Unraveling the groundwater transit time control of permafrost derived dissolved organic carbon processing in a hillslope headwater watershed of Qinghai-Tibetan Plateau

Climate warming leads to massive thaws of the northern permafrost that has increased the release of soil organic carbon (SOC) to streams and rivers partly as dissolved organic carbon (DOC). The transport pathways of SOC releasing into porewater and entering into stream are undergoing profound hydrological changes triggered by permafrost thawing, yet the role that the groundwater plays in processing the permafrost derived DOC is ambiguous. Unravelling how subsurface flow affects permafrost sourced DOC processing is important especially in alpine watersheds of high-altitude permafrost region with extensive surface – groundwater interaction. Here, eight types of water were sampled from a small (25 km²), alpine (elevation 2960 to 4820 m a.s.l) watershed named Hulugou watershed (HLGW) with variably degraded permafrost in the Qinghai-Tibetan Plateau (QTP) in July and September of 2012, 2013 and 2018. The three end-members (glacier-snow meltwater, precipitation, and frozen soil meltwater) analysis suggested contribution of frozen soil meltwater to all types of water with variable DOC levels (0.4 to 22.6 mg L^-1, n = 113), as constrained by δ¹⁸O and electrical conductivity (EC). Spatial patterns of DOC quantity and quality between stream and subsurface waters (groundwater, spring, and seepage-II) point to differences in surface – groundwater exchanges in the upper-, mid- and lower stretch of the watershed. To evaluate the extent of DOC loss (ΔDOC), ΔDOC is calculated using an initial DOC (DOC₀) estimated from mixing of three endmembers, minus the measured DOC concentration. The significant correlations between ΔDOC with proportion of protein-like fluorophores (r = -0.69, p < 0.01) and relatively aromatic C levels (r = -0.62, p = 0.02) indicate ΔDOC corresponding to the extent of microbial utilization of DOC in subsurface environment. Using previously established DOC biodegradation kinetics of 0.25 d^-1 in headwaters of QTP, the mean transit time of groundwater is estimated to be 6 and 20 days based on changes in subsurface ΔDOC of 32% and 74% from the outlet of HLGW for July and September, respectively. The more rapid groundwater transit time corresponds to the higher concentration and more boilable DOC in July (3.5 mg L^-1, protein-like: 98%) than in September (1.0 mg L^-1, protein-like: 53±26%). Together with the mass balance of DOC input and export fluxes showing half loss of C in HLGW, our results indicate that rapid groundwater transit time is associated with permafrost derived DOC processing in alpine hillslope subject to warming.