Influence of geogenic CO2 on mineral and organic soil constituents on a mofette site in the NW Czech Republic

Geogenic CO2 emission on mofette sites may be a factor in soil formation. To demonstrate a CO2 effect, we studied soils (0-60 cm depth) along a transect across a mofette in the NW Czech Republic. We determined CO2 partial pressures (p(CO2)), and the contents in the soil of carbon (C), nitrogen (N), sulphur and dithionite- and oxalate-extractable iron and manganese. X-ray diffractometry (XRD) and Fourier-transform infrared (FTIR) spectroscopy methods were applied to the soils' particle-size fractions. The CO2 partial pressures varied considerably (0.001-1) along the transect and were positively correlated with both the C-org contents (5.5-432.9 g kg(-1)) and the C:N ratio (9.3-32.2), indicating a decreased turnover of organic parent material with increasing CO2. When the soil atmosphere was entirely composed of CO2, pedogenic Fe oxide contents were small (minimum 0.5 g dithionite-extractable Fe kg(-1)) and poorly crystalline. XRD and FTIR spectroscopy revealed primary and secondary minerals such as quartz, feldspars, mica, illite, kaolinite and halloysite irrespective of CO2 contents. A pronounced effect of CO2 was found for soil organic matter (SOM), because the FTIR spectra did not reveal a normal accumulation of alkyl C and lipids of microbial origin in the clay fraction. This indicates that microbial synthesis and/or degradation of plant-derived aliphatic species were reduced. We did not detect more organo-mineral associations, microbially formed polypeptides or pectin in clay fractions in comparison with the clay-plus-silt fractions at large p(CO2). This indicates relatively unaltered particulate OM in the clay fraction. At large p(CO2) values, the IR bands indicative of lignin became detectable and that of aryl ketones in lignin was positively correlated with p(CO2). Thus, we suggest that microbial formation of SOM and degradation of lignin is restricted under an increased CO2 atmosphere. We attribute less humification at increased CO2 in the soil atmosphere to a decrease in oxidative transformations and decreased microbial activity.