Importance of nondiffusive transport for soil CO(2) efflux in a temperate mountain grassland

Soil respiration and its biotic and abiotic drivers have been an important research topic in recent years. While the bulk of these efforts has focused on the emission of CO(2) from soils, the production and subsequent transport of CO(2) from soil to atmosphere received far less attention. However, to understand processes underlying emissions of CO(2) from terrestrial ecosystems, both processes need to be fully evaluated. In this study, we tested to what extent the transport of CO(2) in a grassland site in the Austrian Alps could be modeled based on the common assumption that diffusion is the main transport mechanism for trace gases in soils. Therefore, we compared the CO(2) efflux calculated from the soil CO(2) concentration gradient with the CO(2) efflux from chamber measurements. We used four commonly used diffusion‐driven models for the flux‐gradient approach. Models generally underestimated the soil chamber effluxes and their amplitudes, indicating that processes other than diffusion were responsible for the transport of CO(2). We further observed that transport rates correlated well with irradiation and, below a soil moisture content of 33%, with wind speed. This suggests that mechanisms such as bulk soil air transport, due to pressure pumping or thermal expansion of soil air due to local surface heating, considerably influence soil CO(2) transport at this site. Our results suggest that nondiffusive transport may be an important mechanism influencing diel and day‐to‐day dynamics of soil CO(2) emissions, leading to a significant mismatch (10–87% depending on the model used) between the two approaches at short time scales.