Importance of nondiffusive transport for soil CO₂ efflux in a temperate mountain grassland

Abstract

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 CO2 from soils, the production and subsequent transport of CO2 from soil to atmosphere received far less attention. However, to understand processes underlying emissions of CO2 from terrestrial ecosystems, both processes need to be fully evaluated. In this study, we tested to what extent the transport of CO2 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 CO2 efflux calculated from the soil CO2 concentration gradient with the CO2 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 CO2. 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 CO2 transport at this site. Our results suggest that nondiffusive transport may be an important mechanism influencing diel and day‐to‐day dynamics of soil CO2 emissions, leading to a significant mismatch (10–87% depending on the model used) between the two approaches at short time scales.

Environmental forcing does not induce diel or synoptic variation in the carbon isotope content of forest soil respiration

“…Variability was also unrelated to sunlight, humidity, soil temperature, soil moisture, soil efflux, and CO 2 mole fraction increase during chamber closure. This contrasts with other studies that have shown correlations between soil CO 2 and/or soil efflux with these environmental forcing mechanisms (e.g., Davidson et al, 1998;Roland et al, 2015;Sánchez-Cañete et al, 2013;Savage et al, 2013;Subke et al, 2003), highlighting the variability in these processes across the landscape.…”

“…4 to dramatic changes in Fig. 3 of Sánchez-Cañete et al, 2013), and soil CO 2 was not correlated with wind speed, friction velocity, or pressure. Large changes in soil CO 2 were related to rain events (Fig.…”

“…Several studies have shown that trace gas transport in porous soils in a variety of ecosystems is subject to ventilation by wind or pressure pumping (e.g., Flechard et al, 2007;Fujiyoshi et al, 2010;Maier et al, 2010;Sánchez-Cañete et al, 2013) and even by thermal convection (Ganot et al, 2014). When diffusion initially dominates transport and a ventilation event follows, there should be a readily detectible isotopic effect as ventilation removes some of the small pool of diffusively enriched CO 2 within the soil (as described in Fig.…”

“…However, soil efflux and production are not always tightly coupled. Permeable soils are subject to ventilation by wind or pressure changes (e.g., Flechard et al, 2007), with sometimes dramatic changes in soil CO 2 over just a few hours or days (Sánchez-Cañete et al, 2013). Sometimes ventilation is less obvious, but a comparison of measurements of soil efflux and soil CO 2 with models of diffusive transport suggests that ventilation may be persistent in some soils (Roland et al, 2015).…”

“…Permeable soils are subject to ventilation by wind or pressure changes (e.g., Flechard et al, 2007), with sometimes dramatic changes in soil CO 2 over just a few hours or days (Sánchez-Cañete et al, 2013). Sometimes ventilation is less obvious, but a comparison of measurements of soil efflux and soil CO 2 with models of diffusive transport suggests that ventilation may be persistent in some soils (Roland et al, 2015). Because aerobic respiration involves CO 2 production that is stoichiometrically related to O 2 consumption, the ratio of these two fluxes (apparent respiratory quotient or ARQ) can be used to identify the presence of other processes in the soil that can lead to decoupling between CO 2 efflux and production.…”