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.

Figure 1. The diurnal courses of soil CO₂ flux under the canopy (hollow circles) and in inter–plant spaces (solid circles) over 10 clear sampling dates in 2009.

Figure 2. Verification of the measuring reliability using different matrices (quartz sand only, quartz sand with distilled water and quartz sand with sterilized soil solution).

Figure 3. The contribution of inorganic CO₂ flux to total soil CO₂ flux in soils varying in pH and EC at soil moisture content of 10% and the overall contribution of inorganic flux at different pHs [pHs for (a)–(e) were 7.19, 7.80, 8.30, 8.55 and 8.85, respectively].

The shaded parts indicate the period in which the inorganic CO₂ fluxes are positive and the instantaneous ratio of inorganic CO₂ flux to total CO₂ flux were calculated (f). Error bars represent standard error of the mean.

Figure 4. The contribution of inorganic CO₂ flux to total soil CO₂ flux in soils varying in pH and EC in air-dried condition.

Error bars represent standard error of the mean.

Figure 5. Diurnal patterns for simulated (solid lines) and directly measured (hollow circles) inorganic soil CO₂ flux in soils with different pH and soil salt content (EC) at soil moisture content of 10%.

(a–c) pH 7.80; (d–f) pH 8.00; and (g–i) pH = 8.30.

Figure 6. The modeling output of daily inorganic CO₂ exchange for soils with different pH and EC at soil moisture content of 10%.

The black stars indicate the inorganic processes that were validated by directly measured data.