Origins of hard carbonate nodules in arable Chernozems in the Central Russian Upland

摘要

The genesis of hard carbonate nodules in the lowermost horizons (180-200 cm) of arable soils in the southern part of the forest-steppe region of the Central Russian Upland was associated with a change in soil water regime. The conversion of forest to arable lands was studied in three agro-chronosequences located on flat interfluves and consisted of undisturbed soils under deciduous forests and arable soils with different durations of agricultural use. Due to arable agricultural activity, the upper soil horizons become drier in the summer during the growing season, whereas the lowermost parts get wetter in the spring and autumn after harvests. As a result, two types of hard carbonate nodules, which differed in morphology, origin and age, formed in the arable soils. The first type of hard nodules had a dense cryptocrystalline fabric in thin sections and colloform morphology viewed under an electron microscope, consisting of calcite with Si, Al and Fe peaks in EDS spectra, and had a C-14-age from 16,410 +/- 200 to 13,570 +/- 150 years BP. Their formation occurred due to an ascending of "old" carbonate matter in colloidal suspensions through capillary pores from parent rocks in the periods of strong heating of the soil surface; these nodules had an evaporative origin. The second type of hard nodules consisted of crystalline pure calcite and had a C-14-age of < 4,500 years BP. They had a hydromorphic genesis and developed in periods of water stagnation in the deep horizons and can be considered to be markers of a seasonal hydromorphism of arable soils in the studied area.