Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

Abstract

Abstract. CO2 exchange between terrestrial ecosystems and the atmosphere is key to understanding the feedbacks between climate change and the land surface. In regions with carbonaceous parent material, CO2 exchange patterns occur that cannot be explained by biological processes, such as disproportionate outgassing during the daytime or nighttime CO2 uptake during periods when all vegetation is senescent. Neither of these phenomena can be attributed to carbonate weathering reactions, since their CO2 exchange rates are too small. Soil ventilation induced by high atmospheric turbulence is found to explain atypical CO2 exchange between carbonaceous systems and the atmosphere. However, by strongly altering subsurface CO2 concentrations, ventilation can be expected to influence carbonate weathering rates. By imposing ventilation-driven CO2 outgassing in a carbonate weathering model, we show here that carbonate geochemistry is accelerated and does play a surprisingly large role in the observed CO2 exchange pattern of a semi-arid ecosystem. We found that by rapidly depleting soil CO2 during the daytime, ventilation disturbs soil carbonate equilibria and therefore strongly magnifies daytime carbonate precipitation and associated CO2 production. At night, ventilation ceases and the depleted CO2 concentrations increase steadily. Dissolution of carbonate is now enhanced, which consumes CO2 and largely compensates for the enhanced daytime carbonate precipitation. This is why only a relatively small effect on global carbonate weathering rates is to be expected. On the short term, however, ventilation has a drastic effect on synoptic carbonate weathering rates, resulting in a pronounced diel pattern that exacerbates the non-biological behavior of soil–atmosphere CO2 exchanges in dry regions \\mbox{with carbonate soils}.

A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

“…Roland et al . 52 used a chemical carbonate weathering model to explain non-biological fluxes detected at ecosystem scale in a karst, finding that the CO 2 coming from deeper layers at night could be stimulating carbonate dissolution and, thus, consuming CO 2 . Hamerlynck, et al ., 53 found a negative F soil at night in a Chihuahuan desert shrubland, both using an automatic soil chamber and using the gradient method with CO 2 sensors buried in the shallowest layer, similarly attributing the CO 2 consumption to carbonate dissolution.…”

Mind the gap: non‐biological processes contributing to soil CO2 efflux

“…Although its contribution to the annual soil carbon budget is small on medium-to long-time scales (Kuzyakov, 2006), it has been shown that on short-time scales (hourly-daily), the equilibrium of these reactions can be very dynamic explaining, under certain circumstances, diurnal patterns of soil CO 2 exchange (Roland et al, 2013). Using a geochemical model to calculate weathering rates in a shrubland site in the SE of Spain, Roland et al (2013) showed that weathering reactions can contribute considerably to the net ecosystem carbon balance during dry periods. As explained in this study, on carbonate substrates, changes in soil CO 2 concentration within the soil-atmosphere caused by turbulence-driven mass transport affect the equilibrium of carbonate precipitation-dissolution reactions on short-time scales.…”

“…In the current literature, soil CO 2 efflux has been mostly assumed to be equivalent to soil respiration, that is, to biological activity within soils, but non-biological processes contribute to soil CO 2 efflux in many regions of the world (e.g. Rey et al, 2012a;Roland et al, 2013). Soil respiration is defined as the flux of CO 2 resulting from the biological activity of roots, microfauna and microorganisms within the soil.…”

Hidden carbon sink beneath desert

“…For instance, the CO 2 sequestration process proposed in the current study, must have been, and will be intensified as atmospheric CO 2 concentration increases [Hamerlynck et al, 2013], in which CO 2 is pushed into saline/alkaline soil and thus the process itself must constitute a carbon sink. The temperature-driven abiotic (or inorganic, geochemical) CO 2 fluxes observed in desert [Yates et al, 2013] and karst regions [Roland et al, 2013] or Antarctic dry valleys [Shanhun et al, 2012] should also contribute significantly to diurnal, seasonal, or even interannual variations in the global carbon cycle. Overall, we believe that nonbiological processes in the modern global carbon cycle may have been greatly underestimated and further study of these processes is highly warranted.…”