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

• Main Authors: E. P. Sánchez-Cañete, A. S. Kowalski, P. Serrano-Ortiz, O. Pérez-Priego, F. Domingo
• Format: Article
• Language: English
• Published: Copernicus Publications 2013-10-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/10/6591/2013/bg-10-6591-2013.pdf
• ISSN: 1726-4170; 1726-4189

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 (<i>p</i>). Here we examine variations in subterranean CO₂ molar fractions (&chi;_c) over two years within a vertical profile (1.5 m) in a semiarid ecosystem, as influenced by short-timescale <i>p</i> changes. <br><br> Analyses to determine the factors involved in the variations in subterranean &chi;_c were differentiated between the growing period and the dry period. In both periods it was found that variations in deep &chi;_c (0.5–1.5 m) were due predominantly to static <i>p</i> variations and not to wind or biological influences. Within a few hours, the deep &chi;_c can vary by fourfold, showing a pattern with two cycles per day, due to <i>p</i> oscillations caused by atmospheric tides. By contrast, shallow &chi;_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 &chi;_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.