The structure and function of Arctic stream ecosystems are changing because of climate warming, with landscape shifts such as permafrost thaw and shrub expansion altering C and N inputs to receiving waters. Changes in the form and relative availability of C and N are likely to have implications for stream metabolism and nutrient cycling. Here, we assess how future increases in C availability may influence N uptake in Arctic headwater streams by combining solute additions, metabolic tracers, and other ancillary measurements. We conducted 4 short-term, constant-rate solute additions in 2 streams in which we added NH₄⁺ and NO₃⁻ individually, with and without a co-release of labile C (acetate). To better understand the response of metabolic and biogeochemical processes to changes in C and N availability, we also measured resazurin-resorufin transformation, N to Ar ratios, and greenhouse gas concentrations. Additions of labile C increased NH₄⁺ uptake by 61 to 63% but had variable effects on NO₃⁻ uptake, ranging from a 24% decrease to a 14% increase. Our combined data on resazurin and greenhouse gas concentrations suggest that heterotrophic assimilation may be a dominant pathway for dissolved inorganic N uptake in Arctic stream ecosystems. When combined with data on N uptake across Arctic and non-Arctic biomes, we found that higher ratios of dissolved organic C to dissolved inorganic N (DOC∶DIN) corresponded with higher rates of NO₃⁻ uptake globally, whereas NH₄⁺ uptake showed no consistent relationship with DOC∶DIN. Collectively, our results suggest that future increases in C availability in Arctic streams may have more immediate and short-term effects on NH₄⁺ cycling, but as DOC∶DIN ratios shift over longer time periods we can expect a corresponding shift in the capacity for stream biota to process NO₃⁻. Such changes may alter the retention and downstream export of biologically available N, with implications for nutrient availability, primary production, and ecosystem functioning across Arctic freshwater ecosystems.