Elevated CO2 concentration affects leaf photosynthesis–nitrogen relationships in Pinus taeda over nine years in FACE

To investigate whether long-term elevated carbon dioxide concentration ([CO2]) 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 (Na) in Pinus taeda L. growing in a long-term free-air CO2 enrichment (FACE) facility at an N-limited site. We also determined how maximum rates of carboxylation (Vcmax) and electron transport (Jmax) varied with Na under elevated [CO2]. In trees exposed to elevated [CO2] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity (Anet-Ca) and Na 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 Vcmax and Jmax with Na decreased in 1-year-old needles after up to 9 years of growth in elevated [CO2], 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 [CO2] restored the slopes of the relationships of Vcmax and Jmax with Na to those of control trees (i.e., in ambient [CO2]). The Jmax:Vcmax ratio was unaffected by either [CO2] or N fertilization. Changes in the apparent allocation of N to photosynthetic components may be an important adjustment in pines exposed to elevated [CO2] on low-fertility sites. We conclude that fundamental relationships between photosynthesis or its component processes with Na may be altered in aging pine needles after more than 5 years of exposure to elevated atmospheric [CO2].