Regional-Scale Soil Organic Carbon Dynamics Evaluation in Southeastern Siberia Inferred from Stable Carbon Isotopic Values (δ<sup>13</sup>C)

The spatial and intra-profile variability of soil δ13С values is assessed along with corresponding environmental and edaphic variables in Baikal region, a vast region in the south of Eastern Siberia that is highly heterogenous in terms of bioclimatic conditions. Studied sites distributed in elevation range of 403–2315 m a.s.l., which define a strong landscape and climatic gradient encompassing mountain tundra, subalpine grasslands, mountain taiga, subtaiga and steppe landscapes. We found that δ13С values of soil organic matter vary significantly, corresponding with δ13C values of plants with C3 photosynthesis. The topsoil organic matter has δ13C values from –29.50 to –22.98‰. Changes in δ13С values in the altitudinal profile correlate well with changes in landscapes. The highest δ13C values are characteristic of soils in mountain tundra and steppe landscapes. In taiga soils, a depletion of organic matter by 13C is observed. These large regional differences in topsoil carbon isotopic composition are also revealed when differences in soil carbon turnover rates are analyzed across the landscapes. The soils exhibit a SOC decrease and δ13С increase with depth. The regress gradient, termed β, of δ13С and the logarithm of SOC with depth in the soil column ranged from –2.1 to –0.4 for the mountain tundra and steppe soils. Slow carbon turnover in such soils were largely controlled by low temperatures and insignificant precipitation, respectively. Most pronounced slope of the linear regression β (from –6.1 to –1) is observed in mountain taiga soils. Edaphic rather than climatic factors are the dominant controlling factor of C turnover in taiga soils of study region.