Effect of Humidity on the Stable Carbon Isotopic Composition of Soil Organic Matter in the Baikal Region

The stable carbon isotopes of organic matter in humus horizons of soils of the Baikal region have been studied. A wide range of soil forming factors makes it possible to identify the most important ones that determine the carbon isotopic composition of organic matter. Depending on the landscape and climatic conditions, the δ13C values range from –29.91 to –22.98‰. The lowest values are typical of the landscapes with the highest humidity. Analysis of the carbon fractionation factors suggests that the influence of climatic factors on 13C discrimination during the photosynthesis by C3 plants plays the leading role in the observed differences in the isotopic composition of soils. With an increase in precipitation during the vegetation season, the δ13C values in soil organic matter decrease by 1.35‰ for every 100 mm of precipitation. These values are in good agreement with isotopic gradients in the adjacent areas of Mongolia and China and reflect a considerable sensitivity of plants that form soil organic matter to the changes in humidity. Although pairwise linear regressions do not show any significant dependence of the δ13C values on temperature, its effect appears in an indirect manner via moistening index displaying a maximum negative correlation. Thus, not only precipitation, but also heat to moisture ratio during the vegetation season with the most intensive biological and soil processes to the greatest degree influence the carbon isotopic composition of soil organic matter.