Rapid soil CO2 release following wetting governed by physical desorption not biology

Evidence shows large CO2 efflux from rewetted dry soil surfaces peaking within seconds since wetting, too fast to be attributed to the biological “Birch effect”. We conducted experiments with long-term archived soils with different textures and soil organic carbon (SOC) to quantify CO2 release rates during controlled water imbibition. Total soil CO2 efflux varied considerably across textures (that also affected water imbibition rates) with cumulative efflux ranging from 5 (loamy sand) to 35 (loam) mmol/m2 during water imbibition times of 400 s (loamy sand) to 800s (loam). The measured CO2-efflux was reproduced using a physically based model for wetting front displacement, gas diffusion, and wetting-induced CO2 desorption. Repeated rewetting following oven drying of the soil samples resulted in different CO2 release behavior, suggesting kinetic effects of CO2 re-adsorption rates (especially for SOC) and potential bypassing of CO2 bearing surfaces during imbibition. In other words, surface accessibility to water, wettability and nano-porosity play a role in CO2 adsorption and desorption rates. While the measurements suggest only a minor role of this temporary carbon sink in the global carbon balance, there is a potential for a persistent measurement bias by eddy covariance flux towers (missing CO2 release during rainfall) that may lead to a bias in carbon balance of the order of 0.1-0.5 Gton/year globally. Additionally, it highlights a potential role of physical CO2 efflux, which operates at a much faster time scale than biological fluxes.