Lower photorespiration in elevated CO₂ reduces leaf N concentrations in mature Eucalyptus trees in the field

Rising atmospheric CO₂ concentrations is expected to stimulate photosynthesis and carbohydrate production, while inhibiting photorespiration. By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO₂ (eCO₂ ), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO₂ : (a) A "dilution effect" caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO₂ . This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature Eucalyptus forest exposed to Free-Air CO₂ Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate ( $$ ) concentrations, carbohydrates and $$ reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO₂ in new leaves, while the $$ fraction and total carbohydrate concentrations remained unchanged by CO₂ treatment for either new or mature leaves. Photorespiration decreased 31% under eCO₂ supplying less reductant, and in situ $$ reductase activity was concurrently reduced (-34%) in eCO₂ , especially in new leaves during summer periods. Hence, $$ assimilation was inhibited in leaves of E. tereticornis and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of $$ reductase activity due to lower photorespiration in eCO₂ can contribute to understanding how eCO₂ -induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO₂ on N biogeochemistry include both mechanisms, especially where $$ is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.