While high-Arctic landscapes are at the frontlines of climate change, there remain knowledge gaps about how climate-induced warming will modify coupled nutrient and carbon (C) dynamics in headwater streams. Thermokarst processes, including retrogressive thaw slumping (RTS) along Arctic stream banks, can mobilize permafrost C, fundamentally altering stream biogeochemistry. To understand the impact of RTS-mobilized C on nitrogen (N) and phosphorus (P) cycling, we conducted short-term solute additions in paired reaches (RTS-impacted reach vs. upstream reference) of two streams in the Zackenberg Basin (74ºN) in NE Greenland. Both Kaerelv (Q=11-16 L/s) and Graenselv (Q=19-20 L/s) have been shown previously to be co-limited by N+P, but have recently (<2 yrs) exhibited RTS. We hypothesized that RTS-mobilized organic C could stimulate heterotrophic N and P uptake. We conducted ammonium (NH4+-N), nitrate (NO3--N), and soluble reactive phosphorus (SRP) releases, with and without acetate, allowing us to quantify nutrient spiraling metrics, before and after the addition of labile C, in reaches with and without RTS. For NH4+-N, uptake velocities (Vf) were consistently higher in the RTS reach (10-11.5 mm/min), compared to upstream (5.8-7.9), but NH4+-N+C consistently decreased NH4+-N Vf (7.4-7.6). Surprisingly, this suggests that C-priming may stimulate mineralization of benthic NH4+-N and decrease overall N demand. For NO3--N, Vf was slightly higher in RTS reaches (5.9-6.1 mm/min) compared to upstream (4.7-5.1), but co-addition of acetate decreased NO3--N Vf in all but one reach. In general, NO3--N demand was lower than NH4+-N. Finally, SRP Vf was higher in the RTS reach for Kaerelv (13.5 mm/min) than upstream (5.6), and the co-addition of acetate decreased SRP Vf in both reaches (4.2-4.7). In both streams, the ambient chemistry in RTS vs. upstream reaches gave no indication of RTS-related differences. In contrast, short-term solute additions suggest that very small increases in availability can stimulate nutrient demand, and that C, possibly supplemented by RTS events, can alter nutrient demand in surprising ways, stimulating mineralization and decreasing nutrient demand from the water column. These results emphasize the need to understand solute-specific patterns of nutrient uptake, especially given the implications for nutrient-limited coastal waters.