Technical Abstract: While rising atmospheric [CO2] has the potential to enhance plant growth, it has been suggested that this response may be constrained by soil nitrogen (N) availability. Here we demonstrate in a five-year field study conducted in a semiarid grassland that plant growth, plant N uptake, and soil N availability remained significantly higher under elevated than under ambient [CO2] (720 vs. 368 ppm). A novel 15N tracer method revealed that persistently greater plant N uptake throughout five years of elevated [CO2] was due to greater soil N mineralization. Increased soil moisture and root activity under elevated [CO2] likely enhanced soil N mineralization. These results are in stark contrast to several other grassland field studies in which reduced soil N availability has been observed under elevated [CO2]. Our results suggest that increases in soil moisture due to higher plant water use efficiency under elevated [CO2] may increase N mineralization and thus delay the onset of reduced soil N availability seen in wetter climates where N mineralization is less constrained by soil moisture. Climate may therefore mediate the extent to which ecosystems respond to elevated [CO2].