1 Introduction

Geological structure, topography, rainfall and human activities contribute to the occurrence of landslides in the Polish conditions. In Poland, the Carpathians are a region highly predisposed to the occurrence of landslides. The landslides that take place in forest areas cause the destruction of trees, and they break the continuity of soil cover which results in different physical, chemical and biological properties (Shiels et al. 2006; Shiels and Walker 2013). Deposits of landslides are characterised by high variability of properties, especially the distribution and the amount of soil organic matter (Błońska et al. 2016). The contents of soil organic matter are often used as an indicator of soil quality, and it helps determine the physical (Masri and Ryan 2006), chemical (Sebastia et al. 2007) and biological properties of soil (Valarini et al. 2003; Arandá et al. 2015). Soil organic matter is used in the evaluation of soils in addition to biological indicators of soil quality (Flieβbach et al. 2007). The microbial biomass carbon and nitrogen (Błońska et al. 2016) and enzyme activity (Pająk et al. 2016) are primary biological indicators of soil quality. The quantity and quality of soil organic matter can be determined by specifying the total content of soil C (Haynes 2005), or by estimating the available fractions of soil organic matter (Dębska et al. 2016). Humic substances include three main fractions: humin (H)—insoluble fraction of humic substances; humic acid (HA)—fraction soluble in alkaline conditions and fulvic acids (FA) which are soluble under all pH conditions. The relationship between the concentration of humic and fulvic acids (HA/FA ratio) is an indicator of the potential mobility of C in soil. The ratio of (HA + FA)/humin indicates the degree of soil organic matter illuviation (Guimarães et al. 2013). Identification of soil organic matter fraction can serve as an indicator of changes in the carbon store (Leifeld and Kögel-Knabner 2005). Changes in the amount of soil organic matter, nutrients and physical properties have different intensification within the range of landslides and strongly influence the processes of soil cover and vegetation restoration (Pickett et al. 1999; Shiels et al. 2006). Walker et al. (1996) suggest that organic matter and nutrients are a limiting factor for landslide succession.

Many studies have been conducted on the importance and the hydrological impact of water repellency in soils (Dekker et al. 2001; Doerr and Thomas 2003). Burguet et al. (2016) found significant correlation between soil organic matter (SOM) content, and Water Drop Penetration Time (WDPT Täumer et al. (2005) suggested a new approach for calculating wettability of soil by introducing a parameter called ‘critical water content’ (Θcrit) as a function of the amount of soil organic matter (SOM). According to Dekker and Ritsema (1994), a critical soil moisture zone can be important for stability in a landslide. The lower zone determines a limit below which the soil is water repellent; the higher zone determines the water content above which the soil is wettable (Dekker et al. 2001, 2003; Ziogas et al. 2003).

In the Polish part of the Carpathians, the number of landslides is estimated at over 50,000, which accounts for 30% of that area. This is considered to be a significant problem in the mountainous areas of Poland. The previous studies related the variability of physical and chemical properties of soils as a result of the landslides (Wilcke et al. 2003; Shiels and Walker 2013), with a few studies demonstrating the variability of biochemical properties (Mandal 2012; Błońska et al. 2016). The biochemical properties have not been used in assessing the restoring processes of soil cover and vegetation on a landslide. In the previous studies, the authors focused on the assessment of the relationship of soil properties with the possibility of landslides (Meisina 2006; Kitutu et al. 2009). The studies performed mainly concentrated on landslides in the tropical zone (Shiels et al. 2006; Mugagga et al. 2012), while there are no similar studies in the temperate climate zone. The aim of the study was to present variability of content and quality of soil organic matter on the landslide surface. Attempts were made to show the progress of the process of soil cover restoration 7 years after the landslides and the biochemical activity of soil associated with the restoration of soil cover. The physical, chemical and biochemical parameters were used to monitor the changes on the landslide surface and were referred to the characteristics describing reproducing vegetation. Additionally, the fractions of soil organic matter were used in the study as an indicator of soil quality due to the many important interactions of these components in the soil system. Based on the SOM content, the critical water content was calculated and the evaluation of the distribution of this characteristic in landslide zones was carried out.

