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

Rising atmospheric CO 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 CO (eCO ), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO : (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 eCO . 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 CO 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 eCO in new leaves, while the fraction and total carbohydrate concentrations remained unchanged by CO treatment for either new or mature leaves. Photorespiration decreased 31% under eCO supplying less reductant, and in situ reductase activity was concurrently reduced (-34%) in eCO , 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 eCO can contribute to understanding how eCO -induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO 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.