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 CO2 affects root N uptake kinetics. The few available data indicate a highly variable pattern of response to elevated CO2, but it is presently unclear if the observed inconsistencies are caused by differences in experimental protocols or by true species differences. Furthermore, if there are interspecific variations in N uptake responses to elevated CO2, it is not clear whether these are associated with different functional groups. Accordingly, we examined intact root-system NH4+ and NO3- uptake kinetic responses to elevated CO2 in seedlings of six temperate forest tree species, representing (i) fast- vs. slow-growers and (ii) broad-leaves vs. conifers, that were cultured and assayed in otherwise similar conditions. In general, the species tested had a higher uptake capacity (V-max) for NH4+ than for NO3-. Species substantially differed in their NO3- and NH4+ uptake capacities, but the interspecific differences were markedly greater for NO3- than NH4+ uptake. Elevated COP had a species-dependent effect on root uptake capacity for NH4+ ranging from an increase of 215% in Acer negundo L. to a decrease of about 40% in Quercus macrocarpa Michx. In contrast, NO3- uptake capacity responded little to CO2 in all the species except A. negundo in which it was significantly down-regulated at elevated CO2. Across species, the capacity for NH4+ uptake was positively correlated with the relative growth rate (RGR) of species; however, the CO2 effect on NH4+ uptake capacity could not be explained by changes in RGR. The observed variation in NH4+ uptake response to elevated CO2 was also inconsistent with life-form differences. Other possible mechanisms that may explain why elevated CO2 elicits a species-specific response in root N uptake kinetics are discussed. Despite the fact that the exact mechanism(s) for such interspecific variation remains unresolved, these differences may have a significant implication for competitive interactions and community responses to elevated CO2 environment. We suggest that differential species responses in nutrient uptake capacity could be one potential mechanism for the CO,induced shifts in net primary productivity and species composition that have been observed in experimental communities exposed to elevated levels of CO2.