Vertic properties and gilgai-related subsurface features in soils of

Catena 128 (2015) 95–107

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Vertic properties and gilgai-related subsurface features in soils of south-western Poland Cezary Kabala a,⁎, Tomasz Plonka b, Agnieszka Przekora a a b

Wrocław University of Environmental and Life Sciences, Institute of Soil Science and Environmental Protection, ul. Grunwaldzka 53, 50375 Wrocław, Poland University of Wroclaw, Institute of Archeology, ul. Szewska 48, 50-139 Wrocław, Poland

a r t i c l e

i n f o

Article history: Received 18 December 2014 Received in revised form 22 January 2015 Accepted 27 January 2015 Available online xxxx Keywords: Vertic Bowl structures Chernozems Pheozems Planosols

a b s t r a c t More than 650 subsurface concave structures (1–20 m long, 0.5–7 m wide, and 0.1–0.9 m deep) were described in soils developed of clays overlain by loess in a large-scale archeological excavation in SW Poland. The structures are considered to be microlows and microhighs similar to those in gilgaied vertisols, which have never been reported in Central Europe until now. Soils do not have a clayey texture throughout the profile; however, the thick and humus-rich mollic horizons, as well as a clay-enriched argic layers, developed directly above vertic horizons have favored soil cracking and shrink–swell phenomena in the clayey subsoil. As a result, well developed wedge-shaped (lenticular) aggregates have formed in the clayey subsoil. Aggregates have grooved surfaces with an oriented matrix (slickensides) and are ordered under variable angles from 10 to 60°. A periglacial origin of these structures has been rejected and the mid- to late-Holocene period (since the Neolithic epoch) has been considered to be favorable for intense shrink–swell phenomena, vertic horizon development and clayey mass cycling responsible for the subsurface bowl and chimney structure development. A micro-mosaic of soil units related to these structures has been distinguished, including (non- or deep-vertic) Stagnic Chernozems/Phaeozems above the microlow centers, Mollic Planosols (Vertic) above the microslopes, and Mollic Planosols (Protovertic) over the microhighs (chimneys). It is concluded that a temperate climate (as in Central Europe) allows the formation of vertic properties and subsurface mass reorganization similar to those in soils with gilgai microrelief, if specific conditions – clayey substratum, position in landscape, soil moisture regime and the land use connected with vegetation type – support deep seasonal changes of soil moisture and shrink–swell phenomena in the subsoil. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Vertisols are a soil group representing an important resource for many countries, including Australia, India, China, the Caribbean Islands, and the USA (Wilding, 2004) due to high natural soil fertility and suitability for agriculture; however, this resource is sometimes difficult to manage and is locally underutilized (Atasoy, 2008; Bogunovic et al., 1991). The specific physical properties of Vertisols are mainly connected with parent material rich in a clay fraction (Reeve et al., 1980). By convention, more than 30% clay fraction in the fine earths is required (IUSS working group WRB, 2014). Such materials include regolith from the weathering of base-cation rich igneous, metamorphic, and sedimentary rocks (including pyroclastic), both deposited in situ and, most often, in alluvial and colluvial beds (Moustakas, 2012; Mykhaylyuk, 1996). More rarely, the parent materials for Vertisols are glacio-limnic, lacustrine or marine clays (Mocek et al., 2009; Olszewski, 1956). The crucial clay compound of the Vertisol substratum is a smectite, due to its high shrink–swell activity (Righi et al., 1998). ⁎ Corresponding author. E-mail address: [email protected] (C. Kabala).

http://dx.doi.org/10.1016/j.catena.2015.01.025 0341-8162/© 2015 Elsevier B.V. All rights reserved.

The majority of Vertisols is found between 45°N and 45°S (Graham and Southard, 1983; Harris, 1958; Knight, 1980), mostly in tropical and subtropical zones (Nordt et al., 2004; Pal et al., 2012). The intense alternation between wet and dry periods in these zones, in combination with a smectitic substratum, allows shrink–swell phenomena and morphological diversification of the soil profile (Kishné et al., 2009). Upon drying, the clayey soil aggregates shrink and the soil cracks, sometimes down to 80–100 cm, while the surface soil develops a granular or crumb structure (“surface mulch”). Mulch granules fall into cracks during the dry season. Upon re-wetting, the soil increases its volume; however, it cannot expand back to the crack space occupied by the mulch material. Continued swelling generates the pressure that results in the shearing — sliding of soil masses against each other that develops a specific soil structure (lenticular or wedge-shaped) and shear planes (polished surfaces called “slickensides”). Strong development of vertic structures may be accompanied by uneven surface microtopography — “gilgai” (Florinsky and Arlashina, 1998; Kovda and Wilding, 2003; White and Agnew, 1968). Gilgai consists of alternating depressions (microlows, bowls) and mounds (microhighs, chimneys) with a denivelation ranging between 0.02 and 1 m (Knight, 1980; Miller et al., 2010). In some Vertisol landscapes, the chimney material is exposed at the surface and is termed

