Ultraviolet radiation enhances the degradation of humus-like substances in DOC in Permafrost streams of the Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau (QTP) is characterized by high-altitude permafrost, intense ultraviolet (UV) radiation, freeze-thaw cycles, and rapid warming. Permafrost thaw can mobilize large quantities of previously frozen organic carbon into aquatic systems where it can be decomposed as greenhouse gases via biodegradation or photodegradation or both. Yet the degradability mechanisms of DOC in permafrost streams of the QTP are not well understood. Here we sampled riverine water in a typical permafrost catchment named Fenghuoshan (FHS) and explored how the concentration, composition, and degradability of DOC respond to microorganisms and UV radiation, by using laboratory incubations and parallel factor analyses. Our results showed that riverine DOC concentrations varied from 3.59 mg·L-1 to 5.61 mg·L-1 during the initial freezing period. After a 28-day microbial incubation in the dark, 53.3% (41.2%~60.8%) of DOC degrades, which underpins the high biodegradability of DOC. In a 3-day comparison experiment, 30.6% (22.4%~36.7%) of DOC is degraded by UV light after 72h abiotic photodegradation, 1.81 (1.14~2.43) times higher than microbial decomposition. Interestingly, the degradation of DOC rises to 42.9% (32.8%~53.8%) after a microbial incubation under UV radiation, 2.53 (1.85~3.11) times higher than microbial decomposition alone. Spectral analysis showed that microorganisms and UV radiation preferentially degrade non-aromatic compounds and small molecules. DOC components change negligibly during microbial incubations. In contrast, the contribution of terrestrial organic matter and autochthonous dissolved organic matter increased, while the degree of humification and the content of humus-like substances significantly reduced during UV radiation incubations. Overall, these results suggested that UV radiation degrades riverine DOC more intensely than microorganisms. The humus-like substances of DOC are preferentially degraded by UV radiation. Our findings suggest that sunlight is a crucial regulator for DOC degradation in permafrost streams of the QTP.