The mechanism dominating the color of the Calci-Aquic Vertisols in China

Abstract The Calci-Aquic Vertisols comprise one of the largest areas with low crop yields in China normally characterized with black color, high smectite contents but low organic matter contents. This study aims to establish a physical method for extracting black matter from the Calci-Aquic Vertisols, and explore the formation mechanism of black nano-organo-mineral complexes of the soils. The alternation of freezing-thawing and adding dispersant followed by oscillation and multi-level ultrasound(HOMU) method were proposed and tested to disperse soil and extract black matters from the topsoil layers (0−40 cm) and black soil layers (40−70 cm) of three Calci-Aquic Vertisols. Six different components including POM, white, light white, light black, black and nano-black were extracted successively and analyzed for blackness, organic matter content, particle gradation, mineral composition and elemental oxides. The nano-organo-mineral complexes are found be responsible for the Calci-Aquic Vertisols appearing black. Smectite (SM) absorbing organic matters mainly through cation bridges of Fe3+ and Al3+, hydrogen bond, van der Waals force formed the black nano-organo-mineral complexes which consists the main parts of the black matters in the Calci-Aquic Vertisols. Agricultural activities increased the organic matter contents in the topsoil layers, but the difference of formation environment resulted in the poorer crystallinity and higher content smectites and thus higher nano-organo-mineral complex contents and blackness in the black soil layers in the Calci-Aquic Vertisols. The result in this research is of great value for understanding the organic matter turnover and improving the productivity of the Calci-Aquic Vertisols.