1 Introduction

Globally, increasing urbanization and industrialization are responsible for increasing heavy metal content in ecosystems (Chrzan, 2013; Mazurek et al., 2017; Xia et al., 2019). The main anthropogenic sources of heavy metals include mining, agriculture (Antonkiewicz & Gworek, 2023; Marrugo-Negrete et al., 2017), transportation (Chrzan, 2013; Gorlach & Gambuś, 2000; Xia et al., 2019), and atmospheric deposition (Kuźniar et al., 2018; Lenart-Boroń & Boroń, 2014; Minase et al., 2016). Heavy metals are trace elements that are toxic to living organisms in high concentrations and have a high persistence in soil and organic matter (Antonkiewicz & Gworek, 2023; Lasat, 1999; Lv, 2019). Among the trace elements, iron (Fe), chromium (Cr), nickel (Ni), zinc (Zn), copper (Cu), cobalt (Co), lead (Pb), and cadmium (Cd) are the most common in soil (Lasat, 1999; Tóth et al., 2016). Increased human activity has caused these elements to accumulate in the environment, acting toxic to the environment (Chrzan et al., 2009). The high stability of trace elements hinders their flow between soil horizons, causing heavy metals to accumulate in topsoil layers (Antonkiewicz & Gworek, 2023; Mazurek et al., 2017; Pająk et al., 2016).

Heavy metals are highly persistent, meaning they do not biodegrade or leach into the environment (Mazurek et al., 2017). These elements can be taken up from the soil by plants if converted to the appropriate forms (Lenart-Boroń & Boroń, 2014). The mobility and bioavailability of heavy metals in soil depends on the forms they take and the concentrations in which they accumulate (Kabata-Pendias & Pendias, 1993; Mocek-Płóciniak, 2011). Metals can be distinguished as labile or stable. Labile metals are free ions and inorganic complex compounds, which are distinguished by their bioavailability and easily leached from the soil. Stable metals are macromolecular, organic chelate compounds, which are difficult for plants and microorganisms to assimilate (Dziadek & Wacławek, 2005). Stable metals that are less soluble and not assimilated by plants e.g. decrease groundwater quality (Marrugo-Negrete et al., 2017).

The bioavailability of heavy metals also depends on the sorption properties of the soil, which are determined by physical and chemical properties, including soil organic matter (SOM) content (Impellitteri et al., 2002; Wang, 2008), clay fraction, pH (Gäbler, 1997; Gruca-Królikowska & Wacławek, 2006), and soil texture (Dube et al., 2001; Pająk et al., 2016). SOM is a key factor in the behaviour of heavy metals in soils (Pająk et al., 2016; Schnitzer & Kerndorff, 1981), as soils with high SOM content have high trace element binding and retention (Kabata-Pendias & Pendias, 1993; Stefanowicz, 2015). Soils with a high SOM content and clay mineral fraction strongly bind heavy metals and immobilise them (Bååth, 1989). Clay soils, unlike sandy soils, have a high clay mineral content and a weak pH dependence on cation exchange capacity, resulting prone to heavy metal accumulation (Dube et al., 2001).

The chemical composition of organic matter in forest soils is determined primarily by the decomposition of tree litter (Błońska et al., 2016). Therefore, the species composition of the stand and undergrowth can indirectly affect the mobility of trace elements. A study by Błońska et al. (2016) showed that deciduous species have a beneficial effect on SOM. Coniferous monocultures accumulate detritus that is difficult to decompose, producing large amounts of acidic humus that decrease soil pH (Osei et al., 2022; Shen et al., 2023). Soil pH affects cation mobility, heavy metal solubility (Minase et al., 2016), and element uptake by plant roots (Lasat, 1999). Acidic soils promote the solubility of heavy metals, bringing them into biocirculation (Lasat, 1999; Minase et al., 2016). Nutrient-poor litter and acidic soils lead to less microbial biodiversity, causing organic matter and hence, heavy metals, to accumulate (Osei et al., 2023). Compared to coniferous monocultures, mixed stands facilitating nutrient uptake from different soil horizons, improving soil quality (Lutter et al., 2021). Mixed stands increase vegetation diversity (Gosselin et al., 2017), which contributing to the uptake of bioavailable forms of heavy metals.

Undergrowth plays a key role in nutrient cycling and maintaining species diversity (De Groote et al., 2018; Gilliam, 2007; Gracia et al., 2007). The presence of shrubs can increase aboveground biomass, affect soil properties like pH, concentration cations, sorption properties (Small & McCarthy, 2002;; Oh et al., 2012), and affect the distribution and rooting depth of other plants (Hu et al., 2021; Xiang et al., 2018). The amount and composition of aboveground plant parts varies between tree and shrub species (Hobbie et al., 2006; Nordén, 1994), shaping the type of litter and its rate of decomposition (De Groote et al., 2018; Hättenschwiler et al., 2005). Shrubs have a faster decomposition rate than coniferous species (Nilsson & Wardle, 2005), with their leaf litter more easily decomposed by soil microorganisms than coniferous litter (Chen et al., 2021; Niu et al., 2020). Shrubs in coniferous monocultures increase species diversity and thus, alter soil nutrient supplies and the chemical composition of forest litter, affecting soil nutrient availability and other properties like pH (Légaré et al., 2002; Myers-Smith et al., 2011; Ste-Marie & Paré, 1999).

