Soil physical properties of agriculturally reclaimed area after lignite mine: A case study from central Poland

Soil & Tillage Research 163 (2016) 54–63

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Soil physical properties of agriculturally reclaimed area after lignite mine: A case study from central Poland
ska-Jurkiewicza , Krzysztof Otrembab , Beata Kołodzieja,* , Maja Bryka , Anna Słowin b Mirosława Gilewska a b

Institute of Soil Science, Environment Engineering and Management, University of Life Sciences in Lublin, Lublin, Poland
University of Life Sciences, Poznan
, Poland Department of Soil Science and Reclamation, Poznan

A R T I C L E I N F O

Article history: Received 30 December 2015 Received in revised form 25 April 2016 Accepted 2 May 2016 Available online xxx Keywords: Agricultural reclamation Physical properties Mine soil

A B S T R A C T

For the assessment of the extent of post-mine land degradation, arranging management practices for sustainable land use, and evaluation of reclamation success research on soil physical properties plays a significant role. The aim of this study was to quantify and interpret the effects of varied agricultural reclamation methods in order to assess which of the treatments yielded higher soil quality with respect to its physical state. Basic physical and chemical parameters (texture, particle and bulk density, total porosity, total organic carbon and pH), soil water characteristic curves, water and air permeability, structure of the soil damaged by mining in the area of internal dumping ground Pa˛tnów (Central Poland) were therefore measured. The soil samples were taken from the following 5 different variants of over 30year-long reclamation: black fallow (BF), monoculture of winter wheat (WW), monoculture of winter wheat with a single application in 1992 of lignite dust (WW + L), monoculture of alfalfa with orchard grass in the proportion of 90/10% (A + G), and spontaneous succession (SS). We collected from each treatment: 12 soil cores for soil water and air properties; 6-kg composite bulk samples for basic physical and chemical properties of soil; 1 undisturbed soil block for structure analysis. The reclamation methods applied on the post-mining grounds influenced in a diverse manner the physical state of the 0–10 cm layer of the developed technogenic soil. Bulk density values generally decreased with the increase of number of factors potentially loosening the soil structure. The studied plots had sandy loam or loamy sand texture which determined their overall air and water properties. The soils were characterized by high macroporosity and favourable content of water available for plants. Less beneficial conditions of the soil were associated with air and water permeability. The present studies indicated that spontaneous succession, with the highest typological diversity of vegetation, contributed to the most preferred soil physical state. Remaining variants of reclamation (including black fallow without vegetation) also provided sufficient air and water relations in the soils. However, it should be emphasised that plants play an important role in TOC accumulation and ensuring relative stability of soil structure. Therefore rehabilitation of post-mining grounds involving vegetation is recommended. ã 2016 Elsevier B.V. All rights reserved.

1. Introduction Mining has a negative effect on the natural environment, which is manifested as direct or indirect degradation (Krümmelbein et al., 2010; Ussiri and Lal, 2005; Zhang et al., 2015). Large scale of lignite mining greatly impacts the ecosystem, including temporary or permanent destruction of land surface and excluding relatively

* Corresponding author at: Institute of Soil Science, Environment Engineering
skiego 7, 20and Management of the University of Life Sciences in Lublin, Leszczyn 069 Lublin, Poland. E-mail address: [email protected] (B. Kołodziej). http://dx.doi.org/10.1016/j.still.2016.05.001 0167-1987/ã 2016 Elsevier B.V. All rights reserved.

large areas from their former use (Pedrol et al., 2010; Wang et al., 2014; Zhao et al., 2013). Environmental transformations related with the surface mining activities entail changes in the morphology of the area, reduction of vegetation cover and fauna diversity, modification in organic carbon concentration, soil structure and soil water and air relations of the exploited area (Fettweis et al., 2005; Knappe et al., 2004; Sadhu et al., 2012; Zawadzki et al., 2016). Mining leads to increased rates of mineralization, erosion and leaching (Biemelt et al., 2005; Hendrychová et al., 2012; Sharma et al., 2004). Reclamation of mine soils is the process aiming at the restoration of a stable and productive ecosystem. The most

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common method is the rehabilitation of the disturbed areas with plants. Vegetation plays an important role in reversing degradation processes. Various species, which affect soil not only physically but are also a source of soil organic matter, influence in different ways the soil and ecosystem functions (Alday et al., 2014; Doležalová et al., 2012; Evans et al., 2013). The plant appearance can be intentional or spontaneous (Baasch et al., 2012; Hoda9 cová and Prach, 2003). Productivity of soil could be also enhanced by adding various amendments such as fly ash, saw dust, sewage sludge, animal manures, limestone or lignite (Cheng et al., 2014; Krümmelbein and Raab, 2012; Pedrol et al., 2010; Wick et al., 2014). For the assessment of the extent of post-mine land degradation, arranging management practices for sustainable land use, and evaluation of reclamation success research on physical properties of soil plays a significant role (Dexter, 2004). Shrestha and Lal (2008) estimated the effects of 28 years of post-reclamation land uses by means of soil bulk density, water-stable aggregates, meanweight aggregate diameter, moisture retention, cone index, and infiltration rate. Krümmelbein et al. (2010) and Krümmelbein and Raab (2012) investigated the influence of different agricultural reclamation treatments on the development of soil structure on the basis of soil physical properties such as texture, bulk density, water and air capacity, precompression stress, air permeability, and saturated hydraulic conductivity. Cao et al. (2015) applied bulk density, porosity, and water content measurements to compare reconstructed soils in relation to site type and topography of the dump. Singh et al. (2015) studied selected soil physical properties