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a puff (Paton, 1974; Miller et al., 2010). Under natural conditions, the depressions are seasonally filled with water forming shallow ephemeral (micro-)lakes (Paton, 1974). The most common “normal” form of gilgai consists of round microlows. On slightly sloping terrains, “wavy” or “linear” gilgai may occur with parallel ridges and valleys that run with the slope (at right angles to the contours). The “lattice” gilgai is a transitional form on very slight slopes (a less than 0.5% slope). Although Vertisols can be found under temperate climates and in quite cool regions (Dasog et al., 1987), in Europe these are mainly encountered in the Mediterranean zone (Barbera et al., 2008; Bogatyrev, 1958; Filipovsky, 1974; Mirabella et al., 2005; Moustakas, 2012; Shishkov and Kolev, 2014) and in the north Caucasus foreland (Kovda et al., 2010). In the cooler regions of Europe, Vertisols (as Smolnica or Smonitsa) are rarely reported; however, vertic properties have recently been recognized in Cambisols, Luvisols, Chernozems and other soil groups in many countries, including Moldova (Ursu, 2012), Hungary (Novák et al., 2014), Slovakia (Skalský et al., 2009), Germany and Austria (Leitgeb et al., 2013), and Poland (Marcinek and Komisarek, 2004), as well as under the continental climate of southern Ukraine (Mykhaylyuk, 1996) and Russia (Khitrov, 2012; Khitrov and Rogovneva, 2014). Although seasonal water regime alternation and moisture deficit in soils during drier seasons under temperate climate is much lower than in the warmer climates, it seems obvious that the rarity of clay materials at the land surface rather than climate is responsible for the scarcity of Vertisols in these regions (Wilding, 2004). Sandy, loamy and silty textures prevail in the glacial and periglacial surface deposits that cover the huge areas of Central and North Europe affected by the Pleistocene glaciations, whereas clays occur sporadically (Bieganowski et al., 2013; Drewnik et al., 2014; Lasota and Blonska, 2014; Świtoniak, 2014). Clay-enriched residua of pre-Pleistocene rocks, are in most cases covered by coarsertextured materials and only provide conditions for subsurface development of the vertic properties (Gerasimova and Khitrov, 2012; Jakubus et al., 2013; Terhorst, 2007). Recent findings have resulted in a revision of the concept and spatial extent of Vertisols. The diversity and range of soils with vertic properties are much greater than previously recognized, so classification criteria and soil classification systems have been modified to recognize this fact (Wilding, 2004). For a long time, Vertisols were not expected to occur in Poland or surrounding countries due to the inappropriate climate (either too temperate or too cool). Vertic properties (slickensides and lenticular/wedge-shaped structures) have not been listed in guidebooks and the respective soil group was not distinguished until 2011 (Kabala, 2015; Labaz and Kabala, 2014). Soils that potentially fulfilled the criteria for Vertisols (“Smolnice”) were classified as “Black earths”, together with non-clayey soils having a thick humus horizon and stagnic or gleyic properties in the subsoil (Mocek et al., 2009; Olszewski, 1956; Prusinkiewicz, 2001). Identification of vertic properties and Vertisols in Poland is hindered by relic cryoturbation features that are common in materials transformed under the periglacial conditions of the last two glaciations (Jary, 2009). Various kinds of downsinking pockets, updoming diapirs, intensely convoluted forms (“festoons”), and cracks interpreted to be typical cryoturbations (Vliet‐Lanoë, 1991) are morphologically similar to the results of shrink–swell processes, in particular in soils with textural discontinuity (Vandenberghe, 1992). These difficulties lead to skepticism in the identification of vertic properties in soils previously affected by periglacial conditions. Vertisols and soils with vertic properties have not been identified in the lowlands of south Poland, even though favorable conditions for their formation locally exist (Kabala, 2015). Numerous specific structures – shallow pits in clayey subsoil, filled with dark humus material – have been found in the course of archaeological “rescue” excavations near Wroclaw in south-western Poland on a site prepared for a new motorway. The archaeological artifacts, fragments of pottery, and flint pieces, have mainly been found in the humus layer, whereas just single pieces (up to 3 fragments) have been

found in 48 pits (i.e., in 7.4% of all exposed pits). Moreover, the arifacts have mainly been located in the uppermost parts of the pits, so that it can be supposed that they got there by bioturbation. The archaeological materials were dated to the Early Neolithic (the Linear Pottery culture), through the Middle Neolithic (the Funnelbeaker culture), early Bronze Age (the Únětice culture), late Bronze/early Iron Age (the Lusatian culture), Pre-Roman and Roman Iron Age up to the late Middle Ages and modern times (Wiśniewski, data not yet published). The colonization of the Silesian Lowland started as early as in the Middle Paleolithic period (Bobak et al., 2013; Wiśniewski, 2005, 2006) and during the Neolithic period the area was densely inhabited; thus, the remains of settlements and manufacturing activity are very common (Kulczycka-Leciejewiczowa, 2000, 2010). It has been realized that the pits were not settlement objects. Initially, they were interpreted to be relics of clay mining for making pottery in the Neolithic period and later. This opinion was consistent with chemical analyses prepared by Trąbska (2007), who affirmed excellent usefulness of these clays for pottery production. However, unusual characteristics of the pits, including (i) single artifacts in pits, typically at their tops, (ii) uniform fill of the objects without stones and involved sublayers, and (iii) strange planigraphical features, i.e., some pits are very close to one another but they never intersect, drew the attention to the non-anthropogenic origin of these forms. Already during preliminary pedological inspection, wedge-shaped structures and shiny “slickensides” were identified, as well as an outline of microlows and microhighs typical for gilgai microrelief (Paton, 1974). This suggested the natural origin of the structures. The aim of the study was to characterize vertic properties still rarely documented in the soils of Poland and Central Europe. This study also describes the first in the Central Europe findings of subsurface features presumably connected with gilgai microtopography. Also, an attempt was made to explain the origin and age of these forms taking into account possible climate conditions and human impact. 2. Materials and methods 2.1. Description of the area The area is located between Domasław and Tyniec villages, in the Silesian Lowland, 10 km south of the city of Wroclaw in south-western Poland, N 51°01′00″, E 16°56′34″ (Fig. 1), at an elevation of 136.5– 137.8 m ASL. The generally flat surface of the lowland is varied in this area with hills, residuals of the large-scale early-Pleistocene erosion. The archaeological site Domasław 34 is located on the edge of a hill, at the mouth of a poorly marked valley drained by a single stream, presently regulated (channeled). The area has been covered at least twice by the Scandinavian ice sheets (San/Elster glaciations, 700–400 thousand years BP, and Odra/Riss 1/Saale-Drenthe glaciations, 300– 230 thousand years BP), but the thickness of the glacial deposits in this area is generally very shallow. The Neogene clays are present already at a depth of 70–120 cm, mostly with inserts of sand or lignite, sometimes glacio-tectonically distorted. The subsurface pavement of the Scandinavian gravels and stones resting directly over the residual clays proves the former existence of glacial till or glacio-fluvial deposits. Most of the stones are “ventifacts” with a clearly marked edge between the two surfaces polished (abraded) by wind. The entire land surface is presently covered with loess and loess-like materials whose thickness reaches 70–120 cm (locally more than 200 cm), sedimented during the last glacial period (Wisła/Weichsel/Würm), 70–55 thousand years BP (Badura et al., 2013; Jary, 2009). The Silesian Lowland has all the characteristics of a transitional temperate climate. Mean annual air temperature averages 9 °C and ranges from −0.4 °C in January to 18.8 °C in July (Fig. 2). Mean annual precipitation averages 567 mm with the most precipitation falling in June–July (77–80 mm monthly mean) and the minimum in January–February (30–31 mm). Snow covers the soil for 60 days (annual mean) between


97

Fig. 1. Localization of the area under investigation.

November and March. The annual climatic water balance (difference between precipitation and potential evapotranspiration) is slightly positive — ca 10 mm/year due to low evapotranspiration during the winter months (November–February). However, the water balance during the warmer months is negative, up to −20 mm/month (Pawlak, 2008). Soils with thick, very dark, humus-rich, structural, and base saturated mollic horizons predominate in this area. Lime clods are often present in the A horizon, whereas natural carbonates are leached to various depths, mostly below 70–90 cm. All soils are imperfectly drained and have stagnic or gleyic properties, at least in the bottom of soil profile (if deeply drained). Depending on the texture differentiation, carbonate leaching depth and redoximorphic phenomena, the soils were classified (IUSS working group WRB, 2014) as Endostagnic/Endogleyic Chernozems or Phaeozems, Mollic Gleysols or Mollic Planosols (Kabala, 2015). Presently, most of the soils are carefully drained and intensively fertilized for cereal (wheat, barley), corn, sugar beet, and rape production. For economic reasons, cattle breeding and dairy farming collapsed in the early 1990s resulting in the rapid disappearance of pastures and meadows. Forest coverage is among the lowest in Poland and does not 20

