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 the ¹⁸O/¹⁶O ratio of atmospheric CO₂. In the model, we parameterised intra-canopy transport by exploiting soil-borne ²²²Rn 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.