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(texture, bulk density, porosity, water holding capacity) during the reclamation of coal mine spoil to quantify the recultivation effects. Undoubtedly, the water-air relations of soil are of key importance for the creation of an optimum environment for the growth and development of plants. The primary task of reclamation should be to bring post-mining areas to a condition that will permit their further sustainable use (Miao and Marrs, 2000; Shrestha and Lal, 2011; Zanuzzi et al., 2009). In Poland 90 lignite deposits, 11 under exploitation, of 22,684 million Mg geologically documented resources have been recognized (Central Statistical Office, 2014). One of the lignite mining centres is the Lignite Mine in Konin which covers an area of about 14,500 ha. More than half of this area (7,243 ha) is utilized for mining activity, additional 1,362 ha are not deformed but related to mining, and the area of devastated post-mining lands is 5,627 ha. Within the Konin Mine, lignite excavation entailed radical changes in the environment. The restoration of the ecological values in the region is therefore a complex problem. Mining activity created many earthworks in the form of large external and internal dumps, often elevated above the surrounding area. Consequently, twenty heaps differing in height, volume and architecture were formed on this post-mining land. One of them was Pa˛tnów where exploitation of coal began in 1962 and finished in 2001. By the time, almost 130 million Mg of lignite were excavated. In the process of opencast mining the upper soil layer was removed and internal dumping ground was formed by depositing the overlay inside the excavation; afterwards the area was agriculturally reclaimed.

Fig. 1. Location of the study area on the background of a Landsat 8 band 7-5-3 image (scene LC81900242015111LGN00 acquired April 21, 2015; courtesy of the U.S. Geological Survey).

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B. Kołodziej et al. / Soil & Tillage Research 163 (2016) 54–63

The aim of this study was to quantify and interpret the effects of varied agricultural variants of reclamation in order to assess which of the treatments yielded higher soil quality with respect to its physical state. Basic physical and chemical parameters, soil water characteristic curves, water and air permeability, structure of the soil damaged by mining in the area of internal dumping ground Pa˛tnów were therefore measured. 2. Materials and methods 2.1. Study site The samples used in the analysis came from the experimental field established in 1978 by the Department of Soil Science and
University of Life Sciences on the Reclamation of the Poznan internal dumping ground Pa˛tnów (52
180 4600 N, 18
150 3200 E) near the city of Konin (Fig. 1). This area is located in mesoregion
skie Lake District (315.54) (Kondracki, 2011) with mean Gnie
znien annual temperature 9.2
C and mean annual precipitation of approximately 518 mm. The reclamation was carried out starting from 1978 in accordance with the principles of the target species concept developed by Bender (1995). After reclamation the soil is classified as the Spolic Technosol, TC-sp (IUSS Working Group WRB, 2015). The soil samples were collected from the following 5 different variants of reclamation: 1) black fallow (BF), experimental field dimensions 12  50 m— plants were eliminated continuously with mechanical cultivation. Ploughing was applied once a year in autumn. Basic treatments like harrowing and disking were done in autumn and in spring, with the frequency depending on the weather conditions. 2) monoculture of winter wheat (Triticum aestivum L.) (WW), 20  50 m—straw as well as post-harvest residues were ploughed into the soil every year as a source of organic matter for the superficial soil layer. 3) monoculture of winter wheat (Triticum aestivum L.) with application of lignite dust (WW + L), 20  50 m—winter wheat had been cultivated continuously as explained above. Lignite dust (waste in coal briquette production) was applied once in 1992 for reclamation processes in the post-mining land in the amount of 1,000 Mg ha1. This material was then mixed with the soil to a depth of ca. 30 cm using cultivation tools, which resulted in lignite content of 33.3% by volume (28.5% by weight). Physico-chemical properties of lignite from Konin Mine can be found in Kwiatkowska et al. (2006). 4) monoculture of alfalfa (Medicago sativa L.) with orchard grass (Dactylis glomerata L.) in the proportion of 90/10% (A + G), 12  50 m—ploughing was applied every 10 years to preserve the planned balance between alfalfa and orchard grass. 5) spontaneous succession (SS), 12  50 m—with predominating species: alfalfa (Medicago sativa L.), orchard grass (Dactylis glomerata L.), sorrel (Rumex acetosa L.), wormwood (Artemisia absinthium L.), common yarrow (Achillea millefolium L.) and wood small-reed (Calamagrostis epigejos (L.) Roth).

