SoilSci13_10KovdaLO.fm

Abstract-Soil organic matter (SOM) was studied in relation to vertic processes (i.e., shrinking/swelling, cracking, vertical turbation, lateral shearing, gilgai formation) in Vertisols and vertic soils of the North Cau casus in Russia, and Texas and Louisiana in the USA. Their impact on SOM properties and distribution was analyzed according to various levels of soil organization, such as soil cover, profile, horizon, and aggregate structure using chemical methods, micromorphology, isotopic analyses, and physical fractionation. The greatest variations both in the distribution and properties of SOM were found in mature Vertisols at the level of soil cover including C tot , organic carbon stocks, stable carbon isotopic composition, and SOM 14 C age, chemical composition. The distribution of SOM at the profile and horizon levels was related to the function ing of Vertisols during wet dry cycles. The isotopic and chemical study of densi granulometric fractions at the aggregate level reflected the minor role of vertic processes