Ultraviolet radiation stimulates the degradability of groundwater-fed DOC during the baseflow period of streams on the Qinghai-Tibet Plateau permafrost region

Abstract

As the climate warms and permafrost thaws, large quantities of dissolved organic carbon (DOC) enter streams via surface or subsurface flows and undergo biodegradation and photodegradation. Deciphering the transformation mechanisms of riverine DOC are crucial to understand the riverine carbon cycle and carbon-climate feedback. Yet the degradation characteristics of groundwater-fed DOC in streams of the Qinghai-Tibet Plateau (QTP) are not well understood. Here we sampled riverine DOC in a catchment of the QTP during the baseflow period and explored how the concentration, composition, and degradation of DOC respond to microorganisms and UV radiation, by combining laboratory incubations, ultraviolet–visible absorption, fluorescence spectroscopy and parallel factor analyses. Photo-, bio- and photo-bio-degradation experiments were conducted in parallel. Our results showed that riverine DOC concentrations varied from 3.59 mg·L^-1 to 5.61 mg·L^-1 during the baseflow period. After 28-day biodegradation experiments, 53.3 % (41.2 %–60.8 %) of DOC were degraded, which underpins the high biodegradability of DOC. In 3-day photodegradation and photo-biodegradation experiments, 30.6 % (22.4 %–36.7 %) and 42.9 % (32.8 %–53.8 %) of DOC were degraded, respectively. We found that microorganisms and UV radiation degraded non-aromatic compounds and small molecules. The relative abundances of C1 and C2 changed negligibly by microorganisms. In contrast, UV radiation reduced the degree of humification as a result of significant degradation of terrestrial humic-like substances (C2). Synergistic effects were also found in the tandem photo- and microbial degradation experiments within 48 h. Our findings demonstrated that UV radiation enhances the degradability of groundwater-fed DOC by preferentially degrading the terrestrial humic-like substances, and sunlight is a crucial regulator for groundwater-fed DOC degradation in streams of the QTP.