Soil formation in Greyzems in Moscow district: micromorphology, chemistry,clay mineralogy and particle size distribution

Greyzems (Grey Forest Soils) are zonal soils of the forest-steppe, in Russi a geographically situated between the (Podzo) Luvisols of the southern taig a forest and the (Luvic) Chernozems of the steppe. Greyzems are characteriz ed by a dark mollic horizon, with uncoated (bleached) silt and sand grains on pedfaces, and an argic horizon as diagnostic horizons. The FAO-Unesco so il map of the world shows Greyzems and Luvisols in Russia at this transitio n (the Russian soil map shows only Greyzems), while in similar geographic p osition in the USA and Canada the proportion of Greyzems is very small and Luvic Phaeozems/Chemozems and Albic Luvisols occupy those transitional zone s of the grassland-forest interface. Three Greyzem profiles, presently unde r forest, and developed on loess-like mantle loams of Late Weichselian (Val day) age in the northern forest-steppe zone of the East European plain (Mid dle Russian Upland) were described and sampled near Pushchino, some 100 km south of Moscow. Micromorphology, particle size data, chemical data and cla y mineralogy were studied. Based on the particle size distribution and the occurrence of fragments of a second humus horizon (SHH) the presence of two , rather similar, deposits in the solum is advocated. The following process es have been deduced from the study: (i) decalcification and secondary accu mulation of carbonates; (ii) humus accumulation, including the significance of the SHH; (iii) clay illuviation, presumably two main phases; (iv) biolo gical activity; (v) degradation of the mollic A: occurrence of bleached gra ins; (vi) downward migration of textural components and organic matter, in the Bt horizon along major pedfaces: occurrence of black organo-clay coatin gs and uncoated silt/sand grains; (vii) gleying. The tentative sequence of these processes during Late Weichselian and Holocene times leads us to conc lude that Greyzems are polygenetic. They formed as Podzo(Luvisols) under fo rest, with fine clay coatings in the fine pores inside the blocky and prism atic peds. in the Late Glacial and Early Holocene. The change to tall grass steppe in the Atlanticum created a mollic horizon, that degrades when fore st re-invades during the Subatlanticum. Fine clay, combined with organic ma tter forms black coatings on the major pedfaces. Uncoated silt and sand par ticles also migrate downward along those major pedfaces. Biological activit y is involved in the very complex pattern of the transitional AhE and EBt h orizons. Active gleying only occurs in the profile on the lowest topographi c position. These latter processes are stilt active today. Similar soils do occur in the grassland-forest interface in North America, except where the younger age of the landscape and high CaCO3 content at shallow depth preve nted their full development. (C) 1999 Elsevier Science B.V. All rights rese rved.