Interspecies variation in nitrogen uptake kinetic responses of temperate forest species to elevated CO 2 : potential causes and consequences A Y A L S E W Z E R I H U N and H O R M O Z B A S S I R I R A D *Department of Biological Sciences, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA Abstract 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 interspeci®c variations in N uptake responses to elevated CO 2 , it is not clear whether these are associated with different functional groups. Accordingly, we exam- ined 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 capaci- ties, but the interspeci®c 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 signi®cantly 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 differ- ences. Other possible mechanisms that may explain why elevated CO 2 elicits a species-speci®c response in root N uptake kinetics are discussed. Despite the fact that the exact mechanism(s) for such interspeci®c variation remains unresolved, these dif- ferences may have a signi®cant implication for competitive interactions and commu- nity 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 . Keywords: elevated CO 2 , fast- and slow-growing trees, NH 4 + , NO 3 ± , uptake kinetics Received 3 January 2000; revised version received 4 June 2000 and accepted 5 June 2000 Introduction In the last three decades, extensive research has been carried out to examine the effect of elevated CO 2 on a variety of plant processes. Surprisingly, very little of this effort has been directed to evaluating how elevated CO 2 affects plant capacity to acquire belowground resources such as nitrogen (N) (Luo et al. 1999). This is despite the growing recognition that the responsiveness of plant growth to elevated CO 2 is contingent on plant N acquisition (e.g. Bazzaz 1990; Bernston et al. 1998; Poorter 1998). Both soil factors (e.g. bulk soil solution Correspondence and present address: CSIRO Plant Industry ± Horticulture Unit, Private Mail Bag, Merbein, VIC 3505, Australia, fax + 61 (0) 35051 3111, e.mail: Ayalsew.zerihun@pi. csiro.au Global Change Biology (2001) 7, 211±222 # 2001 Blackwell Science Ltd 211