Research on the value of shrub admixtures in pine monocultures on heavy metal content in soil surface horizons is lacking. Our research attempts to fill this gap by seeking to explain how shrub admixtures in the lower canopy of single-species pine stands affects soil properties and heavy metal content. Taking into account the need to examine the level to address soil heavy metal contamination in order to look for future methods of managing forest ecosystems, taking into account admixtures of different species of shrubs, the following objectives of this study were formulated: 1) to evaluate the effect of alder buckthorn (Frangula alnus) and European hazelnut (Coryllus avellana) admixtures in pine monocultures on the accumulation of heavy metals; 2) to evaluate vertical distribution of heavy metals in tested soils.

2 Materials and Methods

2.1 Study Sites and Soil Sampling

This study was conducted in the Rybnik Forest District in southern Poland (50° 05′ 55″ N; 18° 32′ 42″ E), which has the region's average annual temperature of 8.4°C and an average annual precipitation of 705 mm. The soils in this area were formed from glacial sand and are primarily Brunic Arenosols. The soils covered by the study were made from the same parent material and characterized by the sandy loam grain size. Three stand compositions were selected for study: pine stands (Pinus sylvestris) with a shrub admixture of alder buckthorn (Frangula alnus), pine stands (Pinus sylvestris) with a shrub admixture of European hazelnut (Coryllus avellana), and pine monocultures (Pinus sylvestris) without a shrub admixture as a control. All stands were 80 years old and of a uniform tree canopy density which guarantee equal light conditions. Each of the three stand compositions comprised 10 study plots, resulting in 30 plots in total. Soil samples were gathered from the organic (O) horizon (0 to 7 cm depth) and the humus mineral (A) horizon (7 to 15 cm depth). Composite soil samples consisting of three subsamples from different points, were collected from each horizon.

2.2 Laboratory Analysis

Soil samples were dried and sieved through a 2 mm diameter sieve. Soil pHH2O was determined using the potentiometric method. In pH analysis the ration of 1:2.5 soil:water was used. Organic carbon (C) and total nitrogen (N) contents were determined using an elemental analyser (LECO CNS TrueMac Analyzer; Leco, St. Joseph, MI, USA). A pure oxygen environment in a high-temperature horizontal ceramic combustion furnace was used to determine C and N. The soil was free from carbonates and total C equalled organic C. Base cation concentrations (Ca2+, Mg2+, K+, and Na+) were extracted with ammonium acetate. 10 g of soil with 30 ml extractand was shaken for 1 h. Inductively coupled plasma-optical emission spectrometry (ICP-OES Thermo iCAP 6500 DUO, Thermo Fisher Scientific, Cambridge, UK) was used for determination of base cations concentration. Elemental concentrations (Cd, Co, Mn, Ni, Cr, Cu, Pb, and Zn) were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES Thermo iCAP 6500 DUO, Thermo Fisher Scientific, Cambridge, UK) after prior mineralisation with a concentrated nitric and perchloric acid mixture at the ratio 2:1.

2.3 Statistical Analysis

Spearman's correlation coefficient was used to determine the relationship between soil properties for the organic and mineral horizons separately. The Shapiro-Wilk test was used to verify the normal distribution of the data. Variations in soil properties by stand composition were assessed using the Kruskal-Wallis test, while the Mann-Whitney U test was used to determine differences in properties by soil horizon. The dependence among soil properties in relation to stand composition and soil horizon was examined using principal component analysis (PCA). A generalized linear model (GLM) analysis was used to determine the significance (alpha of 0.05) of the influence stand composition and soil horizon on heavy metal content. All statistical analysis was conducted using the R programming language (R Core Team, 2020) in R Studio (Posit team, 2020).

3 Results

The organic and mineral horizons differed significantly in basic soil chemical characteristics and heavy metal content (Figs. 1 and 2). N content, C content, and C/N ratios were significantly greater in organic horizons than mineral horizons. N content was greater in the organic horizon of the pine monoculture (1.25 %) than the alder buckthorn stand (0.96 %) or European hazelnut stand (1.03%), although this difference was not statistically significant. C content was significantly greater in the organic horizon of the pine monoculture (28.48 %) than the alder buckthorn stand (20.76 %) and the European hazelnut stand (21.43 %). The C/N ratio in the organic horizon was significantly greater for the pine monoculture (22.85) than the alder buckthorn stand (21.76) and the European hazelnut stand (20.75). N content in the mineral horizon was significantly lower for the pine monoculture (0.16 %) than the alder buckthorn stand (0.30 %) and the European hazelnut stand (0.44 %). The C content of the mineral horizon was the lowest for the pine monoculture at 4.25 %, but only differed significantly from the European hazelnut stand at 10.29 %. The C/N ratio did not differ significantly by stand composition for the mineral horizon for the mineral horizon and for the pine monoculture was 26,40 %, alder buckthorn stand at 23,89 %, European hazelnut stand at 23,34 %. For the organic horizon pHH2O was significantly greater for the European hazelnut stand (4.23) than the other the other stand compositions. Only the alder buckthorn stand did not differ significantly in pHH2O between the organic and mineral horizons (Fig. 1). In the mineral horizon, pHH2O did not differ significantly by stand composition, with values of 3.67 for the pine monoculture, 3.80 for the alder buckthorn stand, and 3.79 for the European hazelnut stand (Fig. 1).

