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The most prominent paleosol unit in the Chinese Loess Plateau (CLP) is the fifth paleosol complex (S5) with its well-developed very thick and dark colored pedons. To provide more insight in the formation of S5 and its environmental significance, the pedogenesis and clay mineral transformation in the S5 of the Wugong section (Shaanxi Province) on the southernmost CLP are analyzed. S5 at the Wugong section is essentially composed of three well-developed reddish pedons (i.e., S5 -1, S5 -2, S5 -3) which signify three glacial-interglacial climatic fluctuations during its formation. Complete decalcification in each pedon and a calcic horizon of only 30-50 cm in thickness beneath each of the three pedons suggests that after deposition the pedons developed with a relatively stable surface in a sustained warm and humid climate. Clay formation in the S5 includes neogenesis of clay materials by in situ post-depositional weathering and mechanical migration of the fine fraction after complete decalcification. Complete leaching of CaCO3, intensive clay formation (with 60-100% higher clay content than that in the overlying and underlying loess (L5 and L6)) and extremely high magnetic susceptibility in the S5 pedons reflected a warmer, more humid climate and soil environment for pedogenesis than in the 'optimum' Holocene. However, the chemical alteration of the phyllosilicate minerals was weak and restrained by the hard calcic horizon, the compact argillic horizon and the flat terrain. The major clay mineral weathering processes during the formation of the S5 pedons at the Wugong section were depotassication, hydrolysis of primary minerals and degradation of chlorite. The pedogenesis in a loess-paleosol sequence and its pedogenic environment can best be deduced from combined data on pedogenic properties, and geochemical and mineralogical characteristics. [ABSTRACT FROM AUTHOR]

The soil carbon reservoir is the largest carbon reservoir in terrestrial ecosystems and consists of soil organic and inorganic carbon stocks. Previous studies have mainly focused on the soil organic carbon (SOC) stock, and limited information is available about the soil inorganic carbon (SIC) stock. The Chinese Loess Plateau (CLP), which is located in the arid and semi-arid region of China, is an important inorganic carbon reservoir, with a thick soil layer that is rich in calcium carbonate. However, there are few reports on the SIC stock and its spatial distribution in this region. In the current study, the SIC densities and stocks for various soil types and land use patterns were evaluated based on 495 profiles with 2470 soil samples across the CLP, which were collected from the Chinese Second National Soil Survey. The results showed that in the top 1m of soil across the CLP, the average SIC density is 17.04kg/m², and the total SIC stock is approximately 10.20 Pg C (1 Pg=1015g). The SIC stock of the CLP accounts for approximately 18.4% of the total SIC stock throughout China. The average values of the SIC stock in the 0-20, 20-50 and 50-100cm depths of the CLP are 2.39, 2.92 and 4.89 Pg, respectively. Under different land use patterns, the order of the average SIC density is farmland≈grassland > forest in all soil layers. For the various soil types, the SIC density in the 0-100cm layer is the highest in alkaline soil and lowest in subalpine meadow soil, whereas the SIC stock is highest in loessial soil, eolian sandy soil and sierozem, and the lowest in subalpine meadow soil. These differences are largely a result of the area occupied by each soil type and the climate conditions. The results of this study provide basic information about carbon reservoir in China and contribute to our understanding of the SIC stock on the CLP as it relates to the carbon balance of terrestrial ecosystems. [ABSTRACT FROM AUTHOR]