Climate warming has accelerated thawing of frozen soil in the northern permafrost, supplying dissolved organic carbon (DOC) to streams and rivers with uncertain fate. Although recent incubation experiments have established that permafrost derived DOC is labile, field evidence is rare and ambiguous, with the linkage to surface – groundwater interaction poorly illustrated. Here, we quantify and characterize DOC for eight types of water sampled from a small (25km), alpine (elevation 2960 to 4820 m a.s.l) watershed with variably degraded permafrost in the Qinghai-Tibetan Plateau (QTP) in July and September of 2012, 2013 and 2018. Spatially variable dissolved organic carbon (DOC) concentrations with high percentages of protein-like fluorophores (48± 41%, n=91), attributable to frozen soil based on tracers, are detected throughout the watershed. Increasing DDOC (loss of DOC) in subsurface waters corresponds to decreasing proportion of protein-like fluorophores and SUVA. Assuming microbial processing of subsurface DOC and using previously established DOC biodegradation kinetics, the mean transit time of groundwater is estimated to be ~ 7 and 25 days based on changes in DDOC of 32% and 74% for July and September, respectively. In addition to providing field evidence for prevalence of labile DOC derived from permafrost in surface and subsurface waters of the QTP, the study establishes that very young groundwater participates in alpine hillslope hydrological and biogeochemical processes. Mass balance of DOC input and export fluxes shows a loss of nearly half of the carbon, indicating that hillslopes are hotspots for DOC processing, with subsurface environment playing a key role.

Climate warming has accelerated thawing of northern permafrost, resulting in changes to the supply of dissolved organic carbon (DOC) to inland waters with uncertain fate. Extensive surface – groundwater interactions occur in alpine permafrost watersheds and likely influence DOC processing differently than systems with limited interactions. Here, we quantify and characterize DOC in waters collected from eight water types sampled across a small (25 km2) alpine (elevation 2960 to 4820 m a.s.l) watershed in the Qinghai-Tibetan Plateau (QTP) containing variably degraded permafrost. Three types of water (thermokarst ponds, red mud gully and seepage-I) contained high DOC concentrations (5.2 to 22.6 mg/L, n=38), with C contributions predominantly from frozen soil meltwater. Spatial patterns of DOC in stream (0.3 to 4.8 mg L-1, n=41), and subsurface waters (0.4 to 3.8 mg/L, n=34), all contained frozen soil meltwater C as constrained by δ18O and electrical conductivity, reflecting surface – groundwater exchanges in the upper-, mid- and lower stretches of the watershed. Further, patterns of increasing DOC loss (ΔDOC) in subsurface waters with decreased proportions of protein-like organic matter and SUVA254, suggest subsurface microbial processing. Using previously established biodegradation DOC kinetics (0.06 d-1) from the QTP, the groundwater transit time is estimated to be between 6 and 20 days based on ΔDOC changes of 32% and 74% for July and September, respectively. Mass balance of DOC inputs and export fluxes demonstrate nearly half of all DOC was lost in this small watershed, indicating hillslopes are hotspots for DOC processing, with subsurface environments playing a key role.