2.2. Sampling and analysis methods For the study of the physical properties of the soil, samples with undisturbed structure were taken in Spring 2009, in 12 replicates per treatment, in metal cylinders with 100 cm3 volume. Since the studied soil was in its initial phase of development, it was characterized by a thin A horizon. Moreover, the annual cultivation measures on fields BF, WW, and WW + L involved only the upper 15

centimetres, consequently the samples were taken from the 0– 10 cm layer. Six soil cores were used for soil water characteristic determination. The soil samples were brought to the state of full saturation with water, 0 kPa. Next, measurements of soil water content were performed, at the potentials: 0.98, 3.10, 9.81, 15.54, 30.99, and 49.03 kPa in low-pressure chambers and at the potentials: 155.4 kPa, 490.3 kPa, and 1554 kPa in high-pressure chambers, on porous ceramic plates (Eijkelkamp, The Netherlands; SoilMoisture Equipment Co., USA), following the Richards’ method. The volumetric soil water content, WC, was determined by a standard thermogravimetric method and expressed in m3 m3. Water retentions were calculated on the basis of water contents at the appropriate soil water potentials (kPa): gravitational water GW = WC0  WC15.54, water available for plants AW = WC15.54  WC1554, water unavailable for plants UW = WC1554, where WC0— maximum water capacity, WC15.54—field water capacity for a soil with a deep groundwater table (FC), WC1554—permanent wilting point (PWP). The volumetric gravitational water corresponded to the volume of macropores and field air capacity, while available and unavailable water equated mesopore and micropore volume, respectively. The available water content values, AW, were categorized according to Paluszek (2011) into 5 classes: 0.080—very low; 0.081–0.120—low; 0.121–0.170—medium; 0.171–0.210—high; >0.210 m3 m3—very high. At each state of soil water saturation corresponding with water potentials from 3.10 to 30.99 kPa, air permeability, AP, was also measured for the samples in the metal cylinders in the device designed to test the permeability of moulding sands (type LPiR-2e, Multiserw-Morek, Poland). These measurements were carried out at the constant ambient temperature (20  0.5
C), therefore the dynamic viscosity of air did not require consideration. The results of the air permeability were given in 108 m2 Pa1 s1. The values of air permeability at 15.54 kPa, AP15.54, were categorized into 5 classes: 1.8–10.0—very low; 10.1–100—low; 100.1–1000—medium; >1000  108 m2 Pa1 s1—high. The classification was elaborated
ska-Jurkiewicz on the basis of by B. Kołodziej, M. Bryk and A. Słowin long-term air permeability measurements of diverse soils in the Institute of Soil Science, Environment Engineering and Management (University of Life Sciences in Lublin). Utilizing the other 6 soil cores collected into the metal cylinders, saturated hydraulic conductivity, KS (m d1) was measured with the ICW laboratory permeameter (Eijkelkamp, The Netherlands) by a constant head method. The values of saturated hydraulic conductivity, KS, were categorized into 5 classes: 0.100—very low; 0.101–0.500—low; 0.501–2.000—medium; 2.001–10—high; >10 m d1—very high (Paluszek, 2011). The RETC computer programme was then used to fit soil water characteristic curves to the observed water retention data (WC at 10 soil water potentials) and along with the saturated hydraulic conductivity value (KS) to predict the unsaturated hydraulic conductivity function (RETC 6.02, www.hydrus3d.com; van Genuchten et al., 1991) with single-porosity van GenuchtenMualem (van Genuchten, 1980) or dual-porosity Durner-Mualem (Durner, 1994) models. RETC was run with the fitting parameters WCS, a, and n. WCR and w2 were chosen to maximize R2 for regression of observed vs. fitted values and minimize a and n standard deviations (Table 2). The obtained fitted values of water contents were used to plot the water retention curves of the studied soils in coordinates volumetric water content vs. modulus of soil water potential, WC vs. |c| (Fig. 3a). Pore-size distribution in m3 m3 was resolved on the basis of the fitted water retention curve. Pore volume (m3 m3) was plotted as the function of equivalent pore diameter, dE (mm), estimated from dE = 294/|C|, where |C| was modulus of soil water potential expressed in kPa (Fig. 3b). Hydraulic conductivity, K (m

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d1), versus reduced water content, WCE = (WC  WCR)/(WCS  WCR), was presented in Fig. 3c. An amount of 6-kg composite bulk samples with disturbed structure were taken from each treatment at the same depth and point as cylinders. They were used to determine soil texture (sand 0.05–2 mm, cS; silt 0.002–0.05 mm, cSi; and clay <0.002 mm fraction content, cC; g g1) by a combination of the hydrometer and the wet-sieve methods (Polish Society of Soil Science, 2009), particle density, rS (Mg m3, by the pycnometer method; PN-ISO 11508, 2001), total organic carbon (TOC, mg g1, PN-ISO 14235, 2003), pH (by the potentiometric method with a glass electrode in 1:5 (V/V) suspension of soil in distilled water and in a 1 mol dm3 solution of KCl; PN-ISO 10390, 1997). Bulk density of the soil, r (Mg m3) was determined with the thermogravimetric method, on the basis of the ratio of the mass of soil dried at 105
C to the initial volume of the soil (100 cm3) in the 6 metal cylinders, after the soil water characteristics had been assessed. The soil bulk density