100 90 80

15

70 10

50 40

5

30 20

T (oC)

P / E (mm)

60

0

10 0

-5 J

F

M

A

M

I

J

Month

A

S

O

N

D

T P E

Fig. 2. Climatic water balance (P = Precipitation, E = Potential evapotranspiration) and air temperatures (T) in Wroclaw.

exceed 7% of the county area. The remnants of broadleaf stands mainly occupy the river embankments and other low-lying sites that are too wet for farming. 2.2. Field and laboratory methods Pedological research was conducted within and on the edge of the archaeological site Domasław 34 delineated in 2007/2008 on the grounds planned for the S-8 motorway. Archeological investigation has covered an elongated irregular rectangle, 270 m long and 70– 75 m wide (total area of excavation — 19,630 m2), extended along the N–S direction (Fig. 1). Initially, the humus layer was removed from the whole area to the depth of 30–45 cm, which revealed a mosaic of lighter and darker spots in the subsoil. In cross-section, these “dark spots” were small “cavities” in the clayey subsoil, filled with humusrich silt-loamy material. All the “cavities” have been precisely located, measured, and drawn on a map using geodetic methods, including the determination of altitude ASL, length and width, and the orientation of the longer axis. After extraction of the humus material infilling the cavities, their maximum depth was registered. A total number of 651 “cavities” were characterized (Fig. 3). Four soil profiles were located in relatively small “cavities” situated in the northern, central and southern sections of the archaeological site. All these pits are “truncated” due to the prior removal of the humus layer. The soils have a similar morphology, texture, and chemical characteristics; thus, the description of only two somewhat different profiles (No 1 and 2) is provided in this paper (Table 1). For comparison, one complete soil profile (that includes the original humus horizon) was described at the border of the archaeological site (No 3), where the “cavities” were absent. All the soil layers were individually sampled for laboratory analysis. Soil samples were dried, crushed and sieved. All further analyses were conducted in fine earths (diameter ≤ 2 mm). Particle-size distribution, after removing the organic matter and carbonates, and sample dispersion with hexametaphosphate–bicarbonate, were conducted using sieves for sand separation and the hydrometer for silt and clay fractions (Van Reeuwijk, 2002). Soil pH was potentiometrically measured in a 1:2.5 suspension (soil:distilled water, v/v). Calcium carbonate

98


separated by centrifugation preceded by ultrasonic sample dispersion. Clay samples were treated with 10% H2O2 to remove organic matter. The Mg2 +-saturated slides were scanned before (symbol N in Fig. 7) and after treatment with glycerol (symbol Gl). The slides were finally heated to 550 °C and rescanned (symbol 550). Diffraction pattern were obtained using CuKα radiation and step-scanning between 3 and 40° 2θ, using 0.05° 2θ increments with a 3 s counting time. 3. Results 3.1. Humus-infilled “cavities” (microlows) More than 650 “dark spots” that were in fact the outlines of microlows infilled with humus soil were discovered within an area of ca. 2 ha after removal of the humus layer (Fig. 3). The area occupied by microlows was the widest in the northern, nearly flat section (slope inclination: 0.5%) and became narrower in the southern, more sloped section (inclination: 1.5%). The microlow outlines have both spherical and elongated shapes. Large spherical forms dominated in the flat section, whereas the elongated forms occurred in the more inclined sections. Their longer axes were always roughly parallel to contour lines (perpendicular to the slope). Numerous little spherical forms were scattered between and on the sides of large elongated forms, also in more sloped sections. More than 20% of the forms had irregular shapes: bifurcated (Y-shaped), U-shaped, pear-shaped, etc. The mean length of the microlow longer axis was 2.8 m in the range 1–20 m (Fig. 4b). The mean dimension of the shorter axis was 1.5 m and varied between 0.5 and 7 m (Fig. 4a). The ratio of the longer to shorter axis (Fig. 4c) ranged between 1 (circular) and 9 (strongly elongated microlow) and a mean value was ca 1.8 due to the general prevalence of spherical and irregular forms. The depth of microlows was measured from the truncated surface (after removal of the plow layer) to the bottom of humus infilling at its maximum thickness. The thus measured depth of the microlows varied between 10 and 90 cm and amounted to 29 cm on average (Fig. 4d). In fact, the apparent depth of microlows down to contact with the clay subsoil was 20–40 cm higher (Fig. 5), but the methodology of archeological investigation involved only humus-enriched layer measurement. 3.2. Soil morphology and texture

Fig. 3. A sketch of microlow outlines in the archeological site after humus layer removal. Soil profile location indicated (numbers in the circles).

content (equivalent) was determined using the gas volumetric method, based on carbonate dissolution in reaction with 10% HCl (Van Reeuwijk, 2002). Soil organic carbon (TOC) was determined by wet oxidation using a potassium dichromate–H2SO4 mixture with an external heating. Total nitrogen (Nt) in the humus horizons was measured using the Kjeldahl semi-automated method. Due to a neutral or alkaline reaction, soil acidity and the concentration of exchangeable aluminum were not measured, and the base cations (Ca, Mg, K, and Na) were determined, for comparison, using ammonium acetate at pH 7 and ammonium chloride at pH 8.2 (Van Reeuwijk, 2002). The concentration of elements in all extracts was measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The mineralogical composition of the clay fraction (b 0.002 mm) was determined by X-ray diffractometry (XRD). The clay fraction was

All soils have clearly differentiated texture — silt loam in the topsoil and in the microlow infilling, and clay or heavy clay in the subsoil and in the microhighs (chimneys) separating the microlows (Table 2). The thickness of the silty-textured top-layer is quite uniform in the adjacent area (ca 50–60 cm, profile 3), whereas it is very variable (45–130 cm) in the area with microlows. Thin gravelly layer consisting of a Scandinavian material (red granite, quartzite, limestone, etc.) is present at the loess–clay contact (Fig. 5). The layer is continuous on the microslopes and in the bowls. Larger gravels and stones have wind-polished upper surfaces (ventifacts). Not the entire loess volume is transformed into a black (10YR 1-2/12) humus horizon, probably due to seasonally excessive soil wetness above the clayey subsoil that may suppress activity of earthworms and other burrowing animals. The bottom part of the humus horizon often has redoximorphic features (reddish spots on ped surfaces) and a higher clay content as compared to the topsoil (Table 1). Accordingly, clay illuviation is often recognizable directly below the humus horizon (as clay cutans on ped surfaces). This layer, resting directly above the clayey subsoil, has strong redoximorphic features due to seasonal water stagnation, in particular in bowls. Humus horizons have a fine to medium granular structure in the topsoil (profile 3) and a medium to coarse granular and blocky subangular structure in the microlows (Table 1). Earthworms are very abundant and the features of their activity (casts, coprolites, channels, etc.) constitute up to 25% of the humus horizon volume. Numerous


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Table 1 Morphological features of representative soil profiles. Horizon Depth, cm

Colour (moist)

