The characteristics of soil CO 2 fluxes at a site with natural CO 2 enrichment

Abstract

Soil CO 2 degassing was studied at the Stavešinci mofette by small-scale measurements of fluxes, soil CO 2 concentrations (depth of 20 cm) and measurements of soil water and moisture. A relatively high correlation between soil CO 2 concentrations and fluxes was found ( r = 0.76–0.88, p = 0.0001). At the sites that are most exposed to mofette exhalations, CO 2 displaced oxygen from the soil, creating hypoxic conditions. The temporal variability of CO 2 fluxes was followed at the sites that differ in the rate of soil CO 2 enrichment (low-, medium- and high rate of CO 2 degassing from the soil). Throughout the two growing seasons, 2005 and 2006, the daily average CO 2 efflux rates varied from 4.6 to 27.9 µmol CO 2 m − 2 s − 1 in the low-CO 2, 6.1–92.4 µmol CO 2 m − 2 s − 1 in medium CO 2 and from 68 to 268.2 µmol CO 2 m − 2 s − 1 in high CO 2. At the sites with high CO 2 concentrations, the biogenesis of CO 2 is known to be suppressed and geological CO 2 flux exceeds the respiratory one by several orders of magnitude. A seasonal pattern and diurnal pattern of CO 2 degassing could be observed only in low-CO 2 measuring sites, where high CO 2 flux rates during mid-season (midday) can be related to temperature-promoted biogenesis of CO 2. No clear seasonal trend could be observed in locations where the geological CO 2 dominated the efflux. The effect of soil moisture on CO 2 fluxes was negligible for both, soils enriched with geological CO 2 and for control low-CO 2 soils. An indirect effect of CO 2 enrichment on soil water content was observed. On the sites where elevated CO 2 concentrations limited plant growth, the moisture of the upper soil layers also remained high at the time of the year when a high transpirational water loss was induced from the soils with normally developed vegetation. In these low CO 2 soils, the first-order exponential relationship between soil CO 2 flux and soil temperature accounted for more than 60% of the CO 2 flux variability. In contrast, the CO 2 fluxes were independent of soil temperature at the sites with geological CO 2 degassing.