5 Special Feature Ecology, 87(1), 2006, pp. 5–14  2006 by the Ecological Society of America NITROGEN UPTAKE, DISTRIBUTION, TURNOVER, AND EFFICIENCY OF USE IN A CO 2 -ENRICHED SWEETGUM FOREST RICHARD J. NORBY 1,3 AND COLLEEN M. IVERSEN 2 1 Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6422 USA 2 Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996 USA Abstract. The Progressive Nitrogen Limitation (PNL) hypothesis suggests that eco- systems in a CO 2 -enriched atmosphere will sequester C and N in long-lived biomass and soil organic pools, thereby limiting available N and constraining the continued response of net primary productivity to elevated [CO 2 ]. Here, we present a six-year record of N dynamics of a sweetgum (Liquidambar styraciflua) stand exposed to elevated [CO 2 ] in the free-air CO 2 enrichment (FACE) experiment at Oak Ridge, Tennessee, USA. We also eval- uate the concept of PNL for this ecosystem from the perspective of N uptake, content, distribution, and turnover, and N-use efficiency. Leaf N content was 11% lower on a leaf mass basis (N M ) and 7% lower on a leaf area basis (N A ) in CO 2 -enriched trees. However, there was no effect of [CO 2 ] on total canopy N content. Resorption of N during senescence was not altered by [CO 2 ], so N M of litter, but not total N content, was reduced. The N M of fine roots was not affected, but the total amount of N required for fine-root production increased significantly, reflecting the large stimulation of fine-root production in this stand. Hence, total N requirement of the trees was higher in elevated [CO 2 ], and the increased requirement was met through an increase in N uptake rather than increased retranslocation of stored reserves. Increased N uptake was correlated with increased net primary produc- tivity (NPP). N-use efficiency, however, did not change with CO 2 enrichment because increased N productivity was offset by lower mean residence time of N in the trees. None of the measured responses of plant N dynamics in this ecosystem indicated the occurrence of PNL, and the stimulation of NPP by elevated [CO 2 ] was sustained for the first six years of the experiment. Although there are some indications of developing changes in the N economy, the N supply in the soil at this site may be sufficient to meet an increasing demand for available N, especially as the roots of CO 2 -enriched trees explore deeper in the soil profile. Key words: carbon dioxide; FACE; free-air CO 2 enrichment; Liquidambar styraciflua; net primary productivity; nitrogen uptake; nitrogen-use efficiency; Oak Ridge, Tennessee; plant N dynamics; pro- gressive nitrogen limitation; retranslocation of reserves; sweetgum. INTRODUCTION Forest response to CO 2 enrichment of the atmosphere has long been thought to be controlled by N availability (Kramer 1981, Strain and Bazzaz 1983, Oren et al. 2001, Hungate et al. 2003). Free-air CO 2 enrichment (FACE) experiments support the conclusions from smaller scale experiments that the N status of an eco- system can influence the responses of C assimilation and plant production to elevated [CO 2 ] (Nowak et al. 2004). Conversely, widespread occurrence of increased net fractionation of 15 N between soil and leaf in FACE experiments provides evidence that elevated [CO 2 ] also has an important impact on N dynamics of diverse eco- systems (BassiriRad et al. 2003). Ecosystem models also indicate an important modifying role of N in CO 2 response through interactions involving litter quality and decomposition, litter quantity and soil N immo- Manuscript received 23 December 2004; revised 31 March 2005; accepted 6 May 2005. Corresponding Editor: Y. Luo. For reprints of this Special Feature, see footnote 1, p. 3. 3 E-mail: rjn@ornl.gov bilization, and root production and N uptake or avail- ability (McMurtrie et al. 2000). Luo et al. (2004) have proposed the concept of pro- gressive N limitation (PNL): the process by which available soil N becomes increasingly limited in CO 2 - enriched ecosystems as C and N are allocated to long- lived biomass or soil organic matter (SOM) pools. In- creased productivity and the corresponding increased demand for N are predicted to exacerbate any existing N limitation within the ecosystem and eventually limit the productivity response to elevated [CO 2 ]. Results of CO 2 enrichment experiments have been ambiguous with respect to the occurrence of PNL, and evaluation of forest responses has been particularly problematic. The vast majority of data on C–N interactions in CO 2 - enriched trees comes from individual plants or ex- panding systems that are acquiring increasing amounts of N. Mature or non-expanding forests, however, re- cycle most of their N (Johnson et al. 2004), and this feature cannot be simulated in short-term studies with seedlings and saplings (Norby et al. 1999). FACE ex-