Lower Texture Structure Consistence boundary class Dry Moist

Slickensides Coatings Carbonates Earthworms Mottles

Profile 1 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–60 10YR GD SiL BS, KO, f SHA FR, st, pl – – SL vc 1/1 Ag 60–100 10YR CL, gl SiL BS, m SHA FR, st, pl – – SL co 2/2 Btg 100–115 10YR CL, w SiL BA, m HA FI, st, pl – CL N fw 5/4 115–120 (Gravel-) stone line (Scandinavian red granite, sandstone, quartzite), continuous, enriched in sand fraction, wet 2Big 120–150 2.5Y 7/1 GD C WE, c VHA FI, vst, + H SL – vpl 2Ckl 150–180 5GY 7/1 n.d. C MA VHA FR, vst, – – SL – vpl Profile 2 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–55 10YR GD SiL BS, KO, f SHA FR, st, pl – – SL 1/1 Ag 55–80 10YR CL, gl SiL BS, c SHA FR, st, pl – – SL 2/2 SL Btg 80–100 10YR CL, w SiL BA, m HA FI, vst, pl – CL, H 6/3 100–105 (Gravel-) stone line, continuous, enriched in sand fraction, wet 2Big 105–120 2.5Y 7/1 GD C WE, c VHA FI, vst, + H SL vpl 2Ckl 120–160 5GY 7/1 n.d. C MA VHA FR, vst, – – MO vpl Profile 3 (horizon depths measured from original soil surface) Ap 0–30 10YR CL, w SiL GR, KO, SHA FR, st, pl – 2/2 m A 30–45 10YR GD SiL BS, KO, c SHA FR, st, pl – 2/1 ABg 45–55 10YR CL, w, gl SiL BA, m SHA FR, st, pl – 3/3 55–60 (Gravel-) stone line, continuous, enriched in sand fraction, wet 2Big 60–90 10YR GD C PR-BA, c HA FI, st, vpl + 5/7 2Cg 90–115 2.5YR AB, sm SiC BA, c VHA FI, vst, – 7/1 vpl ca. 115 Stone line (exclusively rounded stones), discontinuous 3Ckl 115–160 2.5YR n.d. C MA VHA FR, vst, – 7/1 vpl

Fe–Mn conc.

Reductic

Oxic

–

–

so, f, vf

2.5Y 4/1, 5%, as 2.5Y 7/2, 10%, as

–

so, f, vf

60%, as

10YR 5/6, 40%, so, f, vf ai – so, m, co

N95%

10YR 5/6, 20%, so, f, vf ai

vc

–

–

–

co

2.5Y 4/1, b5%, as 2.5Y 7/2, 15%, as

–

so, f, vf

fw

–

60%, as

–

N95%

10YR 6/8, 20%, so, f, ai fw 10YR 4/8, 40%, so, m, ai co – so, m, co

–

SL

vc

–

–

–

–

SL

vc

–

–

–

–

SL

co

2.5Y 7/2, 10%, as

–

so, f, vf

CL, H

SL

fw

–

SL

–

2.5Y 7/2, 20%, as 60%, as

10YR 6/8, 20%, ai 10YR 6/8, 40%, ai

so, f, fw so, m, co

–

MO

–

N95%

–

so, m, co

Lower boundary: AB — abrupt, CL — clear, GD — gradual, gl — glossic (tonguing), sm — smooth, w — wavy; Texture class: C — clay, SiC — silty clay, SiL — silt loam; Structure: GR — granular, KO — koprolitic, BA — blocky angular, BS — blocky subangular, PR — prismatic, WE — wedge-shaped (lenticular), MA — massive; Consistence (dry): SHA — slightly hard, HA — hard, VHA — very hard; Consistence, stickiness and plasticity (moist): FR — friable, FI — firm, st — sticky, vst — very sticky, pl — plastic, vpl — very plastic; Coatings: CL — clay, H — humus; Carbonates: N — non-calcareous, SL — slightly calcareous, MO — moderately calcareous; Fe–Mn concentrations: so — soft accumulations; Redox mottles: ai — interior of aggregates, as — surface aggregate. Dimension (all features): f — fine, m — medium, c — coarse; Abundance (all features): vf — very few, fw — few, co — common, vc — very common (abundant); n.d. — not described; “–” means lack of feature.

black humus “tongues” dissect the illuvial layer below the A horizon and continue down to the clay subsoil. The tongues are wider (up to 5 cm) in the microslopes. In bowls, the tongues are thinner (up to 2 cm), irregular, and often recognizable as thick humus “skins” on the vertical walls of larger angular peds. It should be noted that cracks regularly open in late summer on the soil surface on the fields adjacent to the archeological site, where they form irregular semi-polygonal nets. The upper part of the clayey subsoil in the area occupied by microlows has a lenticular (wedge-shaped) structure. Individual peds have lengths up to 15 cm and thicknesses up to 6–8 cm (Fig. 6). The peds are embedded at very low angles in the bottom part of microlows (bowls), while the inclination of their longer axis reaches 60° at the microslopes. Most wedge-shaped peds have lengthwise grooved, shining surfaces (slickensides), whose colors are strongly reductimorphic; whereas, the rusty oximorphic colors predominate in the internal part of peds. The vertic horizon is dissected by only a few humus-filled vertical tongues that start in the A horizon (in particular, in the microslopes). The thickness of the vertic horizon varies between 50 and 60 cm in the microslopes and ca. 30 cm in bowls. Wedge-shaped structures disappear in the top part of the microhighs, where angular peds with the slickensides were identified.

No permanent, free groundwater table was registered in the profiles during archeological and pedological investigations; however, the clayey subsoil (below the vertic horizon) is permanently saturated with capillary water, and has a massive plastic structure, reductimorphic colors (5GY 7/1) and abundant Fe–Mn concentrations (not cemented). There are no cracks or tongues with humus infilling below the vertic horizon; also, the roots and zoogenic features are absent there. Soils in the area surrounding the archeological site (profile 3) have a black (10YR 2/1-2), 40–50 cm thick structural humus horizon developed of loess. Its bottom part has a higher color value and chroma (10YR 3/2-3), and an enhanced clay content. Numerous humus-infilled tongues start in the A horizon and continue to a depth of 100–130 cm. The horizons developed from loess (A + AB) are separated from the underlying clay by the gravelly pavement. Moreover, one additional discontinuous pavement layer was identified within the clayey subsoil, at the depth of ca. 115 cm (Fig. 5). There are common slickensides on the walls of angular blocky-columnar peds in the upper clay layer; wedgeshaped structures are absent. The bottom clay layer (below the second pavement) has a massive plastic structure without slickensides. Vertical or nearly vertical wedges (pseudomorphs) start at the upper limit of

100


Fig. 4. Width (A), length (B) of microlows, ratio length:width (C), and the depth of microlows (D).

