Fabric of topsoil horizons in aridic soils of Central Asia .

Abstract

Two surface microhorizons – vesicular and platy – were found to be typical of the fabric of topsoil horizons in gravely loamy soils located along an arid/continental climatic gradient in the Subboreal zone of Central Asia (from southern Russia to Uzbekistan, Kazakhstan, and Mongolia). Their genesis is related to active vesiculation (for vesicular А<sub>1</sub> microhorizon) and cryogenesis (for platy А<sub>2</sub> microhorizon) against a background of high clay mobility and/or eolian accumulation of fine material. New mechanisms responsible for the formation of vesicular pores are suggested: (a) the leaching of dense complete crystal infillings (moldic voids) and (b) the development of biogenic capsules typical of iron bacteria that were diagnosed by the microbiological method of fouling Agar-covered glasses under natural conditions. In a series of studied soils, Yermic Regosols are characterized by additional specific microfeatures: (1) iron depletion from the material around the pores and formation of iron concentrations in the intrapedal mass owing to biogenic mobilization of iron with participation of sulfate- and iron-reducing bacteria and (2) the accumulation of organic matter (residues of microbial cells) in the vesicular pores.