (C1) in surface water. Conversely, low-flow periods marked a transition to biogeochemical control, where prolonged hydraulic residence times promoted in-situ microbial metabolism, transforming allochthonous precursors into biostable, autochthonous forms (C4). Integrated multiscale geographically weighted regression and structural equation modeling revealed that groundwater DOM dynamics were primarily governed by meteorological forcing. During high-flow periods, evaporation/precipitation dominated DOM regulation (47.8%), indirectly reshaping DOM structure through modulation of meltwater inputs and water-level fluctuations. In contrast, during low-flow periods, regulatory control shifted toward hydrological connectivity and salinity enrichment, with groundwater level emerging as the primary driver (49.7%). These results highlighted a seasonal "on-off" switching role of SGI in controlling DOM transport, provided process-based insights into water quality management and carbon budget assessment in climate-sensitive inland ecosystems.