Isotopic corrections for the radiocarbon composition of CO2 in the soil gas environment must include diffusion and mixing

Earth system scientists working with radiocarbon in organic samples use a stable carbon isotope (δC) correction to account for mass-dependent fractionation caused primarily by 10 photosynthesis. Although researchers apply this correction routinely, it has not been evaluated for the soil gas environment, where both diffusive gas transport and diffusive mixing are important. Towards this end we applied an analytical soil gas transport model across a range of soil diffusivities and biological CO2 production rates, allowing us to control the radiocarbon (ΔC) and stable isotope (δC) compositions of modeled soil CO2 production and atmospheric CO2. This approach allowed us to assess 15 the bias that results from applying the conventional correction method for estimating ΔC of soil production. We found that the conventional correction is inappropriate for interpreting the radioisotopic composition of CO2 from biological production, because it does not account for diffusion and diffusive mixing. The resultant ΔC bias associated with the traditional correction is highest (up to 150 ‰) in soils with low biological production and/or high soil diffusion rates. We propose a new solution 20 for radiocarbon applications in the soil gas environment that fully accounts for diffusion and diffusive mixing.