Temporal changes in soil pore space CO2 concentration and storage under permanent grassland

Carbon dioxide concentrations were measured semi-continuously in soil pore space under permanent grassland for a 3-year period, using gas-permeable membrane tubes buried at various depths in the soil, in an attempt to quantify temporal variations of soil CO2 storage. Diurnal wavelike variations in CO2 concentration were observed systematically through the soil profile down to 50 cm, and the patterns and amplitude of mean diurnal waves depended on soil- and environmental conditions. For moderate to high soil water contents (SWC), soil CO2 concentrations were positively correlated with soil temperature on a diurnal basis, resulting from an exponential increase of soil respiration with temperature and a shift in thermodynamic gaseous/aqueous phase equilibria. In dry conditions, however, air-filled porosity CO2 concentrations tended to be lowest in the late afternoon and highest in the early morning, and thus negatively correlated with soil temperature. The storage of CO2 in soil was calculated in the air-filled and in the water-filled pore fractions using Henry's law and carbonate chemistry. Storage estimates were extremely sensitive to soil pH and a parametrisation of soil pH as a function of soil water content was derived from field measurements and used in the storage calculation scheme. Instantaneous rates of soil CO2 storage change with time were negatively correlated with atmospheric friction velocity in dry and moderately wet conditions, though in very wet conditions storage changes were dominated by vertical movements of soil water. The data suggest that CO2 can accumulate in the soil during the night when transport is restricted to molecular diffusion through soil layers, and that wind-induced pressure pumping results in a gradual daytime flushing out. These findings may help account for the oft-reported u*-dependence of night-time eddy covariance respiration fluxes.