Partitioning and efficiency of use of N in Dactylis glomerata as affected by elevated CO2:: Interaction with N supply

We applied allometry to examine the effects of elevated [CO2] on N use efficiency and N partitioning within Dactylis glomerata L. plants, independent of ontogenetic drift. The plants were grown hydroponically at either ambient (360 muL L-1) or elevated (680 muL L-1) atmospheric [CO2] and four concentrations of NO3-. Independent of N supply, elevated [CO2] increased whole-plant dry matter production compared with N accumulation during very early stages of growth only, indicating a transient increase in plant N use efficiency (i.e., net dry mass gain per net N gain). However, an increase in N supply resulted generally in a decrease in plant N use efficiency throughout ontogeny. When compared at the same age, elevated [CO2] reduced the N concentration (mass percentage) of the whole plant and shoot but did not affect the N concentration of the root. However, CO2 enrichment had hardly any effect on the partitioning of N between shoot and root when analyzed allometrically and independent of the partitioning of dry matter. In contrast, enhanced N supply reduced the allocation of N to roots throughout ontogeny. When compared at the same age, CO2 enrichment reduced the N concentration of leaves when expressed per unit dry mass but not when expressed per unit leaf area, which is related to a decrease in specific leaf area induced by elevated [CO2] during early stages of growth. We conclude that atmospheric CO2 enrichment can increase plant N use efficiency, independent of N availability. This increase may result from physiological changes in plant N use efficiency during very early stages of growth and is more than a size-dependent phenomenon resulting from accelerated dry matter production.