Air temperature controls diel oscillation of the CO2 concentration in a desert soil

The factors that control the soil CO₂ concentration are not yet well understood. Therefore, the objective of this study was to explore what factors control the soil CO₂ concentration and its dynamic in a desert soil. For this purpose, CO₂ concentration and temperature were measured in six soil depths (ranging from 15 to 185 cm) in a deeply weathered, coarse-textured desert soil in the North of Chile at high frequency (every 60 minutes) together with precipitation and air temperature for one year. The mean CO₂ concentration calculated across the whole measuring period increased linearly with soil depth from 463 ppm in 15 cm to 1542 ppm in 185 cm soil depth. We observed a diel oscillation of the CO₂ concentration that decreased with soil depth and a hysteretic relationship between the topsoil CO₂ concentration and both air and soil temperature. A small precipitation event increased the CO₂ concentrations in 15, 30, and 50 cm soil depths for several days but did not alter the amplitude of the diel oscillation of the CO₂ concentration. The diel oscillation was very likely caused by strong differences between the soil and the air temperature at night, in particular in summer, causing transport of topsoil air to the atmosphere by thermal convection. Our results have important implications as they show that the soil CO₂ concentration can be controlled by air temperature through thermal convection, rather than by soil temperature, and that the hysteretic relationship between soil CO₂ concentration and temperature can be caused by physical factors alone.