Can decreased transpiration limit plant nitrogen acquisition in elevated CO ?

N acquisition often lags behind accelerated C gain in plants exposed to CO 2 -enriched atmospheres. To help resolve the causes of this lag, we considered its possible link with stomatal closure, a common first-order response to elevated CO 2 that can decrease transpiration. Specifically, we tested the hypothesis that declines in transpiration, and hence mass flow of soil solution, can decrease delivery of mobile N to the root and thereby limit plant N acquisition. We altered transpiration by manipulating relative humidity (RH) and atmospheric [CO 2 ]. During a 7-d period, we grew potted cottonwood ( Populus deltoides Bartr.) trees in humidified (76% RH) and non-humidified (43% RH) glasshouses ventilated with either CO 2 -enriched or non-enriched air (~1000 vs ~380 μmol mol –1 ). We monitored effects of elevated humidity and/or CO 2 on stomatal conductance, whole-plant transpiration, plant biomass gain, and N accumulation. To facilitate the latter, NO 3 – enriched in 15 N (5 atom%) was added to all pots at the outset of the experiment. Transpiration and 15 N accumulation decreased when either CO 2 or humidity were elevated. The disparity between N accumulation and accelerated C gain in elevated CO 2 led to a 19% decrease in shoot N concentration relative to ambient CO 2 . Across all treatments, 15 N gain was positively correlated with root mass ( P