Modelling of biospheric CO2 gross fluxes via oxygen isotopes in a spruce forest canopy: a 222Rn calibrated box model approach

One-dimensional box model estimates of biospheric CO2 gross fluxes are presented. The simulations are based on a set of measurements performed during the EUROSIBERIAN CARBONFLUX intensive campaign between 27 July and 1 August 1999 in a natural Picea abies forest in Russia. CO2 mixing ratios and stable isotope ratios of CO2 were measured on flask samples taken in two heights within the canopy. Simultaneously, soil and leaf samples were collected and analysed to derive the 18O/16O ratio of the respective water reservoirs and the 13C/12C ratio of the leaf tissue. The main objective of this project was to investigate biospheric gas exchange with soil and vegetation, and thereby take advantage of the potential of the 18O/16O ratio in atmospheric CO2. Via exchange of oxygen isotopes with associated liquid water reservoirs, leaf CO2 assimilation fluxes generally enrich while soil CO2 respiration fluxes generally deplete the18O/16O ratio of atmospheric CO. In the model, we parameterised intra-canopy transport by exploiting soil-borne Rn as a tracer for turbulent transport. Our model approach showed that, using oxygen isotopes, the net ecosystem CO2 flux can be separated into assimilation and respiration yielding fluxes comparable with those derived by other methods. However, partitioning is highly sensitive to the respective discrimination factors, and therefore also on the parameterisation of internal leaf CO2 concentrations and gradients