Production and stable isotopic composition of CO₂ in a soil near Bern, Switzerland

Abstract

During a period of one year, we measured the concentration and the stable isotopic composition of CO₂ in weekly soil air samples from two depths in a grass-covered soil. The CO₂ production rate, as estimated from the diffusion equation and the observed concentration gradient, was 15 mmol/m^-2/h^-1, as an annual mean. During the cold season, the production rate varied with soil temperature, but no correlation existed with temperature above about 10 °C. Concentrations and fluxes of N₂O in the soil were also determined. Observed δ¹³C values were in the expected range, and δ¹⁸O of Soil CO₂ was found to be in isotopic equilibrium with the soil water. δ¹⁸O of the CO₂ released to the atmosphere is expected to be lower by roughly 8‰ due to diffusive isotope fractionation; the precise value depends on the competition between CO₂ production, equilibrium with soil water and diffusion.