Nitrogen and carbon concentrations, and stable isotope ratios in Mediterranean shrubs growing in the proximity of a CO2 spring

Seasonal changes in foliage nitrogen (N) and carbon (C) concentrations and δ15N and δ13C ratios were monitored during a year in Erica arborea, Myrtus communis and Juniperus communis co-occurring at a natural CO2 spring (elevated [CO2], about 700 μmol mol−1) and at a nearby control site (ambient [CO2], 360 μmol mol−1) in a Mediterranean environment. Leaf N concentration was lower in elevated [CO2] than in ambient [CO2] for M. communis, higher for J. communis, and dependent on the season for E. arborea. Leaf C concentration was negatively affected by atmospheric CO2 enrichment, regardless of the species. C/N ratio varied concomitantly to N. Leaves in elevated [CO2] showed lower δ13C, and therefore likely lower water use efficiencies than leaves at the control site, regardless of the species, suggesting substantial photosynthetic acclimation under long-term CO2-enriched atmosphere. Leaves of E. arborea showed lower values of δ15N under elevated [CO2], but this was not the case of M. communis and J. communis foliage. The use of the resources and leaf chemical composition are affected by elevated [CO2], but such an effect varies during the year, and is species-dependent. The seasonal dependency and species specificity suggest that plants are able to exploit different available water and N resources within Mediterranean sites