57

values were categorized according to Paluszek’s (2011) classification for sandy and loamy arable topsoil (1.4–very low; 1.41–1.50— low; 1.51–1.60—medium; 1.61–1.70—high; >1.70 Mg m3—very high). The total porosity of the soil, PO, was calculated on the basis of the results of particle and bulk densities and expressed in m3 m3. The total porosity values were categorized according to Paluszek (2011) into 5 classes: 0.320—very low; 0.321–0.360— low; 0.361–0.400—medium; 0.401–0.430—high; >0.430 m3 m3— very high. From the 0–10 cm layer of the studied soils, one sample with preserved structure per each treatment was also taken in the vertical plane into metal boxes measuring ca. 8  9  4 cm. Using _ the method described earlier (Bryk and Kołodziej, 2014; Domzał
ska-Jurkiewicz, 1987; Słowin
ska-Jurkiewicz et al., 2012) and Słowin ca. 8  9 cm in size resin-impregnated opaque soil blocks were made. Afterwards, the soil opaque block faces were scanned in colour at a 600 dpi resolution. On the basis of the soil blocks and

Fig. 2. Photographs of the opaque soil blocks: BF—black fallow; WW—monoculture of winter wheat; WW + L—monoculture of winter wheat with application of lignite dust; A + G—monoculture of alfalfa with orchard grass; SS—spontaneous succession. Symbols (a–z) explained in Results and Discussion.

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B. Kołodziej et al. / Soil & Tillage Research 163 (2016) 54–63 reduced water content, WCE. BF—black fallow; WW—monoculture of winter wheat; WW + L—monoculture of winter wheat with application of lignite dust; A + G— monoculture of alfalfa with orchard grass; SS—spontaneous succession.

their enlarged photos observed on the computer screen at a desired magnification, morphographic and morphological structure analyses of the tested soils were performed. The structure was
ska-Jurkiewicz described using the terminology given by Słowin et al. (2012) incorporating the concepts of Brewer and Sleeman (1960), Beckmann and Geyger (1967), Jongerius and Rutherford (1979), FitzPatrick (1984), and Aguilar et al. (2015). Photographs of the soil opaque blocks for each treatment were presented in Fig. 2. 2.3. Statistical analysis Soil physical and chemical variables were tested for normality of variance using the Kolmogorov-Smirnov test. The permeability parameters (AP and KS) were not normally distributed, thus data were log-transformed prior to statistical analyses. Nevertheless, in Table 4 non-logarithmic KS and AP values were presented. Mean values (X) and coefficients of variation (the ratios of the standard deviation to the mean, VX, %) were estimated for the measured parameters. The coefficients of variation VX were classified as small (0–10%), medium (10.1–50%), large (50.1–100%) or very large (>100%). One-way ANOVA was used to compare the physical properties by objects representing variants of reclamation (P < 0.05). Subsequently Tukey’s least significant difference test was performed at P < 0.05. 3. Results and discussion 3.1. Texture The studied formations of the inner dumping ground had mostly the texture of sandy loam, except for the WW + L field which was loamy sand (Table 1). High content of sand fraction in the latter soil resulted from the application of lignite at a dose of 1,000 t ha1 corresponding to 100 kg m2 and the fact that lignite dust had a diameter typical of sand fraction. The estimated content of lignite was ca. 28% by weight, assuming that the mass of the 0– 30 cm soil layer of a 1.17 Mg m3 bulk density at 1 m2 was 351 kg. Nevertheless, the texture of the studied fields in view of the statistical analysis was comparable. Generally, plots were characterized with a high proportion of sand fraction (cS, 0.62–0.80 g g1) and a low amount of clay (cC, 0.03–0.15 g g1), which determined their overall air and water properties. Texture of the newly formed soil in the studied area was related with the origin of the material used in the formation of the dumping ground. The reclaimed post-mining lands were built of various sediments, mixed at various amounts and proportions, that occurred in the composition of lignite deposit: grey boulder loams of the Middle Polish glaciation (Weichsel) and yellow ones of the
clays, and Baltic glaciation (Saale), Quaternary sands, Poznan Miocene sands and loams. The grey boulder loam dominating in the overburden had the strongest impact on the properties of soils formed from post-mining grounds (Gilewska, 2008). The texture and mineralogical composition, as well as the alkaline reaction (Table 1, pH) related with the presence of carbonate minerals, should be considered as favourable properties of that rock from the agronomic point of view. 3.2. Structure Fig. 3. (a) Observed (dots) and fitted (lines) soil water characteristic curves; FC— field water capacity, PWP—permanent wilting point. (b) Pore-size distributions from fitted soil water characteristic curves. (c) Hydraulic conductivity, K (m d1) vs.

The other factor influencing air and water properties of the studied area was soil structure which was an effect of a long-term

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thus had loose arrangement and an aggregate structure with fine aggregates up to 2 mm in diameter (Fig. 2, e). In the soil mass there were also visible rock fragments 2–5 mm in diameter which could

combined action of internal (TOC, texture) and external (vegetation, lignite, tillage) factors (Fig. 2). In the upper 0–3 cm layer of the black fallow soil (BF) large Table 1 Selected physical and chemical properties of the reclaimed post-mining grounds. Parameter

Black fallow (BF)

Winter wheat (WW)

Winter wheat + lignite (WW + L)

Alfalfa + grass (A + G)

Spontaneous succession (SS)

LSD0.05c

Sand, cS, g g1 Silt, cSi, g g1 Clay, cC, g g1 Texture Total organic carbon, TOC, mg g1