this layer (below the second pavement). The wedges have a thickness of up to 10 cm and a length of up to 70 cm, sandy infilling, and sides (walls at the contact with clay) stained with iron oxides. 3.3. Soil mineralogy and chemical properties TOC content reached 2.5% (and more) in the upper and 2% (and more) in the bottom part of the A horizons in the microlows. The vertic horizons contained 0.22–0.26% of TOC in bowls and up to 1.3% of TOC in profile 3 (where the microlows are absent). Clayey subsoil (below the vertic horizon) contained 0.18–0.20% of TOC when not cracked or up to 0.8% if wedges with humus-rich infilling were present (profile 3). The concentration of total nitrogen in humus horizons was between 0.10 and 0.29% and this produced low C:N ratios in a range between 9:1 and 12:1 (Table 3). At least traces of dispersed carbonates were present throughout the soil profiles. In the clayey subsoil (below the vertic horizon) carbonates reached the highest concentrations and formed numerous small, hard nodules. The nodules were present also in the microhigh (chimney) clays. Clods of the lime (of anthropogenic origin) were common in the plow layers. Due to the carbonate presence, the soil reaction was neutral

or slightly alkaline throughout the profile (Table 3), and the exchangeable acidity was below detection limits (not presented). The sum of basic cations measured in acetate extract at pH 7 (Table 4) ranged between 10.7 and 13.3 cmol(+) kg− 1 in A horizons, between 15.6 and 19.3 cmol(+) kg−1 in vertic horizons, and between 23.8 and 24.2 cmol(+) kg−1 in clayey subsoil, in a clear relation to carbonate presence. Ammonium chloride (at pH 8.2) extracted significantly lower amounts of calcium and this resulted in a smaller sum of exchangeable cations, in particular in horizons enriched in secondary carbonates. The extraction method has no importance for soil classification at such a high pH; however, extraction at pH 8.2 seems more reasonable for these soils as it does not release calcium from secondary carbonates; and thus, it does not overestimate the content of exchangeable calcium. Clay fraction mineralogy in the loess-derived top-soil was dominated by illite and illite–smectite mixed layer minerals with a minor addition of kaolinite and other phyllosilicates, typical for the Lower-Silesian and other south-Polish loess (Drewnik et al., 2014; Uziak et al., 1987). Despite organic matter removal, the diffractogram of the A horizon is visibly influenced by amorphic substances (Fig. 7). The clay fraction in the 2Big and 2Ckl horizons, both developed of Neogene clays, had similar characteristics, reasonable for vertic properties development. Their


primary feature is the prevalence of swelling clay minerals (smectitegroup), accompanied by micaceous minerals of illite and illite–smectite groups. The presence of kaolinite (the peak disappearing after heating at 500 °C), well pronounced in both horizons, is typical for the clays of this region and is related to the advanced Neogene weathering of source rocks in the Sudeten Mountains (Dyjor et al., 1968; Jakubus et al., 2013). 4. Discussion Although the influence of swelling–shrinkage phenomena on the morphology and properties of the soils under study seems obvious, scientific reliability requires an analysis and rejection of the opposite concepts. The anthropogenic origin of the microlows as pits of primitive clay mining for pottery production was challenged right at the beginning of

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the pedological investigation. Further observations have reinforced the concept of their natural origin. Some important arguments are as follows: (1) the microlows are small, shallow, only slightly explore the clay resource, and do not cross or adhere to each other, which cannot be logically explained by any, even primitive technology of clay exploitation; (2) continuous gravel pavement occurs at the contact of topsoil loess and subsoil clay, but the coarse rock fragments are absent in the microlow infillings; (3) humus content in the upper and bottom parts of the material that fills the microlows differs significantly which belies its homogenization, which would be expected in the abandoned pits; (4) the few anthropogenic artifacts have only been found in the topsoil, while they are absent in the deeper layers of microlow infillings. The Silesian Lowland was covered at least twice by ice sheets and several times was a subject to periglacial conditions. Relic glacio-tectonic

Fig. 5. Sketch of representative soil profiles.

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Fig. 6. Wedge-shaped (disrupted) aggregates with grooved surface and slickensides in vertic horizon of the profile 1.

without permafrost (Bockheim and Tarnocai, 1998). The development of cryoturbation deformations (involutions), like waves (festoons), diapirs, and pockets is explained by at least three mechanisms: (1) loading of the partially liquefied (plasticized) sediments during the degradation of permafrost; (2) cryohydrostatic pressure that can result in the extrusion of sediment-laden water; and (3) cryostatic pressure (including frost heaving) caused by ice-segregation that may introduce involutions in fine sediments by repeated freeze–thaw cycles (Vandenberghe, 1992). However, none of these mechanisms accommodates the neoformation of an aggregate structure, but rather leads to the destruction of previous soil structure and to soil plasticization, commonly observed in currently cryoturbated soils (Haugland, 2004; Kabala and Zapart, 2009). Also, no slickensides or aggregate formation was observed in the numerous experiments conducted by Cegla and Dzulynski (1970) who simulated artificial cryoturbations in various soils. The only periglacial cryoturbation features found in the soils under study were the pseudomorphs of thick ice wedges with sandy infilling, like in profile 3. The feature starts at the lithologic discontinuity below the upper clay layer and must be older than this material which is also exemplified by the infilling sand, the relic of the former cover sands eroded totally before subsequent sedimentation of colluvial clays. All other cracks dissecting the profiles have a silt-humus infilling identical with the present-day humus horizons. Therefore, the origin of these cracks may be only explained by the

Fig. 5 (continued).

forms (related to older glaciations) and cryoturbation forms (related to the periglacial conditions of the last two glaciations) are quite common in the region and are undoubtedly present in profile 3 (pseudomorphs of ice-wedges with sand infilling). Therefore, the possibility of the cryogenic origin of subsurface wavy forms (microlows and microhighs) must be considered. These forms could not have arisen before loess sedimentation, because the older cryoturbation would have changed the position of the stones and gravels in the pavement. The continuous and unaffected pavement layer at the loess–clay contact could only have survived the cryoturbation and all other movements under protective loess cover. According to Badura et al. (2013), loess was deposited in the upper Pleniglacial period, thus implying the origin of wavy structures only in the last cold Pleistocene stages — the Dryas period (if considered to be of periglacial origin). This period was cold and long enough to develop various cryoturbation forms (Jary, 2009), which are common in present-day permafrost zones, but may also occur in other cold areas Table 2 Particle-size distribution (soils sampled in microlows). Soil horizon

Depth

Percentage of particle-size fraction (mm)

cm

N2

Texture class 0.005–0.002

b0.002

Profile 1 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–60 0 0.05 0.40 1.25 2.30 9 28 30 Ag 60–100 0 0 0.30 0.55 1.15 9 24 32 Btg 100–120 0 0 0.50 0.80 2.70 6 22 28 2Big 120–150 0 0.05 0.55 1.50 1.90 5 7 16 2Ckl 150–180 0 0 0.10 0.10 0.80 8 9 19

13 12 10 13 12

16 21 30 55 51

SiL SiL SiL C C

Profile 2 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–55 0 0.20 0.70 1.35 1.75 9 33 22 Ag 55–80 0 0.20 0.60 1.00 1.20 8 37 19 Btg 80–105 0 0.15 0.30 0.60 0.95 9 35 18 2Big 105–120 0 0 0.20 0.55 1.25 4 9 18 2Ckl 120–160 0 0.05 0.85 2.00 2.10 5 11 15

