Physical controls on the isotopic composition of soil-respired CO2

Measurement of the isotopic composition of soil and soil-respired CO2 (δ13CO2) has become an invaluable tool in understanding ecosystem carbon-cycling processes. While steady state work has been indispensable in understanding the effects of diffusive transport on soil CO2 isotopic composition, it is crucial that researchers studying temporally dependent processes, such as soil CO2 efflux, realize that these systems are rarely at steady state. Non-steady-state effects could result in misinterpretation of isotopic data, but have not been addressed in the literature, despite their fundamental importance to researchers who use isotopes in diffusive, non-steady-state environments. Here, we use an isotopologue-based model to study dynamic fractionation, which we propose is a byproduct of transient changes in environmental variables. Time varying soil characteristics and processes such as biological production rate, soil pore space, diffusivity and atmospheric concentration were all found to induce non-steady-state gas transport conditions in the soil leading to transient changes in the isotopic composition of soil CO2 flux. The main driving force behind this transport related fractionation of CO2 is the rate of the change in 12CO2 gradient compared to that of 13CO2. These numerical simulations show that dynamic fractionation exists under non-steady-state diffusive conditions and suggest that isotopic data collected in non-steady-state, natural environments, cannot be properly interpreted without considering dynamic fractionation effects.