Influence of freezing and thawing on the carbon isotope composition in soil CO2

The carbon isotopic composition of soil CO2 was measured during the winter of 1990/91 at two sites in the Lublin Upland (poland). In that season a period with very low air temperature occurred, during which snow covered soils were frozen. We observed variations of delta(13)C up to 3 permil. During freezing periods the variations depended on soil physical properties. In sandy soil the C-13/C-12 ratio during freezing periods increased, while in loess soil it decreased. This appears to be connected with the granulometrical composition of the soil and the size of the pores, which influence the rate of CO2 diffusion into the atmosphere. During thawing periods, when the snow was melting, the CO2 concentration increased and the C-13/C-12 ratios decreased in both types of the soil under investigation due to worse-contact of soil horizons with the atmosphere. The major winter variations in delta(13)CO(2) of soil may be useful natural markers of infiltrating waters.