An inorganic CO2 diffusion and dissolution process explains negative CO2 fluxes in saline/alkaline soils

Abstract

An ‘anomalous' negative flux, in which carbon dioxide (CO2) enters rather than is released from the ground, was studied in a saline/alkaline soil. Soil sterilization disclosed an inorganic process of CO2 dissolution into (during the night) and out of (during the day) the soil solution, driven by variation in soil temperature. Experimental and modeling analysis revealed that pH and soil moisture were the most important determinants of the magnitude of this inorganic CO2 flux. In the extreme cases of air-dried saline/alkaline soils, this inorganic process was predominant. While the diurnal flux measured was zero sum, leaching of the dissolved inorganic carbon in the soil solution could potentially effect net carbon ecosystem exchange. This finding implies that an inorganic module should be incorporated when dealing with the CO2 flux of saline/alkaline land. Neglecting this inorganic flux may induce erroneous or misleading conclusions in interpreting CO2 fluxes of these ecosystems.

Hidden carbon sink beneath desert

“…The strong CO 2 absorption found in sterilized saline/alkaline soils [Xie et al, 2009] is simply a recovery of CO 2 , as all dissolved CO 2 in the soil solution is driven off [Lindsay, 1979;Xie et al, 2009] during sterilization by steam vapor at >100°C. In fact, in unvegetated sandy desert like the Taklamagan, diurnal variation of temperature itself may create a night-in/day-out CO 2 flux due to dissolution and diffusion, resulting in zero daily mean flux and no net sink or source [Ma et al, 2013] (see also Figure S1). …”

“…However, the use of saline/alkaline water for irrigation is still in the experimental stage and has never been and will most likely never be (in the foreseeable future) practiced on a large scale [Beltrán, 1999], as saline/alkaline water is not usable by crop plants. Diffusion of CO 2 back into the atmosphere through the coarse-textured sandy layer is possible but has been shown to be negligible, as surface CO 2 flux has a zero mean [Ma et al, 2013] (see Figure S1). Freshwater aquifers in arid regions are rare and are usually old geological waters buried long ago [Leaney et al, 2003].…”

Hydration status and diurnal trophic interactions shape microbial community function in desert biocrusts

“…Furthermore, for alkaline or saline soils (typical desert soils), abiotic contributions to gaseous efflux may account for up to 40 % of total CO 2 emissions (Ma et al, 2013). Thus, to separate biotic and abiotic contributions for gaseous efflux, reliable estimates of pH are needed.…”

Contribution of soil respiration to the global carbon equation

“…The soil CO 2 efflux measured at the soil's surface is often assumed to approximate the "true soil respiration," though this assumption has been recently challenged by the finding that respired CO 2 may be dissolved in soil solutions (Emmerich, 2003;Hastings et al, 2005;Stevenson and Verburg, 2006;Schlesinger et al, 2009;Tamir et al, 2011;Ma et al, 2013;Roland et al, 2013;Angert et al, 2015) or root xylem water and carried upward in the transpiration stream (Aubrey and Teskey, 2009;Bloemen et al, 2013). Nevertheless, some studies have shown that the ratio of soil surface CO 2 efflux to O 2 influx (a better indicator for biological activities) is around 1.0 (0.9-1.06) in various ecosystems (Seibt et al, 2004;Angert et al, 2012;Ishidoya et al, 2013).…”