Deep CO<sub>2</sub> soil inhalation / exhalation induced by synoptic pressure changes and atmospheric tides in a carbonated semiarid steppe

Abstract

Abstract. Knowledge of all the mechanisms and processes involved in soil CO 2 emissions is essential to close the global carbon cycle. Apart from molecular diffusion, the main physical component of such CO 2 exchange is soil ventilation. Advective CO 2 transport, through soil or snow, has been correlated with the wind speed, friction velocity or pressure (p). Here we examine variations in subterranean CO 2 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 CO 2 was released; days with rising barometric pressure -when air accumulated belowground, including soil-respired CO 2 -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 CO 2 effluxes from profile data.

Annual CO<sub>2</sub> budget and seasonal CO<sub>2</sub> exchange signals at a high Arctic permafrost site on Spitsbergen, Svalbard archipelago

“…It is also possible that the changes in atmospheric pressure recorded shortly prior and during strong flux events resulted in an additional pumping effect, which led to mixing of gas within the snow pack and a release/uptake at the snow surface. Such pressure effects have been observed for other ecosystems, like peatlands and methane emissions in sub-tropical Japan (Tokida et al, 2007) or over a grass steppe (CO 2 ) in southeast Spain (Rey et al, 2012;Sánchez-Cañete et al, 2013). These mesoscale turbulence-driven "wind and pressure pumping" events therefore suggest a large convective gas transport into, within, or out of the snow pack as modeled by Bowling and Massman (2011), or earlier by Björkmann et al (2010b), Seok et al (2009), Massman (2006, and Massman and Frank (2006), none of whom had any direct atmospheric trace gas flux measurements from above the snow surface that they could use to observe these severe exchange events.…”

Nocturnal soil CO2 uptake and its relationship to subsurface soil and ecosystem carbon fluxes in a Chihuahuan Desert shrubland

“…[3] A fundamental shift in ecosystem processes could potentially occur under prolonged dry periods. There is increasing evidence that abiotic carbon dynamics affect arid land ecosystem-level gas exchange over prolonged, dry plant dormant periods [Emmerich, 2003;Kowalski et al, 2008;Serrano-Ortiz et al, 2010;Plestenjak et al, 2012;Rey et al, 2012;Roland et al, 2013;Sánchez-Cañete et al, 2013a]. Recently, Roland et al [2013] have used a chemical carbonate weathering model to demonstrate how turbulence-induced ventilation drives soil CO 2 outgassing and carbonate precipitation during the day, while nocturnal replenishment of soil CO 2 and dissolution of carbonates can drive soil CO 2 uptake over dry periods in Spanish mattoral.…”

Improving the accuracy of the gradient method for determining soil carbon dioxide efflux

“…Numerous authors have found that advective transport driven by the wind can provoke changes in the soil CO 2 molar fraction [Drewitt et al, 2005;Seok et al, 2009;Bowling and Massman, 2011;Goffin et al, 2014], soil CO 2 effluxes [Subke et al, 2003;Risk et al, 2013;Roland et al, 2015], or in the atmosphere [Kowalski et al, 2008;Sanchez-Canete et al, 2011;Nachshon et al, 2012;Rey et al, 2012]. Nondiffusive transport has been associated both with small changes in pressure induced by the wind, commonly called pressure pumping [Massman et al, 1997;Takle et al, 2004;Maier et al, 2010], or from synoptic atmospheric pressure changes [Rogie et al, 2001;Fujiyoshi et al, 2010;Comas et al, 2011;Sanchez-Canete et al, 2013b]. The soil-atmosphere thermal gradient also can generate convective transport provoking the exchange of the air between soil and atmosphere, both in fractures [Weisbrod et al, 2009;Moore et al, 2011] and in caves [Serrano-Ortiz et al, 2010].…”