9 9 8 10 10

23 24 28 57 54

SiL SiL SiL C C

Profile 3 (horizon depths measured from original soil surface) Ap 0–30 1.40 0.80 2.25 A 30–45 0.60 0.60 1.80 ABg 45–60 0.50 0.50 1.00 2Big 60–90 0 0.05 0.10 2Cg 90–115 0 0 0 3Ckl 115–160 0 0.15 0.85

11 12 9 9 8 10

18 18 24 53 48 53

SiL SiL SiL C SiC C

2–1

1–0.5

0.5–0.25

5.60 5.05 4.00 0.55 0.05 2.00

0.25–0.1

7.35 6.55 4.50 1.30 0.95 2.00

0.1–0.05

5 8 9 6 8 6

0.05–0.02

29 26 26 11 17 9

0.02–0.005

21 22 22 19 18 17

C. Kabala et al. / Catena 128 (2015) 95–107 Table 3 TOC, total nitrogen, carbonates and soil pH (soil sampled in microlows). Soil horizon

Depth

CaCO3

cm

%

pH

TOC

Nt

C:N

%

Profile 1 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–60 0.6 7.8 2.34 0.25 9.7 Ag 60–100 0.3 7.9 1.82 0.15 12.1 Btg 100–120 0.2 7.9 0.40 na na 2Big 120–150 0.5 8.0 0.26 na na 2Ckl 150–180 5.5 8.1 0.20 na na Profile 2 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–55 0.6 7.8 2.10 0.22 9.5 Ag 55–80 0.3 7.9 1.84 0.17 8.3 Btg 80–105 0.4 7.9 0.35 na na 2Big 105–120 0.8 8.0 0.22 na na 2Ckl 120–160 6.4 8.1 0.18 na na Profile 3 (horizon depths measured from original soil surface) Ap 0–30 0.5 7.0 2.65 A 30–45 0.6 7.6 1.96 ABg 45–60 0.5 7.9 1.33 2Big 60–90 0.6 7.9 1.31 2Cg 90–115 0.7 8.0 0.86 3Ckl 115–160 4.8 8.1 0.78

0.29 0.16 0.10 na na na

9.1 12.3 13.3 na na na

desiccation of the modern soil (Nikorych et al., 2014), and falling the humus mulch into the seasonally open cracks. The only acceptable explanation of the wavy subsurface structures, accompanied by coarse wedge-shaped aggregates with grooved surfaces and slickensides, as well as numerous thin cracks filled with topsoil material is connected with the shrink–swell phenomena in the clayey material. This widely accepted soil mechanics model (Wilding, 2004) includes the following major elements: (1) cracking of the soil upon desiccation, (2) development of swelling pressures and closure of cracks upon wetting; (3) upward movement of the soil not confined by overburden pressures; (4) oblique shear failure in the subsoil when the swelling pressures exceed the shear strength; (5) lateral translation of materials along the grooved shear planes, and (6) formation of slickenside-outlined, bowl-shaped structures in the microlows of the

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gilgai surface network. The soil mechanics model would accommodate: rapid formation of slickensides and presence of the gilgai topography, the presence of slickensides below major crack zones, and the presence of E or Bt horizons above zones of slickenside activity (Wilding, 2004). All the requirements of a diagnostic vertic horizon are fulfilled in the upper part of the clayey material: (1) more than 30% of clay throughout, (2) the presence of wedge-shaped aggregates and slickensides, (3) shrink-swell cracks, and (4) a thickness of at least 25 cm (IUSS working group WRB, 2014). However, due to the silt-loam texture of the topsoil, the soils cannot be classified as Vertisols and, as previously concluded by Kovda et al. (2010), various sections of the soil profile may be classified as different soil groups — at the highest level of classification. The soil in a central part of a microlow has a very thick mollic (chernic) horizon and clayey subsoil with a vertic horizon starting at a depth greater than 100 cm. The soil may be classified as Chernozems, e.g., Luvic Chernozems (Drainic, Pachic, Stagnic) or (less often) as Phaeozems, e.g., Luvic Stagnic Chernozems (Drainic, Pachic), if the layer enriched in secondary carbonates starts deeper than 50 cm below the lower limit of the mollic horizon (additional qualifiers referring to soil texture were omitted). It must be stressed that the recognition of chernic horizon was impossible in the archeological site because the topsoil was removed and only the bottom part of the humus horizon was preserved. However, the granular (and coprolite) structures are typical for the plow layer of all soil on adjacent fields (as represented by profile 3); thus, the granular structure of the topsoil was also assumed in the truncated soils in the archeological area. The soils above the microslopes have thick mollic horizon, abrupt textural difference (at the loess–clay contact) within 100 cm from the soil surface, strong stagnic properties above and below the textural change, and the vertic horizon. Based on the key to WRB (IUSS working group WRB, 2014), such soils were classified as Planosols. The vertic qualifier is listed among the supplementary qualifiers in Planosols; thus, the most common denomination of these soils – Eutric Luvic Mollic Planosols (Drainic, Pachic, Ruptic, Vertic) – does not properly reflect the crucial morphological features of these soils. The soil profiles distinguished over the subsurface microhighs have mollic horizon, abrupt textural difference within 100 cm of the soil surface, and strong stagnic properties above and below the textural change.

Table 4 Exchangeable base cations as extracted at pH 7.0 and 8.2 (soil sampled in microlows). Soil horizon

Depth

Ca2+

Mg2+

NH4OAc 7.0

NH4Cl 8.2

NH4OAc 7.0

NH4Cl 8.2

Sum of cations NH4Cl 8.2

NH4OAc 7.0

NH4Cl 8.2

Profile 1 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–60 9.80 10.2 1.39 1.48 0.50 0.19 Ag 60–100 8.60 6.00 3.51 3.09 0.40 0.30 Btg 100–120 9.10 5.90 3.85 2.80 0.28 0.33 2Big 120–150 8.40 6.40 6.42 7.27 0.50 0.56 2Ckl 150–180 17.2 9.40 5.95 7.34 0.50 0.38

0.35 0.29 0.27 0.29 0.23

0.24 0.32 0.27 0.27 0.21

12.0 12.8 13.5 15.6 23.9

12.1 9.71 9.30 14.5 17.3

Profile 2 (horizon depths measured in a central part of micro-depression, 30 cm of the truncated humus horizon is added) A 30–55 9.80 6.40 1.64 1.56 0.47 0.54 Ag 55–80 8.40 5.60 1.78 1.74 0.42 0.42 Btg 80–105 8.40 6.30 1.65 1.54 0.30 0.21 2Big 105–120 11.8 8.40 3.69 3.62 0.59 0.31 2Ckl 120–160 19.0 10.2 3.85 3.25 0.57 0.50

