VARIATIONS IN THE LATERAL MORPHOLOGICAL DIFFERENTIATION OF GILGAI SOIL COMPLEXES

The structure of gilgai soil complexes is a combination of 3-D units compli-menting, entering and/or crossing each other and having different morphological features: colour, slickensides, dispersed calcium carbonate, calcareous and gypsum pedofeatures, eluvial (leaching) and solonetzic features. The 3-D units with these morphological features are described. Nine variations in the verti-cal-lateral morphological structure of gilgai soil complexes are identified on the basis of reviewing literature on Vertisols of tropics and subtropics. Thirteen variations of gilgai soil complexes are identified on the basis of observations on Vertisols within the East European Plain.


INTRODUCTION
The development of Vertisols with gilgai micro-relief is accompanied by both vertical and horizontal (lateral) differentiation of soil profile (Hallsworth et al., 1955;Paton, 1974;Dudal and Eswaran, 1988;Wilding et al., 1990;Khitrov, 2003;. As a result, gilgai soil complexes are formed. Nine variations in the verticallateral morphological structure of gilgai soil complexes are identified on the basis of reviewing literature on Vertisols of tropics and subtropics . More variations of gilgai soil complexes are identified on the basis of recent observations on Vertisols within the East European Plain .
The aim of the present paper is to describe the diversity of variations in vertical-lateral differentiation of gilgai soil complexes in the south of the East European Plain.

METHODS
For each of the study objects the following methods were applied: soil surface micro-relief survey using a dumpy level, soil trench (cross-section) through the main elements of the micro-relief, soil coring by a hand auger with registration of main soil horizons.
The soil description along the trench was based on the standard guidelines (Kornblyum et al., 1982;Guidelines …, 2006), which included identifying morphons, marking their delineations by coloured nails and recording their relative coordinates at each point: length along the trench and depth from the surface. The latter was subsequently transferred to a relative height above the conventional zero level for the whole trench. For that purpose, the trench surface height was determined using a dumpy level with a 5 cm step and interpolation between the points measured. The relative height of a point on the trench wall was calculated as a difference between the trench surface height at the respective point and the depth of the point required.

RESULTS AND DISCUSSION
To analyse the variations in the vertical-lateral morphological differentiation of gilgai soil complexes, the 3-D units having different morphological features are individually described below. Their images have resulted from recorded parameters of horizontal plane (microrelief), lateral plane (four walls of the trench with intermediate planes recorded in the course of digging) and vertical soil cores taken at different points within the study plot.  combination with pillow-like weakly raised diapiric structures of the lower horizons' material; 1.4. Local surface U-shaped dark-coloured humified structures with eluvial (leaching) features within micro-lows in combination with deeper black concave lens-like structures under micro-lows with rising tongues at the lenses' edges under micro-slopes and weakly-raised diapiric structures of the lower horizons' material under micro-highs; 1.5. Local bowl-shaped bleached (eluvial, solodic) structures in micro-lows in combination with pillow-like diapiric structures of the lower horizons' material; 1.6. Layers non-differentiated by colour along the horizontal plane.
The 3-D units with slickensides (Fig. 2): 2.1. Weakly wavy layer of slightly varied thickness: the minimal thickness within the concave part in soils of micro-lows and the maximal thickness within the convex part in soils of micro-highs. The difference between the depths of the upper boundary of slickensides in microhighs and micro-lows is Z1  15-25 cm. The difference between the depths of the upper boundary of the V horizon and/or the AV horizon in micro-highs and micro-lows is Z2  15-25 cm.
2.2. Weakly wavy layer of moderately varied thickness: 25 < Z1  40 cm and Z2  15-25 cm.  The 3-D units containing dispersed calcium carbonate (effervescence with HCl) (Fig. 3): 3.1. A layer with weakly wavy upper boundary of total effervescence with HCl replicating the gilgai surface shape and having its relative height variation amplitude increased by no more than 1.5 times as compared to that of the gilgai surface as a result of weak leaching of calcium carbonate from soils of micro-lows.
3.2. A layer with strongly wavy boundary of total effervescence with HCl replicating the gilgai surface shape. Amplitude of vertical variation in its relative height is increased more than twice as compared to that of the surface, so that the upper boundary of effervescence coin- cides with the lower boundary of dark-coloured horizons or occurs no more than 10-15 cm above the latter.
3.3. A layer with strongly wavy boundary of effervescence, having a wide concave minimum in soils of micro-lows and a wide convex maximum centred in soils of micro-highs and spreading to microslopes with formation of sagging edge resembling a mushroom cap.
3.4. A 3-D network of soil mass containing dispersed calcium carbonate under convex elements of micro-relief. Soils of micro-highs form thickened nodes of the net. Sides are often strongly deformed because of intrusion of carbonate-free soil tongues rising at an angle. The spaces between nodes are occupied by completely leached soils of micro-lows.
3.5. Absence of disperse calcium carbonate in all soils of gilgai complex.  5.3. Closed local bowl-shaped structures of the EL horizon in soils of micro-lows (Fig. 1). 5.4. Closed local bowl-shaped structures of either the AYel,q and Qek horizons, or the AU/Qox,el horizon, or the AU/Qel horizon in soils of micro-lows in combination with the Qek,ox horizon in soil of micro-slope (Fig. 1). Theoretically, there are over seven thousand combinations of the above-described 3-D units, but many of them are unreal. Various 3-D units with different morphological features are developing in a close relationship with each other under the influence of the five groups of processes. The first group of processessignificant changes in soil volume: Vertisols swell when moistened and shrink when dried, which leads to the formation of wide and deep cracks dividing the soil into vertical blocks of various sizes. The second group of processesinternal lateral-rising migrations of soil material from the lower horizons induced by shear and plastic deformations of the clay material. The third group of processeslateral re-distribution of water and solid particles along the curved soil surface that leads to spatial and temporal differentiation of soil moisture over micro-relief, local erosion of micro-highs and accumulation of run-off products within micro-lows. The fourth group of processesthe organic matter accumulation and transformation, calcium carbonate dissolution, migration and accumulation, iron and manganese reduction and oxidation resulting in red-ox pedofeatures, migration of soluble salts, solonetzization and solodization of different intensities depending on moisture differentiation over micro-relief. The fifth group of processesgravitational fall of surface horizons' fragments into open cracks.
In total, there are 13 variations in gilgai soil complexes identified within the East European Plain. Their structure can be represented by the following combinations of the 3-D units described (Table).
Variations in the morphological structure of gilgai soil complexes represented by combinations of the 3-D units with specific morphological features: colour; vslickensides; cadisperse calcium carbonate; nccalcareous pedofeatures; csgypsum; eleluvial features; snsolonetzic features. The 3-D unit numbers are given in the text above. Soil taxonomic designations correspond to the WRB-2014 codes (IUSS, 2014) in a micro-low  micro-high sequence. No. Trench no.

Soils
3-D unit codes and morphological features colour v ca nc cs el sn