2 Materials and methods

2.1 Soil sampling sites

The landslide area was located in southern Poland, in the Sucha Forest District (Fig. 1), in the Carpathian Foothills region, at altitude 450–490 m a.s.l. on a slope with northern-east exposure, inclined at 30°. The study area is characterised by the following conditions of climate: the average annual rainfall is 940 mm, the average annual temperatures amount to 7.2 °C and the length of the vegetative season lasts 213 days. Sample plots were located in the area with a predominance of Magura tertiary sandstones and schists and with Cambisols (WRB 2014). Research plots were established in areas dominated by eutrophic fir forest (Abies alba–Oxalis acetosella communities). The silver fir (Abies alba) was the dominant species in the study area before the landslide and currently surrounds the landslide. Following the classifications of landslides (Walker and Shiels 2013), it was a geological inactive landslide, which related to the flysch rocks. In the study area, the sandstones and claystones separate highly compacted shale rocks or clay slate rock layers located at a certain depth inclined obliquely or parallel to the slope of the terrain. These structures were dominant. Under the influence of long lasting precipitation, the strongly waterlogged soil cover increased its weight, a smooth surface of the underlying slate or non-weathered rock favoured separation and sliding of the overlying layers. he landslide occurred in 2009. Information about the landslide was confirmed by the National Institute of Geology where the landslide inventory is conducted under the System of Landslide Mitigation. In addition, the landslide was inventoried by the State Forests staff during forest monitoring.

Fig. 1
figure 1

Landslide location in Sucha Forest District (southern Poland)

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The surrounding area of the landslide showed no signs of erosion processes and was characterised by a uniform slope. There was no interruption of the soil cover, while at the same time there was one subtype of the soil and humus type. The surrounding area was covered by homogeneous vegetation, in contrast to the landslide area. In the area of the landslide, the vegetation showed very high diversity of species and quantity. The landslide area can be treated as a separate research object, incomparable to surrounding forest areas, characterised by high homogeneity and formed under stable conditions. The soil samples were collected from the entire landslide area, from locations distributed on a regular grid 10 × 20 m (Fig. 2). Thirty soil samples were collected for the tests. Each sample constituted a cumulative sample from 5 subsamples. The samples were collected from the humus–mineral horizon (0–15 cm). Soil samples for laboratory analysis were taken in September 2016.

Fig. 2
figure 2

Scheme of landslide with soil sampling points

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2.2 Laboratory analysis of soil

Soil samples obtained in the field were dried and sieved through 2.0-mm mesh. Using the potentiometric method, the pH of the samples was analysed in H2O and KCl. The soil texture was determined by laser diffraction (Analysette 22, Fritsch, Idar-Oberstein, Germany). Carbon (Ct) and nitrogen (N) contents were measured with an elemental analyser (LECO CNS TrueMac Analyser) (Leco, St. Joseph, MI, USA). Fractional composition of humus was determined by the method of Kononowa and Bielczikowa where extraction is carried out in a mixture of 0.1 M NaOH i 0.1 M Na4P2O7 (Dziadowiec and Gonet 1999). The chemical fraction was carried out to obtain humin (Hm), humic acid (HA) and fulvic acid (FA) fraction. The relationships HA/FA and (HA + FA)/Hm were also calculated.

In the soil samples, the activity of dehydrogenase (EC 1.1.1.1) was determined in three repetitions. Using Lenhard’s method based on the Casida’s procedure, DH was determined by the reduction of 2,3,5-triphenyltetrazolium chloride (TTC) to triphenyl formazan (TPF) (Alef and Nannipieri 1995). Briefly, 6 g of soil was incubated with 1 ml of 3% TTC for 24 h at 37 °C. TPF was extracted with ethyl alcohol and measured spectrophotometrically. Before the analysis, fresh samples of natural moisture were sieved through a sieve (ø 2 mm) and stored at 4 °C.

To determine microbial biomass carbon (MBC) and nitrogen (MBN), 5 g of soil was weighed and fumigated with CHCl3 in an exsiccator for 24 h at 25 °C. Fumigated and non-fumigated samples were extracted with 0.5 M K2SO4 and then filtered with the Whatman filters (Vance et al. 1987). The amount of organic C and N was determined quantitatively (Jenkinson and Powlson 1976).