0.22 0.24 0.22 0.27 0.36

0.20 0.22 0.22 0.23 0.32

12.1 10.8 10.6 16.4 23.8

8.70 7.98 8.27 12.6 14.3

Profile 3 (horizon depths measured from original soil surface) Ap 0–30 8.60 7.60 1.13 A 30–45 10.4 8.40 1.72 ABg 45–60 10.6 7.40 1.92 2Big 60–90 16.2 9.90 2.71 2Cg 90–115 17.8 9.40 3.82 3Ckl 115–160 19.0 10.6 4.34

0.52 0.24 0.25 0.21 0.27 0.32

0.47 0.21 0.20 0.15 0.25 0.30

10.7 12.9 13.2 19.8 22.5 24.2

9.52 9.75 9.04 13.2 13.3 15.2

cmol(+) kg

NH4OAc 7.0

K+ NH4OAc 7.0

cm

NH4Cl 8.2

Na+

−1

1.10 0.86 1.11 2.70 3.16 3.85

0.47 0.50 0.43 0.70 0.63 0.57

0.35 0.28 0.33 0.43 0.43 0.42

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Fig. 7. XRD diffractograms of the clay fraction from the horizons Ag, 2Big, and 2Ckl of the profile 1. Explanation: N— sample untreated, Gl — sample glycerolized, 500 — sample heated at 500 °C.


Vertic properties in the upper part of the clayey layer are recognizable (slickensides on the angular aggregates), but the thickness of the horizon is disputable, and a protovertic horizon instead of a vertic one was distinguished. Therefore, the soils have commonly been classified as Eutric Luvic Mollic Planosols (Drainic, Protocalcic, Protovertic Ruptic). Similarly, the vertic horizon cannot be recognized in the soils surrounding the archeological site (profile 3), due to its insufficient thickness (Fig. 5). The soils have thick mollic (chernic) horizon, abrupt textural difference within 100 cm of the soil surface, and protovertic horizon in the upper part of clayey subsoil. Stagnic properties occur in all horizons below the Ap layer; however, the redoximorphic mosaic covers less than 50% of the soil surface at the textural contact. This implies soil classification as Phaeozems, commonly as Protovertic Stagnic Chernic Phaeozems (Abruptic, Drainic, Pachic, Ruptic). In adjacent fields, the clayey subsoil often had much stronger stagnic properties that actually favored soil classification as Mollic Planosols. A thick mollic horizon is quite common in Vertisols (Nordt et al., 2004; Novák et al., 2014) and vertic properties are in many areas accompanied by thick mollic horizon in non-Vertisols, e.g., in Mollisols (Brevik et al., 2012; Graham and Southard, 1983) and Chernozems (Filipovsky, 1974; Skalský et al., 2009; Ursu, 2012), where the vertic properties are associated with textural differentiation in the soil profile (Bogunovic et al., 1991; Khitrov, 2012). Vertic horizons co-existing with mollic horizons seem to be more important for soil management and more influential for soil naming (classification based on soil morphology and properties) than the textural difference (Khitrov, 2012), even if connected with stagnic properties, as in Planosols. The present-day order of reference soil groups in the key to the WRB classification (IUSS working group WRB, 2014) enforces distinction of a micro-mosaic of two highly different soil units at the highest classification level in the area under study whereas all these soils had developed of the same parent material, under the same climate conditions, including moisture regime, and under the same vegetation. A thick, structural, fertile and biologically active mollic horizon is considered the most important feature of the soils in all sections of a vertic-influenced micro-mosaic, which justifies their classification as Chernozems/Phaeozems rather than Planosols (Gerasimova and Khitrov, 2012). Also, the origin and age of the soils under study are questionable. An early-Pleistocene erosional origin for the microlows was rejected due to the continuity of the periglacial pavement on the microslopes, stable only under protective cover of the younger, Weichsel loess. Similarly, the periglacial origin of wavy subsurface structures was rejected due to the occurrence of large lenticular aggregates and slickensides, unknown in periglacial loading involutions. These statements have shifted the origin of microhighs and microlows to the Holocene period. The initial Preboreal and Boreal periods were presumably too dry to provide enough moisture for intense shrink–swell phenomena. Moreover, the thick layer of initially homogenous loess has stabilized the soil moisture in the clayey subsoil. The warmer and moister Atlantic period provided favorable conditions for broadleaf forest spread on previously decalcified loess, and for clay eluviation/illuviation within the soil profiles (Dreibrodt et al., 2007). The clay-enriched illuvial horizons in the soils under study are overlain by thick humus horizons. The dark humus cutans in the illuvial horizon are only present on the large pedfaces and in tongues/cracks, whereas small structural aggregates are covered by “pure” clay coatings. This finding makes probable an argic horizon development preceding the mollic formation (Albrecht and Kühn, 2011). The factor initiating a “chernozemic” humus horizon formation could be the widespread human colonization, particularly intense since the early Neolithic period (Kulczycka-Leciejewiczowa, 2000, 2010). Forest decline (probably by burning) on soils suitable for grazing and primitive farming has transformed the vegetation over large areas into open grass-forest communities (Bakker et al., 2004). Such an “artificial” environment has favored the accumulation of biomass from grass vegetation and the activity of burrowing animals, previously hindered by close-forest vegetation. Both elements are crucial for the

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development of “chernozemic” humus horizons (Alexandrovskiy, 2007). The soil moisture status during its formation remains unclear, but the higher soil moisture better preserves organic matter in humus horizon and makes it darker (Kowaliński, 1952). Actually, most “Black earths” in the Silesian Lowland have strong stagnic or gleyic properties, although drained. Continuous (from the Neolithic period until now) agricultural land-use, absence of forest vegetation, and excessive soil moisture have preserved chernozemic humus horizons from the degradation, typically observed after forest succession under temperate climate (Borowiec, 1962). Both the subsurface clay-illuvial and the humus-rich topsoil horizon development are considered important for clayey subsoil transformation into protovertic or vertic horizons. Clay accumulation in the argic horizon (up to 24–30%) made it more similar to the clayey materials susceptible for shrink–swell phenomena, in particular for cracking under summer drought (Johnson et al., 1987; Kishné et al., 2009). Such a soil series, where the development of a vertic horizon was able due to significant clay accumulation in Bt horizon were identified and explained by Smith (1986) in the southern states of the US. Furthermore, a thick humus horizon provides a lot of fine granular material to fill the seasonal cracks, not only from surface self-mulching, but also from the earthworm activity in the topsoil (Graham and Southard, 1983; Johnson and Schaetzl, 2015). Very well developed wedge-shaped aggregates and the microlow/ microhigh structures suggest high intensity of a subsurface mass cycling that, under more favorable climate conditions, may lead to the development of the gilgai microrelief. Presently, all fields adjacent to the archeological site (and in the site before excavation) have a surface completely leveled by regular and deep tillage. Thus, the recognition of any microrelief differentiation is impossible and also presumption that gilgai can form on such soils is only hypothetical. However, the very initial stage of gilgai development (in the historical periods preceding the soil plowing) cannot be completely excluded taking into account the amplitude of subsurface concave and convex forms. There is no direct dating, but the above arguments suggest, that environmental conditions favorable for the development of vertic properties arose not earlier than in the Subboreal–Subatlantic periods, after the development of argic and mollic horizons. Moreover, it seems that both the absence of forest and artificial soil drainage (at the depth ca 70 cm) may promote shrink–swell phenomena by enhancing topsoil drying during late summer (in particular in August and September), while the transitional landscape position (at the border of an upper-located hilly lowland and a lower-located small valley) provides seasonal (winter–spring) excess of moisture. The subsurface wavy structures accompanied by oriented wedgeshaped aggregates, presumably connected with initial formation of gilgai microrelief, are the first such findings and have never been described in Poland and probably in the surrounding countries of Central Europe, even in heavy clay soils with well pronounced slickensides (Mocek et al., 2009; Novák et al., 2014; Olszewski, 1956; Prusinkiewicz, 2001; Skalský et al., 2009). It is highly possible that our finding was allowed by the large scale of archeological excavation and the similar structures in other clayey soils of the region are still undiscovered due to the small scale of a typical pedological pit. Thus, the cooperation with archeologists is highly recommended to enlarge the pedological knowledge. 5. Conclusions Subsurface wavy structures found in the archeological excavation Domaslaw-34 in SW Poland, in soils developed of loess underlain by Neogene clays (composed of smectite, smectite–illite, illite, and kaolinite), are considered to be microlows and microhighs typical for subsurface cycling in vertisols. Soils do not have a clayey texture throughout profile; however, the thick and rich in humus mollic horizon, as well as clay-enriched argic horizon, both developed directly above the clayey vertic horizon, have supported soil cracking and shrink–swell

