Deep CO₂ soil inhalation / exhalation induced by synoptic pressure changes and atmospheric tides in a carbonated semiarid steppe

Abstract. Knowledge of all the mechanisms and processes involved in soil CO₂ emissions is essential to close the global carbon cycle. Apart from molecular diffusion, the main physical component of such CO₂ exchange is soil ventilation. Advective CO₂ transport, through soil or snow, has been correlated with the wind speed, friction velocity or pressure (p). Here we examine variations in subterranean CO₂ molar fractions (χ_c) over two years within a vertical profile (1.5 m) in a semiarid ecosystem, as influenced by short-timescale p changes.

Analyses to determine the factors involved in the variations in subterranean χ_c were differentiated between the growing period and the dry period. In both periods it was found that variations in deep χ_c (0.5–1.5 m) were due predominantly to static p variations and not to wind or biological influences. Within a few hours, the deep χ_c can vary by fourfold, showing a pattern with two cycles per day, due to p oscillations caused by atmospheric tides. By contrast, shallow χ_c (0.15 m) generally has one cycle per day as influenced by biological factors like soil water content and temperature in both periods, while the wind was an important factor in shallow χ_c variations only during the dry period. Evidence of emissions was registered in the atmospheric boundary layer by eddy covariance during synoptic pressure changes when subterranean CO₂ was released; days with rising barometric pressure – when air accumulated belowground, including soil-respired CO₂ – showed greater ecosystem uptake than days with falling pressure. Future assessments of the net ecosystem carbon balance should not rely exclusively on Fick's law to calculate soil CO₂ effluxes from profile data.