Dissolved Organic Matter of Chernozems of Different Use: the Relationship of Structural Features and Mineral Composition

Structural peculiarities of the organic component of dissolved organic matter (DOM) of typical chernozems (Protocalcic Chernozem), differing in the principal direction of soil organic matter transformation (accumulation or mineralization), were analyzed in relation to the mineral composition of DOM. Spectrometry in the ultraviolet and visible range and fluorescence spectroscopy were applied to characterize the structural features of the organic component. These approaches are most often used for these purposes due to the rapidity and accessibility of instruments. The content of mineral elements (Al, B, Ba, Ca, Fe, K, Li, Mg, Mn, Na, P, S, Si, and Zn) was determined by optical emission spectrometry with inductively coupled plasma. It has been revealed that DOM of plowed chernozems, where mineralization is the leading process of organic matter transformation, is characterized by increased indices E2/E3, BIX, and SUVA254, which indicates a smaller molecular mass (MM) and a greater contribution of organic matter of microbial origin and of aromatic structures to the composition of DOM as compared to soils not involved in agricultural use. The analysis of the mineral component has shown that Si and Ca are the main elements in DOM. The involvement of chernozems in agricultural use results in a decrease in Ca content and in an increase in Si, Fe, and Al content, which reflects greater weathering of the mineral matrix of chernozems. The rank correlation analysis has revealed significant relationships between the structural characteristics of DOM (E2/E3, SUVA254, MM, BIX, and T) and the content of mineral elements (Ca, Al, Fe, and Si), which testifies to the influence of weathering processes on the formation of soil DOM.