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 (eCO(2)), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO(2): (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(2). 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 (NO3-) concentrations, carbohydrates and NO3- reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO(2) in new leaves, while the NO3- fraction and total carbohydrate concentrations remained unchanged by CO2 treatment for either new or mature leaves. Photorespiration decreased 31% under eCO(2) supplying less reductant, and in situ NO-3 reductase activity was concurrently reduced (-34%) in eCO(2), especially in new leaves during summer periods. Hence, NO3- 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 NO3- reductase activity due to lower photorespiration in eCO(2) can contribute to understanding how eCO(2)-induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO(2) on N biogeochemistry include both mechanisms, especially where NO3- is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.