Genesis and properties of black soils in Buryatia, southeastern Siberia, Russia

Abstract

Soils with deep and dark brown to black humic surface horizons, derived from sandy-silty aeolian and alluvial sediments, are relatively fertile and store huge amounts of carbon. Globally, they mainly correlate with steppe ecosystems. Because their deep and dark epipedons are frequently rich in Black Carbon, it was recently suggested that they developed due to prevalent burning of the semiarid vegetation. This paper describes six soils with deep and dark humic surface horizons, located in catchments of the Selenga and Uda rivers in Buryatia, southeastern semiarid Siberia. The organic matter of most of these soils not only originates from grass steppe, but in addition from trees and shrubs. Five soils can be classified as Chernozems and Kastanozems ( WRB, 2006); one soil is a deep and black Anthrosol, occurring in patches in the region. All soils have calcic horizons below the mollic epipedon, some with cambic Bw horizons in between. As the parent material does not contain carbonates, aeolian input and hydrolysis of Ca-silicates contribute to the development of the secondary carbonate accumulation, which is frequently cryoturbated. All profiles are more or less stratified due to solifluction. Texture ratios (<6.3/63-2000 μm) show maxima in the dark A horizons, indicating advanced paedogenesis. This is supported by weathering proxies, including SiO 2/Al 2O 3 and Al 2O 3 × 100/Al 2O 3 + Na 2O. Increased concentrations of TiO 2, MgO and Fe 2O 3 presumably indicate advanced formation of chlorite. Radiocarbon analyses indicate that these deep and dark mollic horizons started to develop during the more humid-Holocene Climatic Optimum, ca. 7000–5000 BP. Palynological studies confirm expansion of forests in the Lake Baikal region in this period. This is in agreement with higher alkane ratios nC27/nC31 in most of the deep and dark A horizons, indicating that these epipedons, diagnostic for Chernozems and Kastanozems, did not develop exclusively under grass steppe, thus supporting the hypothesis that a significant part of the soil organic matter is inherited from trees and shrubs. This seems to be in contrast to Middle Europe and the Russian plain where Chernozems presumably developed during the early and mid-Holocene under grass steppe and subsequent degraded with increasing humidity and invasion of forests. High contents of Black Carbon in the Calcic Chernozem P4 and lower ones in the long-lasting occupied Hortic Anthrosol indicate that the deep and dark A horizons of Mollisols most likely do not result from frequent burning, but from biochemical processes.