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phenomena in clayey subsoil. As a result, well developed wedge-shaped (lenticular) aggregates were formed in the upper part of clayey subsoil. Aggregates have typically grooved surfaces with an oriented matrix (slickensides) and are ordered under variable angles (from 10 do 60°). The periglacial origin of the subsurface wavy structures was rejected and the mid- to late-Holocene period (since the Neolithic age) was considered to be favorable for intense shrink–swell phenomena, vertic horizon development and the formation of subsurface features related to gilgai microrelief. Due to the variable development of the diagnostic horizons and properties, a micro-mosaic of soil units correlated with subsurface concave and convex structures was distinguished, from (non-vertic) Stagnic Chernozems/Phaeozems above the microlow centers, through Mollic Planosols (Vertic) above microslopes, to Mollic Planosols (Protovertic) over the microhighs. The Planosol shift in the key to the WRB classification was recommended to reinforce the importance of the mollic and vertic horizons in the soils texturally differentiated, to allow their classification as Vertic Chernozems/Phaeozems. Acknowledgments We would like to thank warmly our archaeological team: Krzysztof Czarniak, Magdalena Konczewska, Dagmara Łaciak, Daniel Moroz, Joanna Nastaszyc, and Sylwia Rodak for their patience and perseverance. The archaeological and geological research was founded by the General Directorate for National Roads and Motorways in Poland. The pedological investigation was done within research project was financed by the National Science Centre of Poland, grant number 2012/ 05/B/NZ9/03389. References Albrecht, C., Kühn, P., 2011. Properties and formation of black soils on the Island of Poel (NE Germany). Quat. Int. 243, 305–312. Alexandrovskiy, A.L., 2007. Rates of soil-forming processes in three main models of pedogenesis. Rev. Mex. Cienc. Geol. 24, 283–292. Atasoy, A.D., 2008. Environmental problems in vertisol soils: the example of the Harran Plain. Fresenius Environ. Bull. 17, 837–843. Badura, J., Jary, Z., Smalley, I., 2013. Sources of loess material for deposits in Poland and parts of Central Europe: The lost Big River. Quat. Int. 296, 15–22. Bakker, E.S., Olff, H., Vandenberghe, C., De Maeyer, K., Smit, R., Gleichman, J.M., Vera, F.W., 2004. Ecological anachronism in the recruitment of temperate light-demanding tree species in wooded pastures. J. Appl. Ecol. 41, 571–582. Barbera, V., Raimondi, S., Egli, M., Plötze, M., 2008. The influence of weathering processes on labile and stable organic matter in Mediterranean volcanic soils. Geoderma 143, 191–205. Bieganowski, A., Witkowska-Walczak, B., Glinski, J., Sokołowska, Z., Sławiński, C., Brzezińska, M., Włodarczyk, T., 2013. Database of Polish arable mineral soils: a review. Int. Agrophys. 27, 335–350. Bobak, D., Płonka, T., Połtowicz-Bobak, M., Wiśniewski, A., 2013. New chronological data for Weichselian sites from Poland and their implications for Palaeolithic. Quat. Int. 296, 23–36. Bockheim, J.G., Tarnocai, C., 1998. Recognition of cryoturbation for classifying permafrostaffected soils. Geoderma 81, 281–293. Bogatyrev, K.P., 1958. Smolnitzi (smonitzi) of Albania. Pochvovedenie 4, 14–22. Bogunovic, M., Koric, A., Racz, Z., Vidacek, 1991. Vertic hydromorphic soils of the Sava river valley and problems of their utilization. Zemljiste i biljka 40, pp. 13–28. Borowiec, S., 1962. On the occurrence of the relic chernozems in the Szczecin region. Przegl. Geogr. 34 (4), 739–747. Brevik, E.C., Fenton, T.E., Jaynes, D.B., 2012. The use of soil electrical conductivity to investigate soil homogeneity in Story County, Iowa, USA. Soil Horiz. 53, 50–54. Cegla, J., Dzulynski, S., 1970. Systems with reversed density gradient and their occurrence in periglacial zones. Acta Univ. Wratislaviensis 124, 17–42. Dasog, G.S., Acton, D.F., Mermut, A.R., 1987. Genesis and classification of clay soils with vertic properties in Saskatchewan. Soil Sci. Soc. Am. J. 51, 1243–1250. Dreibrodt, S., Jarecki, H., Lubos, C., Khamnueva, S.V., Klamm, M., Bork, H.-R., 2007. Holocene soil formation and soil erosion at a slope beneath the Neolithic earthwork Salzmünde (Saxony-Anhalt, Germany). Catena 107, 1–14. Drewnik, M., Skiba, M., Szymański, W., Żyła, M., 2014. Mineral composition vs. soil forming processes in loess soils — a case study from Kraków (Southern Poland). Catena 119, 166–173. Dyjor, S., Bogda, A., Chodak, T., 1968. Preliminary studies on the mineral composition of the Poznan clays. Rocz. Pol. Towarz. Geol. 38 (4), 491–510. Filipovsky, G., 1974. On pedoturbation in some Smolnitzas of Yugoslavia. Pochvovedenie 6, 28–38. Florinsky, I.V., Arlashina, H.A., 1998. Quantitative topographic analysis of gilgai soil morphology. Geoderma 82, 359–380.

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