0.62 a 0.24 a 0.14 a SL 3.24 a

0.63 a 0.28 a 0.09 a SL 3.12 a

0.80 a 0.17 a 0.03 a LS 52.80 d

0.70 a 0.18 a 0.12 a SL 23.04 c

0.62 a 0.23 a 0.15 a SL 12.48 b

0.490 0.285 0.304

7.7 7.5 7.3 1.53 c 3.3 2.66 c 0.4 0.427 a 4.4

1.1 7.6 7.2 1.17 b 2.9 2.39 a 0.4 0.510 b 2.7

1.0 7.6 7.1 1.25 b 7.3 2.62 b 1.0 0.523 b 6.6

0.0 7.6 7.2 0.88 a 8.4 2.65 bc 0.2 0.669 c 4.2

pH(H2O) pH(KCl) Bulk density, r, Mg m3 Particle density, rS, Mg m

3

Total porosity, PO, m3 m3

Xa

VXb 11.1 7.8 7.1 X 1.52 c VX 5.1 X 2.67 c VX 0.2 X 0.430 a VX 6.7

0.934

0.116 0.036 0.0442

SL—sandy loam; LS—loamy sand. a X—mean, in units proper for the parameter. b VX—coefficient of variation, %. c LSD0.05—least significant difference at P < 0.05, in units proper for the parameter. Same letters in one row indicate that there are no significant differences according to the least significant difference method.

Table 2 Parameters of the fitted soil water characteristic curves. Parameter

Black fallow (BF) Winter wheat (WW) Winter wheat + lignite (WW + L) Alfalfa + grass (A + G) Spontaneous succession (SS)

Residual water content, WCR, m3 m3 Saturated water content, WCS, m3 m3 a1, m1 n1 a2, m1 n2 w2 R2

0.066 0.466  0.008a 0.069  0.020 3.232  1.982 9.944  1.690 1.439  0.029 0.85 0.9997

0.000 0.435  0.018 8.712  3.441 1.269  0.029 n/a n/a n/a 0.9968

0.115 0.549  0.011 14.351  4.361 1.459  0.063 0.109  0.026 2.465  0.412 0.39 0.9994

0.131 0.465  0.010 18.001  6.263 1.511  0.093 0.135  0.041 2.142  0.326 0.38 0.9992

0.075 0.639  0.011 10.430  1.374 1.962  0.113 0.125  0.017 4.022  0.755 0.35 0.9997

n/a—not applicable. a Mean  95% confidence limits.

Table 3 Water retention categories of the rehabilitated post-mining grounds. Black fallow (BF)

Winter wheat (WW)

Winter wheat + lignite (WW + L)

Alfalfa + grass (A + G)

Spontaneous succession (SS)

LSD0.05c

0.243 b

0.225 ab

0.209 ab

0.177 a

0.345 c

0.0576

VXb 15.6 X 0.142 a

9.8 0.147 a

11.0 0.214 b

18.1 0.151 a

13.6 0.219 b

0.0320

9.2 0.080 b

8.2 0.065 a

7.0 0.128 c

7.3 0.137 c

15.1 0.076 ab

0.0128

5.0

7.7

7.0

7.3

9.2

Parameter Gravitational water (Macropore volume), GW, m3 m3 Available water (Mesopore volume), AW, m3 m3

Xa

VX Unavailable water (Micropore volume), UW, X 3 3 m m VX a

X—mean, in units proper for the parameter. VX—coefficient of variation, %. c LSD0.05—least significant difference at P < 0.05, in units proper for the parameter. Same letters in one row indicate that there are no significant differences according to the least significant difference method. b

pores were visible (Fig. 2, a) and aggregates ca. 10 mm in diameter (Fig. 2,b) resulted from cultivation measures. The lower 3–8 cm layer was characterized by a massive non-aggregate structure. The soil material had diverse texture: coarser in light-coloured (Fig. 2, c) and finer in dark-coloured zones (Fig. 2, d). Dark-coloured zones were characterized moreover by an enrichment in organic matter

be assigned to the fine gravel fraction characteristic for glacial loams (Fig. 2, f). In the whole 0–8 cm soil layer under the monoculture of winter wheat (WW) there were visible large pores (Fig. 2, g), nevertheless soil mass of a non-aggregate structure with pores—vughs (Fig. 2, h) was noticeably compacted. The largest pores, mainly channels,

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Table 4 Air and water permeability of the rehabilitated post-mining grounds. Parameter

Black fallow (BF)

Winter wheat (WW)

Air permeability, AP (108 m2 Pa1 s1) at the given soil water potential (kPa) AP3.10 Xa 16.0 a 58.7 ab VXb 49.0 14.4 AP9.81 X 436.3 c 76.9 ab VX 13.8 16.6 AP15.54 X 109.6 bc 32.9 ab 22.7 VX 31.8 AP30.99 X 385.8 b 83.5 ab VX 44.3 17.3 1 Hydraulic conductivity, K (m d ) Saturated hydraulic conductivity, KS X 4.0 ab 4.5 ab VX 160.2 10.0 Hydraulic conductivity at 15.54 kPa, 1.3  104 1.7  104 K15.54

Winter wheat + lignite (WW + L)

Alfalfa + grass (A + G)

Spontaneous succession (SS)

