Abstract. Soil CO2 efflux results from the transport of CO2 from several respiration sources within the soil profile. A flux – gradient approach (FGA) was used to assess the vertical profile of CO2 production (P_CO2) and its isotopic composition (δ13P_CO2) from the measurement of the vertical profile of CO2 concentration and CO2 isotopic composition combined with soil CO2 and δ13CO2 effluxes. Variations in P_CO2 and δ13P_CO2 within different soil layers were analyzed at different time scales. In the first soil layers, P_CO2 was probably underestimated and δ13P_CO2 overestimated when CO2 transport was not solely diffusive. At the seasonal scale, a vertical gradient of P_CO2 temperature sensitivity was observed. At the within-day scale, variations in soil temperature were too weak to explain the strong variations in P_CO2. At the daily time scale, δ13P_CO2 of sources located between −10 and −20 cm depth was well correlated with the canopy inherent water use efficiency (IWUE) measured the day before. The strong correlation with IWUE argues in favor of an actual connection between canopy activity and soil autotrophic production. Moreover, including SWC of the current day as a second variable improved the linear regression between δ13P_CO2 and IWUE of the previous day, together explaining 76 % of the daily fluctuations in δ13P_CO2. This highlights the actual contribution of both autotrophic and heterotrophic sources to soil P_CO2. The method used gave consistent and promising results even if we could not disentangle the respective contribution of autotrophic and heterotrophic sources to CO2 production as the differences in their isotopic composition were too small and fluctuated too much. In addition, CO2 transport by turbulent advection and dispersion will need to be considered for the top soil layer.
Seasonal and vertical variations in soil CO<sub>2</sub> production in a beech forest: an isotopic flux-gradient approach
