Significance: How terrestrial plants use N and respond to soil N loading is central to evaluating and predicting changing ecosystem structure and function with climate warming and N pollution. Here, evidence from NO₃⁻in plant tissues has uncovered the uptake and assimilation of soil NO₃⁻by Arctic tundra plants, which has long been assumed negligible. Soil NO₃⁻contributed about one-third of the bulk N used by tundra plants of northern Alaska. Accordingly, the importance of soil NO₃⁻for tundra plants should be considered in future studies on N and C cycling in Arctic ecosystems where C sequestration is strongly determined by N availability.