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.
Highlights
Soil CO2 efflux is the largest source of CO2 from terrestrial ecosystems; annually, approximately 98 Pg CO2 is emitted from soils (Bond-Lamberty and Thomson, 2010)
Soil CO2 concentrations measured with solid state sensors at 5 cm and 10 cm depth and measured with the Li-8150 system at 0 cm depth, were much less variable, both at seasonal and daily timescales
As the CO2 gradient is a driver of the CO2 efflux, we expected a clear relationship between the two types of measurements, but we found that the soil CO2 concentration gradients from 5 to 0 cm and from 10 to 5 cm depth were uncoupled from the efflux measured with the chambers
Summary
Soil CO2 efflux is the largest source of CO2 from terrestrial ecosystems; annually, approximately 98 Pg CO2 is emitted from soils (Bond-Lamberty and Thomson, 2010). In contrast to soil chambers, solid-state CO2 sensors allow continuous high frequency measurements of the CO2 gradient with minimal disturbance of the natural conditions, such as, air pressure or wind velocity (Pingintha et al, 2010; Tang et al, 2003) and soil microclimate For this reason, estimation of the soil CO2 efflux from soil CO2 concentrations, the so-called flux-gradient approach (or FGA), is rapidly gaining popularity (e.g., Hirano et al, 2003; Jassal et al, 2005; Jassal et al, 2004; Pumpanen et al, 2008; Tang et al, 2003; Tang et al, 2005; Turcu et al, 2005; Vargas and Allen, 2008a; Vargas and Allen, 2008b). Soil CO2 efflux F is calculated via the flux-gradient method as:
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