Research note: Can decreased transpiration limit plant nitrogen acquisition in elevated CO2?

Abstract

N acquisition often lags behind accelerated C gain in plants exposed to CO₂-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 CO₂ 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 [CO₂]. 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 CO₂-enriched or non-enriched air (~1000 vs ~380 μmol mol^–1). We monitored effects of elevated humidity and/or CO₂ on stomatal conductance, whole-plant transpiration, plant biomass gain, and N accumulation. To facilitate the latter, NO₃^– enriched in ¹⁵N (5 atom%) was added to all pots at the outset of the experiment. Transpiration and ¹⁵N accumulation decreased when either CO₂ or humidity were elevated. The disparity between N accumulation and accelerated C gain in elevated CO₂ led to a 19% decrease in shoot N concentration relative to ambient CO₂. Across all treatments, ¹⁵N gain was positively correlated with root mass (P<0.0001), and a significant portion of the remaining variation (44%) was positively related to transpiration per unit root mass. At a given humidity, transpiration per unit leaf area was positively related to stomatal conductance. Thus, declines in plant N concentration and/or content under CO₂ enrichment may be attributable in part to associated decreases in stomatal conductance and transpiration.