37.4 a 12.8 22.0 a 17.9 13.7 a 39.8 28.0 a 26.3

246.0 bc 19.6 165.4 bc 18.4 43.9 ab 22.1 125.5 ab 33.7

478.6 14.1 429.2 15.5 534.8 11.7 542.0 13.0

1.7 a 61.6 0.8  104

18.5 b 21.1 4.6  104

23.6 b 34.3 10.4  104

c c c b

a

X—geometric mean, in units proper for the parameter. VX—coefficient of variation of log data, %. Same letters in one row indicate that there are no significant differences according to the least significant difference method at P < 0.05. b

developed due to the presence of wheat stalks and roots—postharvest residues which were each year mixed and covered by soil during ploughing (Fig. 2, i). The rock fragments were 2–10 mm in diameter representing fine and medium gravel (Fig. 2, k). The soil under the monoculture of winter wheat with application of lignite dust (WW + L) was mostly characterized by a non-aggregate and quite homogeneous structure. The layer had a relatively uniform dark colour resulting from the lignite presence and indicating at the same time a good integration of lignite with the parent material. Soil mass was cut by numerous planes (Fig. 2, l). In the upper 0–4 cm layer there were zones of loose arrangement and structure with tiny (ca. 1–2 mm) aggregates (Fig. 2, m). There were also visible infrequent fine gravel fragments 2–5 mm in diameter (Fig. 2, n). Soil mass under the monoculture of alfalfa with orchard grass (A + G) revealed enrichment in organic matter due to the presence of abundant plant residues. Soil was characterized by a nonaggregate structure. In the upper 0–2 cm layer there were visible undecayed fragments of alfalfa and orchard grass which built a dense turf (Fig. 2, o). In the soil mass many root cross-sections were also detected (Fig. 2, p). Biogenic channels (Fig. 2, r) and planes related to them (Fig. 2, s) emerged in the zones penetrated by the well-developed systems of alfalfa roots. Alfalfa enhances also soil nitrogen (Boldt-Burisch et al., 2015), which could stimulate the growth of the accompanying orchard grass. As a result of the various rooting depths, combining legumes with grasses leads to a greater soil structure stability. The upper 0–5 cm soil layer under the spontaneous succession (SS) had mostly an aggregate structure with tiny aggregates up to 5 mm in diameter (Fig. 2, t) developed by growing roots. The superficial 0–1.5 cm layer was strongly penetrated by roots, which resulted in formation of a dense turf. Consequently, in the 0–5 cm layer many stalks and root cross-sections (Fig. 2, u) and the zones of loose arrangement in the vicinity of the plant residues (Fig. 2, w) were visible. The lower 5–8 cm layer had, on the contrary, a nonaggregate structure with fissures (Fig. 2, x). In the soil mass there were also discernible fine gravel fragments 2–5 mm in diameter (Fig. 2, z). The five studied soils, due to their coarse texture, were characterized mostly with a non-aggregate structure. The main factor which stimulated soil loosening via the creation of biogenic pores and small aggregates and then stabilized the developed features was vegetation (Wick et al., 2014). Tillage of BF, WW, and WW + L soils caused mainly mixing of soil material. Aggregate structure created during cultivation measures involving only the

upper 15 cm gradually disappeared over time due to low clay or TOC content (Table 1). It was only visible to some extent in the BF soil which was sampled 5 weeks after the last cultivation. Effects of tillage on the structure of the WW and WW + L treatments were not evident, because soil was sampled 8 months after cultivation measures and consequently at this time was already selfconsolidated. The soil enriched in lignite (WW + L) was moreover prone to fragmentation during wetting-drying cycles when numerous planes could expand. 3.3. Particle density, bulk density and porosity Particle density, rS, varied mostly within the range of 2.62– 2.67 Mg m3 (Table 1). Comparable values recorded in 4 of 5 fields in total were related to the similar mineral composition of the material of which the dumping ground was constructed. On the other hand, in the WW + L field, the particle density was significantly lower. The application of lignite at a dose of 100 kg m2 which, as reported by Kwiatkowska et al. (2006), is characterized by particle density of 1.31 Mg m3, caused a decrease of the value of that parameter to 2.39 Mg m3. Bulk density values, r (Table 1) decreased gradually with the increase of number of factors potentially loosening the soil structure (Fig. 2). Namely, the highest values were noted in the BF (without vegetation) and WW (one plant type of fibrous root system) treatments, intermediate  in the A + G (two plant species with a taproot and fibrous root systems) and WW + L (lignite application) soils, and the lowest—in the SS soil, with the highest typological diversity of vegetation. The observed trend was consistent with the results of Huang et al. (2015) that the average soil bulk density value dropped from 1.56 to 1.10 Mg m3 when the number of herbaceous (grass) species rose from 2 to 11. In the analysed Konin mine area the values of bulk density did not exceed the soil density value critical for plants, which for loams and sands is equal to 1.80 Mg m3 (Pierce et al., 1983; USDA-NRCS, 2003). BF and WW soils were characterized with medium and WW + L, A + G, and SS soils with very low bulk densities, taking into account the sandy or loamy texture of these soils. In the soils analysed, the values of total porosity, PO, were high (BF and WW) or very high (WW + L, A + G, SS) (Table 1) showing at the same time a reverse trend as compared with the bulk density values. The bulk density and total porosity values complied with the morphological soil structure analysis. Total porosities and bulk densities were moreover influenced by TOC (Table 1), although physical impact of vegetation rather than organic matter content

