Interspecies variation in nitrogen uptake kinetic responses of temperate forest species to elevated CO 2 : potential causes and consequences

Summary Despite the recognition that the capacity to acquire N is critical in plant response to CO 2 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 CO 2 , 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 CO 2 , it is not clear whether these are associated with different functional groups. Accordingly, we examined intact root‐system NH 4 + and NO 3 – uptake kinetic responses to elevated CO 2 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 NH 4 + than for NO 3 – . Species substantially differed in their NO 3 – and NH 4 + uptake capacities, but the interspecific differences were markedly greater for NO 3 – than NH 4 + uptake. Elevated CO 2 had a species‐dependent effect on root uptake capacity for NH 4 + ranging from an increase of 215% in Acer negundo L. to a decrease of about 40% in Quercus macrocarpa Michx. In contrast, NO 3 – uptake capacity responded little to CO 2 in all the species except A. negundo in which it was significantly down‐regulated at elevated CO 2 . Across species, the capacity for NH 4 + uptake was positively correlated with the relative growth rate (RGR) of species; however, the CO 2 effect on NH 4 + uptake capacity could not be explained by changes in RGR. The observed variation in NH 4 + uptake response to elevated CO 2 was also inconsistent with life‐form differences. Other possible mechanisms that may explain why elevated CO 2 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 CO 2 environment. We suggest that differential species responses in nutrient uptake capacity could be one potential mechanism for the CO 2 ‐induced shifts in net primary productivity and species composition that have been observed in experimental communities exposed to elevated levels of CO 2 .