2.3 Determination of vegetation parameters

In the grid intersections on the areas (fields 4 m2), a description of ground cover vegetation was carried out with a list of the species and determination of their cover level according to the Braun-Blanquet scale. For the ground vegetation, the Shannon diversity index (H) (Falińska 1997) and Shannon and Weaver dominance index (C) (Sienkiewicz 2010) were determined. For each sampling plot, the contribution of specimens of each species was determined and all trees were measured, determining their height and thickness at the root neck. Several specimens of each species with different sizes were collected for the laboratory, where their aboveground biomass was determined after drying. The aboveground biomass relationship with the size of the root neck served for the determination of the total tree species biomass for each of the studied areas.

2.4 Calculation of a critical water content

SOM content was used to calculate critical water content Θcrit (Täumer et al. 2005). This was based on formulas (1) to count the amount of SOM and then (2) to calculate the Θcrit:

$$ \mathrm{SOM}=\left({C_t}^{\ast }1,724\right)/100 $$
(1)
$$ \varTheta \mathrm{crit}\left(\mathrm{SOM}\right)= a\mathrm{SOM}+{\varTheta}_0 $$
(2)

where Ct—carbon content; a = 1,12; Θ 0 = 0.037 g g−1.

2.5 Statistical analysis

The principal component analysis (PCA) method was applied, in order to reduce the number of variables in the statistical dataset and to visualise the multivariate dataset as a system of coordinates in a high-dimensional data space. The PCA method was also used to interpret other factors, depending on the type of dataset. In PCA analysis the physical, chemical and biochemical properties of soil are utilised. On the basis of Ward’s method, agglomeration of the tested areas into groups differing in the measured soil parameters was conducted. The differences between the mean values were evaluated with the Tukey’s test. Pearson correlation coefficients between soil characteristics were also calculated. The statistical significance of the results was verified at the significance level of alpha = 0.05. All the statistical analyses were performed with Statistica 10 software (2010).

3 Results

Zones differing in the properties of soil cover are distinguished on the analysed landslide. The cluster analysis including the content of clay fraction, organic carbon content, pH, dehydrogenase activity and the amount of soil aggregates allowed the distinction between three areas (zones) coinciding with the components of the landslide distinguished by geomorphology. The first zone is the landslide niche (marked as Z1), the second is the landslide foot–accumulation area (zone Z2) and the third is the edge zone (Z3). The validity of distinguishing these three zones was confirmed in both the cluster and the PCA analysis. Figure 3 presents a dendrogram of the agglomeration result. The PCA analysis based on the soil properties suggests existence of three clusters of points, which can be identified with the zones of landslide (Z1, Z2 and Z3 zones) (Fig. 4).

Fig. 3
figure 3

The result of Ward’s method agglomeration (variables used in the analysis: pH, Clay, Ct, DH, Ag > 10)

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Fig. 4
figure 4

Factorial plan and projection of variables in the soil properties on the factor-plane 1 × 2

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On the area of the landslide niche (Z1 zone), soils are characterised by high clay content (mean 22.8%) and trace content of the sand fraction (1%). Additionally, low content of organic carbon (0.75%) and nitrogen (0.10%) was recorded in this part of the landslide. In the fractions of soil organic matter, humic acids dominate over fulvic acids and humins. The relationship of humic acids with fulvic acid carbon remains in the range from 1.5–8.2 with mean 3.0. The level of humic acid carbon is three times higher than fulvic acid carbon and five times higher than C humin. The relationship of the sum humic and fulvic acids to C humin assumes high values from 2.2 to over 700 with a mean of 161.8 (Table 1). Due to the low value of organic matter, the soil in the niche area is characterised by the least developed aggregate structure. The number of soil aggregates of different sizes was lowest compared to the remaining zones of the landslide. Soil in the niche area is further characterised by the lowest activity of dehydrogenases and relatively low microbial carbon and nitrogen biomass (Table 2). The biomass of trees inhabiting the niche zone is almost two times lower than the value found for the accumulation area (Z2 zone) and almost two times higher from the biomass of vegetation inhabiting the edge area (Z3 zone) (see Table 5).

