Interspecies variation in nitrogen uptake kinetic responses of temperate forest species to elevated CO2: potential causes and consequences

Despite the recognition that the capacity to acquire N is critical in plant response to CO2 enrichment, there is little information on how elevated CO 2 affects root N uptake kinetics. The few available data indicate a highly variable pattern of response to elevated CO2, but it is presently unclear i f the observed inconsistencies are caused by differences in experimental pr otocols or by true species differences. Furthermore, if there are interspec ific variations in N uptake responses to elevated CO2, it is not clear whet her these are associated with different functional groups. Accordingly, we examined intact root-system NH4+ and NO3- uptake kinetic responses to eleva ted CO2 in seedlings of six temperate forest tree species, representing (i) fast- vs. slow-growers and (ii) broad-leaves vs. conifers, that were cultu red and assayed in otherwise similar conditions. In general, the species te sted had a higher uptake capacity (V-max) for NH4+ than for NO3-. Species s ubstantially differed in their NO3- and NH4+ uptake capacities, but the int erspecific differences were markedly greater for NO3- than NH4+ uptake. Ele vated COP had a species-dependent effect on root uptake capacity for NH4+ r anging from an increase of 215% in Acer negundo L. to a decrease of about 4 0% in Quercus macrocarpa Michx. In contrast, NO3- uptake capacity responded little to CO2 in all the species except A. negundo in which it was signifi cantly down-regulated at elevated CO2. Across species, the capacity for NH4 + uptake was positively correlated with the relative growth rate (RGR) of s pecies; however, the CO2 effect on NH4+ uptake capacity could not be explai ned by changes in RGR. The observed variation in NH4+ uptake response to el evated CO2 was also inconsistent with life-form differences. Other possible mechanisms that may explain why elevated CO2 elicits a species-specific re sponse in root N uptake kinetics are discussed. Despite the fact that the e xact mechanism(s) for such interspecific variation remains unresolved, thes e differences may have a significant implication for competitive interactio ns and community responses to elevated CO2 environment. We suggest that dif ferential species responses in nutrient uptake capacity could be one potent ial mechanism for the CO,induced shifts in net primary productivity and spe cies composition that have been observed in experimental communities expose d to elevated levels of CO2.