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

Geogenic CO 2 emission on mofette sites may be a factor in soil formation. To demonstrate a CO 2 effect, we studied soils (0-60 cm depth) along a transect across a mofette in the NW Czech Republic. We determined CO 2 partial pressures (p(CO 2 )), 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 CO 2 partial pressures varied considerably (0.001-1) along the transect and were positively correlated with both the C or g 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 CO 2 . When the soil atmosphere was entirely composed of CO 2 , 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 CO 2 contents. A pronounced effect of CO 2 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(CO 2 ). This indicates relatively unaltered particulate OM in the clay fraction. At large p(CO 2 ) values, the IR bands indicative of lignin became detectable and that of aryl ketones in lignin was positively correlated with p(CO 2 ). Thus, we suggest that microbial formation of SOM and degradation of lignin is restricted under an increased CO 2 atmosphere. We attribute less humification at increased CO 2 in the soil atmosphere to a decrease in oxidative transformations and decreased microbial activity.