ISOTOPIC CARBON DISCRIMINATION AND LEAF NITROGEN-CONTENT OF ERICA-ARBOREA L ALONG A CO2 CONCENTRATION GRADIENT IN A CO2 SPRING IN ITALY

We studied a Mediterranean species (Erica arborea L.) growing in a CO2 spring in Italy that was naturally exposed for generations to a gradient of atmospheric CO2 concentrations. The CO2 concentration gradient to which different individual plants were exposed was determined by an indirect method based on radioisotope analysis. The stable carbon isotope ratio of sampled leaves was determined by mass spectrometry, and isotopic discrimination was then calculated. Leaf nitrogen, specific leaf area, total soil nitrogen, soil organic matter content and soil pH were also measured. In one group of plants, grown on a homogeneous soil and exposed to moderate CO2 enrichment, isotopic discrimination was significantly reduced in response to increasing CO2 concentrations, whereas the intercellular CO2 concentration and leaf nitrogen content were almost unaffected. In a second group of plants, grown along a gradient of CO2 concentration and soil nitrogen content, leaf nitrogen content was reduced when nitrogen availability was limiting. However, when soil nitrogen was available in excess, even very high CO2 concentrations did not result in increased discrimination or reduced leaf nitrogen content in the long term. The results are discussed with respect to current theories about the long-term CO2 response of plants based on several years of experimentation with elevated atmospheric CO2 concentrations under controlled conditions.