The characteristics of soil CO2 fluxes at a site with natural CO2 enrichment

Soil CO2 degassing was studied at the Stavešinci mofette by small-scale measurements of fluxes, soil CO2 concentrations (depth of 20 cm) and measurements of soil water and moisture. A relatively high correlation between soil CO2 concentrations and fluxes was found (r = 0.76–0.88, p = 0.0001). At the sites that are most exposed to mofette exhalations, CO2 displaced oxygen from the soil, creating hypoxic conditions. The temporal variability of CO2 fluxes was followed at the sites that differ in the rate of soil CO2 enrichment (low-, medium- and high rate of CO2 degassing from the soil). Throughout the two growing seasons, 2005 and 2006, the daily average CO2 efflux rates varied from 4.6 to 27.9 µmol CO2 m− 2 s− 1 in the low-CO2, 6.1–92.4 µmol CO2 m− 2 s− 1 in medium CO2 and from 68 to 268.2 µmol CO2 m− 2 s− 1 in high CO2. At the sites with high CO2 concentrations, the biogenesis of CO2 is known to be suppressed and geological CO2 flux exceeds the respiratory one by several orders of magnitude. A seasonal pattern and diurnal pattern of CO2 degassing could be observed only in low-CO2 measuring sites, where high CO2 flux rates during mid-season (midday) can be related to temperature-promoted biogenesis of CO2. No clear seasonal trend could be observed in locations where the geological CO2 dominated the efflux. The effect of soil moisture on CO2 fluxes was negligible for both, soils enriched with geological CO2 and for control low-CO2 soils. An indirect effect of CO2 enrichment on soil water content was observed. On the sites where elevated CO2 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 CO2 soils, the first-order exponential relationship between soil CO2 flux and soil temperature accounted for more than 60% of the CO2 flux variability. In contrast, the CO2 fluxes were independent of soil temperature at the sites with geological CO2 degassing.