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 (N-a) 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 (V-cmax) and electron transport (J(max)) varied with N, under elevated [CO2] In trees exposed to elevated [CO2] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity (A(net)-C-a) and N. was significantly reduced by 37% in I-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 [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 V-max and J(max) with N-a to those of control trees (i.e., in ambient [CO2]). The J(max):V-cmax 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].