Ultraviolet radiation rather than inorganic nitrogen increases dissolved organic carbon biodegradability in a typical thermo-erosion gully on the Tibetan Plateau.

Permafrost thaw could lead to frozen carbon (C) being laterally transferred to aquatic systems as dissolved organic carbon (DOC). If this part of DOC has high biodegradability, it could be decomposed during the delivery process, release greenhouse gases to the atmosphere and trigger positive C-climate feedback. Thermokarst is an abrupt permafrost thaw process that can enhance DOC export and also impact DOC processing through increased inorganic nitrogen (N) and ultraviolet (UV) light exposure. Especially on the Tibetan Plateau, where thermokarst develops widely and suffers from serious UV radiation and N limitation. However, it remains unclear how thermokarst-impacted biodegradable DOC (BDOC) responds to inorganic N addition and UV radiation. Here, we explored the responses of DOC concentration, composition and its biodegradability to inorganic N and UV amendments in a typical thermokarst on the Tibetan Plateau, by using laboratory incubations with spectral analyses (UV-visible absorption and three-dimensional fluorescence spectra) and parallel factor analyses. Our results showed that BDOC in thermokarst outflows was significantly higher than in reference water. Our results also revealed that inorganic N addition had no influence on thermokarst-impacted BDOC, whereas exposure to UV light significantly increased BDOC by as much as 2.3 times higher than the dark-control. Moreover, N addition and UV radiation did not generate additive effects on BDOC. Our results further illustrated that dissolved organic matter (DOM) composition explained more of the variability in BDOC, while the nutrients and other physicochemical properties played a minor role. Overall, these results imply that UV light rather than inorganic N significantly increases thermokarst-derived BDOC, potentially strengthening the positive permafrost C-climate feedback.