Dissolved organic matter (DOM), the mobile form of soil organic matter (SOM), plays an important role in soil C cycling and in nutrient transport. We investigated the effects of 5 years of CO₂ enrichment (370 versus 570 μmol CO₂ mol⁻¹) on DOM dynamics at the alpine treeline, including the analysis of fast-cycling components such as low molecular weight organic acids (LMWOAs), dissolved organic carbon (DOC) biodegradability, and the decomposition of ¹⁴C-labeled oxalate. Concentrations of DOC in canopy throughfall were 20% higher at elevated CO₂, probably driven by higher carbohydrate concentrations in leaves. In the organic soil layer, 5 years of CO₂ enrichment increased water-extractable organic C by 17% and soil solution DOC at 5 cm depth by 20%. The ¹³C tracing of recently assimilated CO₂ revealed that the input of recent plant-derived C (<15% of total DOC) was smaller than the CO₂-induced increase in DOC. This strongly suggests that CO₂ enrichment enhanced the mobilization of native DOC, which is supported by significant increases in dissolved organic nitrogen (DON). We mainly attribute these increases to a stimulated microbial activity as indicated by higher basal and soil respiration rates (+27%). The ¹⁴C-labeled oxalate was more rapidly mineralized from high CO₂ soils. The concentrations of LMWOAs, but also those of “hydrophilic” DOC and biodegradable DOC (6% of total DOC), were, however, not affected by elevated CO₂, suggesting that production and consumption of “labile” DOC were in balance. In summary, our data suggest that 5 years of CO₂ enrichment speeded up the cycling of “labile” DOM and SOM in a late successional treeline ecosystem and increased the mobilization of older DOM through a stimulated microbial activity. Such a “priming effect” implies that elevated CO₂ can accelerate the turnover of native SOM, and thus, it may induce increasing losses of old C from thick organic layers.