Hillslopes in Headwaters of Qinghai‐Tibetan Plateau as Hotspots for Subsurface Dissolved Organic Carbon Processing During Permafrost Thaw

Abstract 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 water and 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 types of water sampled across a small (25 km 2 ) alpine (elevation 2,960–4,820 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 (5.2–22.6 mg L −1 , n = 38), with carbon contributions predominantly from frozen soil meltwater. Spatial patterns of DOC in stream (0.3–4.8 mg L −1 , n = 41), and subsurface waters (0.4–3.8 mg L −1 , n = 34), all contained frozen soil meltwater carbon as constrained by δ 18 O and electrical conductivity, reflecting surface‐groundwater exchanges in the upper‐, middle‐, and lower stretches of the watershed. Further, patterns of increasing DOC loss in subsurface waters with decreased proportions of protein‐like organic matter and specific UV absorbance at 254 nm, suggest subsurface microbial processing. Using previously established biodegradation DOC kinetics (0.06 day −1 ) from the QTP, the groundwater transit time is estimated to be between 6 and 20 days based on DOC loss 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.