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had a stronger loosening effect since for WW + L, A + G, and SS soils total porosity increased with decreasing TOC and increasing rooting intensity. 3.4. Soil water characteristic curves Fig. 3a shows interpolated soil water characteristic (SWC) curves. For BF, WW + L, A + G, and SS soils a dual-porosity model was selected, while for WW soil—a single porosity one to maximize R2 value and minimize a and n standard deviations. Water retention parameters obtained from measured data were presented in Table 2. In view of the fact that the R2 values were in the range 0.9968–0.9997, the applied models generated very good fits of measured data allowing for accurate prediction of soil hydraulic properties and pore-size distribution. Water retention depends in a complex way on soil structure and composition, consequently the shapes of the SWC curves illustrated the effect of the various reclamation treatments on the water content and the water retention of the studied soils. The SWC curves plotted for the BF and WW soils were very similar revealing comparable retention capabilities, air properties, and comparable structure of the two soils (Table S1 and Table 3, Fig. 3a). The maximum water capacity attained the highest value under the SS treatment (Fig. 3a), medium—under WW + L, and the lowest under WW, BF, A + G treatments. The highest capacity of gravitational water had the SS soil and the lowest—A + G soil. Intermediate values of this parameter were measured in WW + L, WW and BF soils. On the other hand, retention of water available for plants was less diverse between treatments: higher for SS and WW + L and lower for the other fields. The retention of unavailable water which corresponds to volume of pores not participating in supply of water to plants under A + G and WW + L was ca. 2 times larger than under BF, SS, and WW treatments. The SWC curves showed no defined relation with TOC in the soil water potential range from 0 to 49.03 kPa. In contrast, water contents at the potentials below 49.03 kPa were higher for WW + L and A + G soils characterized by larger TOC but lower for the other soils. As reported by Rawls et al. (2004), research on the effect of TOC on water retention produced inconsistent results. In our study, water retentions at high potentials were affected mainly by vegetation remodelling soil structure. At lower water potentials, however, the parent material itself determined water retention, and as the texture was comparable for the 5 treatments, the increments of TOC governed water content values. The plots of differential pore-size distributions obtained on the basis of the fitted volumetric SWC curves (Fig. 3b) confirmed the above relations. Pore-size distribution of WW soil was unimodal, with a maximum in the macropore area, ca. 75 mm. On the contrary, pore-size distributions of BF, WW + L, A + G, and SS were bimodal with peaks in the range of macropores, ca. 120–250 mm, and mesopores, ca. 2–3 mm. Dual-porosity characteristics of the last three soils could be associated with the presence of interaggregate macropores: biogenic channels (A + G and SS) or planes (WW + L) and mesopores: pores within aggregates developed by vegetation (A + G and SS) and pores within lignite fragments
(WW + L) (Šimunek et al., 2003). The SS soil was characterized by a higher proportion of macro- than mesopores, whereas the A + G and WW + L soils had relatively balanced volumes of these pores (Fig. 3b, Table 3). Larger volume of macropores in the SS soil could be attributed to the intense impact of the roots of the diverse vegetation. In the soil treated with lignite dust (WW + L), macropores could not develop to a high degree in the presence of winter wheat only. Similarly, lignite dust appeared to be a weak aggregate-stabilization factor; it rather filled voids between mineral particles decreasing the amount of macropores (Fig. 2). The post-mine Technosols in Konin, as in other lignite-bearing

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locations (Gerke et al., 2009), are mixtures of overburden sediments (here loamy and sandy) with highly porous lignite fragments of diverse size, yielding a dual-porosity system of coarse matrix with embedded lignitic components. Since in our experiment the 0–10 cm layer was studied, large-sized lignite fragments were practically absent. In the WW + L field, on the other hand, the application of lignite dust resulted in rearrangement of soil pore geometry and size distribution, especially when compared with the WW treatment. 3.5. Air and hydraulic permeability Air and hydraulic permeability values in samples taken from one soil can cover several orders of magnitude. Such a high variability of both air and hydraulic permeability values is characteristic for soil. Soil permeability depends mostly on soil structure and is a very sensitive indicator of pore connectivity and the presence of large pores which facilitate preferential flow of air and water and, on the other hand, the presence of impermeable layers hindering the flow. Due to the wide range of permeability values, it is applicable to use logarithmic transformation. See also Huang et al. (2016). Logarithmic air and hydraulic permeability values of the 5 studied soils were characterized mostly by medium (11.7–49.0%) coefficients of variation (Table 4). Accordingly, despite the visible differences in mean values detected for each soil, not many of them were statistically valid. Lower air permeability values were detected in general for WW + L and WW soils, intermediate—for BF and A + G soils, and the highest—for SS soil. The WW, WW + L, and SS soils were characterized with relatively homogeneous air permeability values at all soil water potentials. The samples taken from BF and A + G soils, on the other hand, in a diverse way responded to measurements. It could be attributed to the unstable structure of BF soil in the absence of plant roots and a temporal entrapment of water in soil pores of A + G soil. The air permeability values did not rise monotonously with decreasing water content. It could be related to the fact that in this heterogeneous material during soil drying from the state of saturation some of the largest pores could be closed by water menisci, which reduced air exchange. However, once the field water capacity was reached, the macropores were unblocked and air permeability increased. Air permeability values at the field water capacity, AP15.54, were low for WW + L, WW, and A + G and medium for BF and SS soils. Hydraulic permeabilities could be classified as very high for SS and A + G, high for BF and WW, and medium for WW + L treatment. Soils of low and comparable organic carbon content, BF and WW, had at the same time very similar hydraulic permeability. For other treatments, however, hydraulic conductivity decreased with increasing organic carbon content. Results obtained by other authors were ambiguous, i.e. Lado et al. (2004) stated that the higher organic matter content resulted in an increase of KS value, because organic matter promoted the development of soil aggregate structure. Nemes et al. (2005, 2006) and Rawls et al. (2004) after analysis of pedotransfer functions for many soils observed on the other hand that hydraulic conductivity decreased with increasing organic matter content. This phenomenon could be explained by a retention of some proportion of water by the soil organic matter, which diminished the water volume transferred through the soil. Hydraulic conductivity was larger in the A + G and SS soils with intensive vegetation cover and greater species number than under WW, WW + L, and especially BF treatments. Our results support the findings of Huang et al. (2015) for soils of comparable texture (loamy sand, sandy loam, and loam) that the infiltration rate was positively correlated with coverage and richness of grasses. The infiltration rates measured by these authors increased from 3.4 to 19.8 m d1 while the number of

