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To investigate whether long-term elevated carbon dioxide concentration ([CO 2 ]) causes declines in photosynthetic enhancement and leaf nitrogen (N) owing to limited soil fertility, we measured photosynthesis, carboxylation capacity and area-based leaf nitrogen concentration ( N  a ) in  Pinus taeda  L. growing in a long-term free-air CO 2  enrichment (FACE) facility at an N-limited site. We also determined how maximum rates of carboxylation ( V  cmax ) and electron transport ( J  max ) varied with  N  a  under elevated [CO 2 ]. In trees exposed to elevated [CO 2 ] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity ( A  net-Ca ) and  N  a  was significantly reduced by 37% in 1-year-old needles, whereas it was unaffected in current-year needles. The slope of the relationships of both  V  cmax  and  J  max  with  N  a  decreased in 1-year-old needles after up to 9 years of growth in elevated [CO 2 ], which was accompanied by a 15% reduction in N allocation to the carboxylating enzyme. Nitrogen fertilization (110 kg N ha −1 ) in the ninth year of exposure to elevated [CO 2 ] restored the slopes of the relationships of  V  cmax  and  J  max  with  N  a  to those of control trees (i.e., in ambient [CO 2 ]). The  J  max : V  cmax  ratio was unaffected by either [CO 2 ] or N fertilization. Changes in the apparent allocation of N to photosynthetic components may be an important adjustment in pines exposed to elevated [CO 2 ] on low-fertility sites. We conclude that fundamental relationships between photosynthesis or its component processes with  N  a  may be altered in aging pine needles after more than 5 years of exposure to elevated atmospheric [CO 2 ].

Elevated CO 2 concentration affects leaf photosynthesis–nitrogen relationships in Pinus taeda over nine years in FACE