A regional-scale estimate of the soil organic carbon isotopic composition (δ13c) and its environmental drivers: case study of the Baikal region

Modern plants and surface soil δ13С values from 95 sites in the Baikal region were obtained for the first time and were used to establish relationships with regional environmental factors. Studied sites were distributed along the elevation gradient from 403 to 2315 m, which defined a strong landscape and climatic gradients encompassing mountain tundra, subalpine grasslands, mountain taiga, subtaiga, and steppe. δ13С values of soil organic matter (SOM) varied from –29.50 to –22.98‰. This result showed that the stable C isotopic composition of the surface soils was mainly determined by δ13С values of C3 plants (vary from –33.0 to –24.5‰) and C isotope fractionation during stabilization of plant-derived C into SOM. The δ13С values of modern plants and surface soils were negatively correlated with mean annual and growing season precipitation (p<0.05), confirming that precipitation is the primary factor determining SOM’s stable C isotopic composition in the Baikal region. A distinct increase in the δ13С values with decreasing mean annual and growing season precipitation was found with a slope of –0.42‰/100 mm and –0.97‰/100 mm, respectively. Temperature had no significant effect on the spatial distribution of SOM δ13С values at the regional scale but played an important role in the severe environments of mountain tundra (the coldest and wettest) and steppes (the warmest and driest). Such conditions strongly impacted SOM δ13С values by influencing plant species composition and soil microbiological activity. As a result, the organic matter of these soils is characterized by the highest δ13С values. The SOM of taiga soils formed under a favorable combination of temperature and precipitation was characterized by the lowest δ13С values