Table 1 Humus substances, organic carbon and nitrogen content, Θcrit in soil of different parts of landslide
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Table 2 pH and biochemical properties in soil of different parts of landslide
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The Z2 zone of the landslide is characterised by a different texture, structure, characteristics of the soil organic matter and the enzymatic activity in comparison to soils of the niche area (Z1 zone). Silt fraction with a considerable contribution of sand fraction (mean 22.2%) and lower of clay (9.0%) dominates in the soil texture in the accumulation area—landslide foot (Z2). The organic carbon content in the soils of the Z2 zone is highly variable. The mean carbon content is 2.76%, but single points have been recorded, where the Corg content was approx. 4%, and in one point in the middle of Z2 zone, the Corg content exceeded 7%. The C/N ratio of the discussed zone (Z2) assumes higher values than in the Z1 zone (mean 11.6) (Table 1). The fraction composition of the soil organic matter in the Z2 zone differs slightly from the niche area (Z1 zone). Typically, fulvic acids dominate over humic acids. The HA/FA ratio ranges from 0.3 to 1.4 with mean 0.68. The mean C fulvic acid content is 0.99%, whereas the C humic acids are 0.62%. Humins constitute a considerable percentage of the fraction of soil organic matter in the Z2 zone with the mean C humin content exceeding 1%, yet the value is variable. The relationship of the sum of C humic acids to C humin (HA + FA/Hm) assumes the values of 1.0–2.8, with mean 1.6 (Table 1). Soils of the Z2 zone are characterised by the lowest pH (mean pH in H2O 5.01). Soils of this part of the landslide exhibit higher activity of dehydrogenases and higher values of microbial C and N biomass than soils of the Z1 zone (Table 2). Considerably higher content of large grain aggregates (5–10 and >10 mm) is observed in the structure of soil aggregates of the Z2 zone. Smaller aggregates exhibit high variability of the abundance, yet they have the tendency to form in larger quantities in soils of the Z2 zone in comparison with Z1 soils (Table 3).

Table 3 The particle size and aggregate content in soil of different parts of landslide
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The edge area Z3 includes points located in the upper and lower part of the landslide on the edge of the area subjected to the landslide of soil masses. The points included in the zone are characterised by grain size similar to that of the Z2 zone, similar organic matter content yet slightly different composition of the soil organic matter fraction and clearly higher soil pH (mean pH in H2O of the zone is 6.33). The Corg content is similar to that of Z2 zone, and it averages 2.34% with a slightly higher C/N ratio of 14.1 (Table 1). Similarly to the accumulation area (Z2), fulvic acids dominate over humic acids in the edge area (Z3). The ratio of carbon in the fractions of humic acids (HA/FA) ranges from 0.2 to 1.4, with a mean of 0.8. Humins exhibit relatively high contribution as well. This is indicated by the value of the quotient of the sum Corg humic and fulvic acids referred to as Corg humin (HA + FA/Hm) which assumes the mean value of 2.8 for the examined zone (in soils of the niche area, this quotient assumes a considerably higher value of 161.8) (Table 1).

The analysed fractions of humus compounds exhibited a very strong relationship with the total nitrogen content and the total carbon content in soils of the examined landslide. C humic acids, and in particular of humic acids, have a strong positive correlation with the microbial nitrogen biomass, and to a lesser extent with the dehydrogenases activity. Among the physical properties, the abundance of soil aggregates and fine sand fraction is positively correlated with the carbon content of the tested humus compounds, whereas the clay content is negatively correlated with the carbon content of all analysed humus substances (Table 4).

Table 4 Correlation between humus substances and some parameters
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Stands of the common hazel (Corylus avellana) with singular admixtures of other species (willow, sycamore, fir) dominated in the area of the niche (Z1 zone). The accumulation area—landslide foot (Z2 zone) was very diverse in terms of vegetation and species-rich biogroups dominated by the common hornbeam (Carpinus betulus) with admixtures of willow, hazel and linden which could be discerned in the majority of points. In the middle and lower part of the landslide foot, the occurrence of three biogroups was recorded dominated by sycamore (Acer pseudoplatanus) and a single group dominated by elder (Sambucus nigra). The edge zone turned out to be very diverse in terms of woody vegetation. In the points located on the upper edge of the landslide, points devoid of vegetation were found, as well as surfaces with dominance of C. avellana, similarly to the niche area. In the edge zone at the lower part of the landslide, points were found dominated by willow (Salix caprea) and sycamore (A. pseudoplatanus) (Table 5). Herbaceous vegetation was quite well developed on the landslide area, and its species composition did not differ significantly in its individual zones, which is exhibited by similar values of biodiversity indicators (Table 5).

