To examine the impact of elevated CO₂ on root N uptake, soil N availability and the feedbacks between them, we quantified the effects of elevated CO₂ and N additions on root N uptake and leaf C gain in Larrea tridentata seedlings grown in reconstituted Mojave Desert soils. After six months of growth under ambient (380 ppm) or elevated CO₂ (600 ppm), plants were photosynthetically-labeled with ¹³CO₂ while soils were labeled simultaneously with ¹⁵N (glycine, NH₄⁺, or NO₃^-). Plants and soils were then harvested over 7 weeks at 0, 2, 10, 24, and 49 days. Elevated CO₂ decreased both above- and belowground biomass in Larrea tridentata. Nitrogen additions did not improve plant growth, but did increase root N concentrations under elevated CO₂ only. Decreases in root biomass and increases in root N concentrations indicated increased N uptake under elevated CO₂. Elevated CO₂ enhanced the uptake of glycine and NH₄⁺, but not NO₃^- which remained high in both CO₂ treatments. Elevated CO₂ did not affect gross NH₄⁺ fluxes, but did increase soil NH₄⁺ mean residence time and gross NO₃^- consumption rates. Strong correlations between root biomass and gross mineralization and NO₃^- consumption rates reflected the tight coupling between organic matter availability and microbial N cycling in Mojave Desert soils. Decreases in plant biomass under elevated CO₂ could have profound effects on long-term N cycling by negatively affecting C substrate availability for microbes involved in N mineralization and NO₃^- consumption. Decreases in both aboveground and belowground inputs, however, could be offset potentially by increases in litter quality.