Ecosystem discrimination of natural and agricultural systems

Stable isotopes find several applications in the study of gas exchanges between the terrestrial biosphere and the atmosphere. The biosphere-atmosphere CO2 exchange contributes to seasonal and annual variation in atmospheric CO2 concentration and its isotopic composition. Recently, ?13C ratios of CO2 in the atmosphere have been used in general circulation models scaling modelled soil and plant isotopic composition at the global level. The biochemical and physical basis of isotopic discrimination by photosynthesis is well established : leaf ?13C reflects the ?13C of tropospheric and canopy CO2, and it is dependent on the turbulence regime and ecosystem physiology. On the contrary, no discrimination was ever demonstrated associated with respiration processes, thus soil CO2 efflux carries the isotopic signature of soil organic matter (SOM) and roots. The ?13C of SOM reflects the ?13C of the vegetation under which it was formed, and tends to increase with time and soil depth. Ecosystem carbon isotope discrimination ( ?e) integrates all autotrophic and heterotrophic C fluxes providing useful information on the water status, water use efficiency as well as reflecting land-use history, due to mixing of litter and slow turnover rates of soil carbon. Coupling this information with oxygen stable isotopes composition ?18O, provides further information on the water availability to plants as well as on the partitioning of NEE in natural systems, where the carbon isotopic signatures of plants and soil are similar. Little data are available on ?e, which was never determined for Mediterranean ecosystems. In the present work data are reported concerning a case study made on a machia stand in the island of Pianosa. Ecosystem discrimination was determined by the Keeling plot technique and was equal to -17.1?.