Table 5 The vegetation biomass, biodiversity indexes and dominant species of shrubs and trees in the different parts of landslide
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The critical water contents for the Z1 zone obtained on average 0.052 (g g−1); for Z2, a value of 0.090 and for Z3, a value of 0.086 (g g−1) (Table 1). In the Z1 zone, a statistically significant lowest critical water content as compared to Z2 and Z3 zones was noted.

4 Discussion

During the conducted study, the strongest distinctiveness of properties was determined for soils of the landslide niche in comparison to the accumulation area (landslide foot) and the edge area. This distinctiveness was confirmed by physical (texture, structural), chemical (content and quality of organic matter) and biochemical (enzymatic activity, microbial biomass C and N) properties. Higher contribution of clay fraction in relation to sand fraction on the niche area results from the slide of soil layers and the uncovering of deeper and richer clay levels. The initial stage of soil cover restoration is here exhibited by the content of soil organic matter. According to Pietrzykowski and Daniels (2014), soil organic matter (SOM) plays a key role in early soil formation processes and re-establishment of ecosystem functions on reclaimed post-mining sites. The organic carbon content in the surface layer of soils in the Z1 zone does not exceed 1%, which is characteristic of the restoring initial humus accumulation horizons. The advantage of humic acids over fulvic acids determined at the same time in the soil of the area indicates the low mobility of carbon accumulated in this horizon. Soil organic matter quality is often assessed by the humic to fulvic acid ratio (HA/FA) (Mládková et al. 2006). The permanent combinations of humic acids with clay, the formation of which was demonstrated in an earlier study (Mikutta et al. 2006), may contribute to the stabilisation of the soil organic matter in the surface horizon of restoring soil. The humic acid to humin content ratio is treated as the indicator of the degree of soil organic matter illuvium (Guimarães et al. 2013). On the examined landslide, the surface humus accumulation horizon was analysed; however, the niche area of the landslide could be subjected to uncovering of deeper horizons, which were supplied with humus substances resulting from the earlier illuvium processes. The advantage of humic acids in relation to humin content determined in the landslide niche may suggest the low degree of soil organic matter stabilisation as related to the early soil forming processes. Completely different characters were found for the soil organic matter accumulated in the surface horizon in the accumulation area (landslide foot) and on its edge. In these zones, the higher content of soil organic matter is found, and its stabilisation is primarily caused by fine silt fractions and clay, which occurs in the zone at a lower percentage. Fulvic acids dominate over humic acids in the fraction composition of soil organic matter in the accumulation zone (Z2), which is characteristic for forest soil humus (Guggenberger and Zech 1994). The advantage of fulvic acids over humic acids suggest that the soil organic matter found in the landslide foot had accumulated through the slide and mixing of humus, which was formed for a long time in the forest environment prior to the landslide. Additionally, further landslides and mixing of soil masses take place on the landslide foot and in the edge area due to the continuing edge disintegration and the additional runoff of humus substances from the area surrounding the landslide. In none of the points in the examined area, the presence of organic horizons or even concentrations of material that is richer in organic matter was determined, which suggests that the mixing of soil organic matter with mineral substances in the landslide process accelerates its decomposition and transformation into specific humus compounds, which can be stabilised in humus–mineral combinations and within soil aggregates. This complies with the observations of Mikutta et al. (2006). This can be confirmed by the relatively high humin content in the accumulation area and in the edge zone. The proportion of the sum of humic acids to humins (HA + FA/Hm) assumes lower values in the accumulation and edge area compared to the landslide niche (in soils of the accumulation area, values below 2.0 prevail, the edge zone is more variable in these terms and the mean of the humic acids to humins ratio (HA + FA/Hm) is over 3.0).

