Seasonal and vertical variations in soil CO₂ production in a beech forest: an isotopic flux-gradient approach

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.