The 14C variations of pedogenetic CaCO3 in Steppe soils under climatic sequence, Mongolia
Authors: Maki Asano, Kenji Tamura, Yuji Maejima, Yasuhito Shirato, Hiroyuki Matsuzaki and Teruo Higashi

Soil horizon enriched with calcium carbonate (CaCO3) is prominent feature in Mongolian steppe soils. Such a horizon is called as calcic horizon (Bk or Ck horizon). The carbonate accumulated in soils by calcification which is one of major soil forming processes in arid and semiarid regions. In general, the depth of calcic horizon is related to the annual precipitation (1). It is important to clarify the forming process of pedogenic CaCO3 accumulation from the viewpoint of the carbon cycle in terrestrial ecosystem. However, that was reported only in the limited areas, such as the southern and northern parts of the United States. Recently, it came to be able to measure the small amount of 14C by the Accelerator Mass Spectrometry (AMS), and radiocarbon isotope ratio of Bk horizons can be considered as an indicator of carbonate movement in soils. The purpose of this study is to clarify the forming process of Bk horizons by using radiocarbon isotope analysis.
Soil samples were taken from six points at forest steppe, steppe and govi steppe in Mongolia. The soil samples of Bk horizons were reacted with 85% phosphoric acid under a high vacuum to release CO2 and it was reduced to graphite with the hydrogen over the high purity iron. The 14C/12C isotope ratios were measured by the MALT-AMS system at the University of Tokyo (2). The carbonate contents in soils were calculated from inorganic carbon content measured by wet combustion method.
There was a remarkable difference in the carbonate content and 14C/12C isotope ratio distributions between forest steppe and govi steppe soils. The A horizons of forest steppe and steppe soils donft include carbonate and the vertical distribution of the 14C/12C isotope ratios show various tendencies and vary widely. On the other hand, govi steppe soils contain carbonate over the entire soil profile and the 14C/12C isotope ratios decrease with depth linearly. The results of the 14C/12C isotope ratio of Bk horizons suggested that the carbonate accumulation process and rate was affected by vegetation and climatic factors.

(1) Arkley, R., Soil Sci, 96.239-248 (1963)
(2) Matsuzaki, H., Nakano, C., Yamashita, H., Maejima, Y., Miyairi Y., Wakasa, S., Horiuchi, K., Nucl. Instr. and Meth. B 223-224, 92-99(2004)

This work is supported by a CREST project of Japan Science and Technology Agency and Grobal Environment Research found of Japan environment ministry.