The quality of stabilisation of humus substances in soil may also depend on the species composition of vegetation inhabiting the soil (Guimarães et al. 2013). Plants provide organic matter in the form of dead debris and root secretions. Detritus may differ in terms of the content of i.e. cellulose, lignins and polyphenol compounds. The latter compounds have long been attributed with the key role in the formation of the rate and direction of soil organic matter changes (Kuiters 1990). In the conducted study, certain differences in the composition of vegetation were found inhabiting individual zones of the landslide. It is difficult to unambiguously determine the influence of the woody vegetation on the quality of soil organic matter. Niche soils were dominated by humic acids. The following question should be asked: Is the common occurrence of hazel able to influence the current structure of fractions of humus substances? Undoubtedly, it is a pioneer species, which may influence the development of soil microorganisms and the rate of soil organic matter changes by different nitrogen content in the debris provided to the soil (Esperschűtz et al. 2013). The European hornbeam and sycamore maple were common in the accumulation area (Z2). Increased SOC storage under Fraxinus exelcior and Acer spp. stands compared to other species was noted by Edmondson et al. (2014). The forest floor nutrient concentrations appeared to be dependent on the presence of deciduous tree species (Sycamore Maple and small-leaved Linden) (Kacálek et al. 2013). Hornbeam is regarded in forestry as “phytomelioration species”. Decomposition of leaves is usually faster in the case of Hornbeam in comparison with oak or beech leaves. According to Błońska (2015), hornbeam provided organic matter which is more favourable for microbial decomposition. The process of forest floor formation is an important component of forest environment restoration.

In the performed study, we have confirmed the strong relationship between the concentration of humic acid carbon and the total nitrogen content in soil. The formation of humic acids is affected by nitrogen contained in the soil, which is one of the main components of these compounds. Moreover, it can be inferred that the analysed humus compound fractions possess a stimulating effect on the activity of soil microorganisms. This is confirmed by the strong positive relationship between the amount of humic acid carbon (in particular of HA) and the microbial nitrogen biomass. A weaker relationship between the microbial biomass nitrogen with the C humin may stem from the stronger binding of these compounds by the mineral substance of soil, resulting in their greater resistance to microbial decomposition and stronger stabilisation (Canellas et al. 2008).

Statistically lowest activity of dehydrogenases and microbial biomass of carbon was recorded in the niche of the landslide as compared to the accumulation area (landslide foot). The result obtained confirmed that dehydrogenase activity could be a soil quality indicator because it provides an indication of changes in organic matter status. Dehydrogenase activity correlated with fractions of humus compounds, and the PCA analysis confirmed the relationship of the enzyme with the organic carbon content. Enzymatic activity responds relatively quickly to changes in soil conditions and can reflect the changes in soil quality before they can be detected by other soil analyses (Jin et al. 2009; Klamerus-Iwan et al. 2015). Soil microbial biomass carbon is usually used to characterise the activity of soil organic carbon (Yang et al. 2016). Our study demonstrated that soil microbial biomass carbon increased with restoration of landslide soils. This finding was consistent with the study conducted by Liu and Wang (2010).

Soil water repellency poses an important problem for landslides. It causes low infiltration rates and increased surface runoff resulting in less soil water storage as a supply to plant growth. It was found that the landslide area is characterised by a stronger diversification of critical water content (Θcrit). The higher value of Θcrit in the middle of the landslide (Z2) provides higher stability and resistance to the emergence of new shifts of soil. Θcrit translates into the capacity to retain water and limit the risk of runoff on the slope (Blackwell 2000). Critical soil water content of limited water availability can be used as an indicator for the stability of slopes.

5 Conclusions

The landslide area is characterised by a stronger variability of physical, chemical and biochemical properties. The upper part of the landslide is strongly eroded and characterised by the least advanced soil cover recovery. Soil organic matter plays a crucial role in the early stages of the formation of soil cover and vegetation. The amount of soil organic matter had a positive effect on the microbial biomasses C and N as well as the dehydrogenase activity. The biochemical parameters utilised, i.e. dehydrogenase activity and microbial biomass C and N, turned out to be useful tools for the evaluation of changes taking place in the soil after a landslide. The soil organic matter fraction can be used as an indicator of changes in the soil. The amount of soil organic matter fractions reflects the potential mobility of C in the soil and the rate of soil organic matter decomposition.