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herbaceous species, including Medicago sativa, increased during the reclamation period. Lower hydraulic conductivity values in the WW + L, WW, and BF soils could be attributed moreover to the low stability of macropores in the coarse material, which were not substantially supported by plant roots. According to Dexter and Richard (2009), macropores disappear quickly after tillage due to soil settlement as a result of time or rainfall and become isolated voids thus do not contribute to hydraulic conductivity. In our study hydraulic conductivity, K, decreased most rapidly with decreasing effective degree of saturation, WCE, for WW soil and most slowly—for WW + L soil, where the curve slope was the smallest, undoubtedly due to the high organic matter content (Fig. 3c, K vs. WCE). Curves plotted for A + G and SS soils were characterized with similar shape. The saturated hydraulic conductivity was also comparable for both sites, revealing equivalent impact of different plant cover on hydraulic properties. It could be related to the similar soil structure which for both soils was cocreated by a well-developed root system of alfalfa (Fig. 2, A + G and SS). Field water capacity (WC15.54, m3 m3) of the five studied soils was in the range from 0.39 (SS) and 0.40 (BF) through 0.48 and 0.49 (for A + G and WW soils, respectively) to 0.53 (WW + L) of reduced water contend value (WCE). At this state of effective saturation with water, which is typical in field conditions since the state of full saturation lasts up to 2–3 days after intense precipitation or irrigation, hydraulic conductivities K15.54 showed comparable trend to KS values (Table 4). 4. Summary and conclusions The reclamation variants applied on the post-mining areas in the dumping ground Pa˛tnów influenced in a diverse manner the physical state of the 0–10 cm layer of the developed technogenic soil. Krümmelbein et al. (2012) after Katzur and Böcker (2010) showed total carbon and bulk density values that should be reached after reclamation, which for mixed Quaternary and Tertiary substrate were 10–15 mg g1 and 1.5 Mg m3, respectively. In all our studied treatments the soil bulk density achieved essentially the recommended value. Bulk density values generally decreased with the increase of number of factors potentially loosening the soil structure. As regards TOC, under the BF and WW treatments involving only simple tillage measures without vegetation or monoculture of annual plant, accumulation of organic matter was not sufficient. Under the SS and A + G treatments TOC values were, on the contrary, much higher. Our results confirmed that after the preliminary basic substrate amelioration the introduction of appropriate plants was indispensable. For that purpose combination of legumes with grasses is often recommended since they produce high amounts of biomass to be incorporated into the developing soil. The studied plots had sandy loam or loamy sand texture which determined their overall air and water properties. The soils were characterized by high macropore volume, providing sufficient soil aeration. The content of water available for plants was also favourable in reclaimed soils, the values were classified as very high (WW + L, SS) or medium (BF, WW, A + G). Less beneficial conditions of the soil were associated with air and water permeability. Low air permeabilities were observed in the soils of the WW + L, WW, A + G; medium in the BF and SS treatments. The lowest values of this attribute detected for the soil enriched in lignite in the amount of 1,000 Mg ha1, at each studied soil water potential, signify a possibility of insufficient soil aeration, particularly for the period of excessive water contents. Hydraulic permeability of the analysed soils was classified as medium to very high. The present studies indicated that spontaneous succession, with the highest typological diversity of vegetation, contributed to

the most preferred soil physical state. Remaining variants of reclamation (including black fallow without vegetation) also provided sufficient air and water relations in the soils. However, it should be emphasized that plants play an important role in TOC accumulation and ensuring relative stability of soil structure, therefore rehabilitation of post-mining grounds involving vegetation is recommended. Acknowledgements This work was carried out within the scope of the statutory research at the University of Life Sciences in Lublin (Institute of Soil Science, Environment Engineering and Management) and financed from the budget of the Ministry of Science and Higher Education in Poland. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.still.2016.05.001. References


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