Fig. 1
figure 1

Basic properties of the tested soils (C and N content in %, C/N, pH; AB – alder buckthorn, EH – European hazelnut, SP – Scots pine; small letters a,b mean significant differences between different species, large letters A,B mean significant differences between different soil horizons)

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Fig. 2
figure 2

Base cations content in the tested soils (Ca, K, Mg and Na content in mg.kg-1; AB – alder buckthorn, EH – European hazelnut, SP – Scots pine; small letters a,b mean significant differences between different species, large letters A,B mean significant differences between different soil horizons)

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Base cation contents were significantly greater in the organic horizon than the mineral horizon for each stand composition (Fig. 2). The pine monoculture had the lowest K content for both soil horizons compared to the other stand compositions, but this difference was only significant relative to the European hazelnut stand (Fig. 2). In the organic horizon, the average K content was 267.6 mg.kg-1 for the pine monoculture and 420.2 mg.kg-1 for the European hazelnut stand. In the mineral horizon, the average K content was 30.5 mg.kg-1 for the pine monoculture and 110.6 mg.kg-1 for the European hazelnut stand. Stands with shrub admixtures had greater Mg content in both soil horizons than the pine monoculture. In the organic horizon, the average Mg content of the pine monoculture (94.1 mg.kg-1) was significantly lower than either the alder buckthorn stand (136.9 mg.kg-1) or the European hazelnut stand (145.4 mg.kg-1). In the mineral horizon, the difference in Mg content was only significant between the European hazelnut stand (25.5 mg.kg-1) and the pine monoculture (10.8 mg.kg-1). Ca and Na contents were greater in the organic horizon than the mineral horizon for each stand composition. In both the organic and mineral horizons, Ca did not differ significantly by stand composition. Na content differed significantly between the alder buckthorn stand and the pine monoculture in the organic horizon, as well as between the alder buckthorn stand and the European hazelnut stand in the mineral horizon (Fig. 2).

GLM analysis confirmed the significance of stand composition and soil horizon in shaping soil heavy metal accumulation (Table 1). Stand composition influenced the accumulation of all metals except Cr, Cu and Pb. The genetic horizon significantly influenced the content of all metals. The interaction between species and genetic horizons significantly influenced the content of all heavy metals. All average heavy metal contents (Cd, Co, Mn, Ni, Cr, Cu and Zn) were greater in organic horizons than mineral horizons for all stand compositions (Fig. 3). The organic horizon of the European hazelnut stand had significantly lower contents of Ni, Cr, Pb and Zn than the alder buckthorn stand or the pine monoculture. The organic horizon Co content differed significantly only between alder buckthorn and the European hazelnut stand compositions, and the Cu content did not differ significantly by stand composition, with average Cu concentrations of 30.21 mg·kg-1 for the alder buckthorn stand, 24.65 mg·kg-1 for the European hazelnut stand, and 33.79 mg·kg-1 for the pine monoculture. The alder buckthorn stand had significantly greater Cd content (1.55 mg·kg-1) than the European hazelnut stand (0.90 mg·kg-1) and the pine monoculture (0.89 mg·kg-1) in the organic horizon. In the mineral horizon, the pine monoculture contained the lowest average values of all analysed heavy metals (Fig. 3).

Table 1 GLM analysis for heavy metal content
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Fig. 3
figure 3

Content of heavy metals (mg.kg-1) in the tested soil samples (AB – alder buckthorn, EH – European hazelnut, SP – Scots pine; small letters a,b mean significant differences between different species, large letters A,B mean significant differences between different soil horizons)

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Spearman correlation analysis revealed strong correlations between soil properties and heavy metal content. The heavy metal content was negatively correlated negatively with pH and the content of base cations in the organic horizon, while positively correlated with C content (Fig. 4). In the mineral horizon, the heavy metal content was correlated with pH, N content, C content, C/N ratio, and the contents of Mg, Na, and K (Fig. 5). PCA's first and second distinguished factors accounted for 69.9 % of the variability of the examined features for the organic horizon, and 88.1% for the mineral horizon (Figs. 6 and 7). This analysis confirmed the existence of a relationship between the studied soil properties at both the organic and mineral horizons. Additionally, PCA analysis confirmed that soil properties varied by stand composition. The European hazelnut variant was characterized by distinctive properties at the organic horizons (Fig. 6). This species showed similar distinctiveness in relation to the properties of the mineral horizons (Fig. 7).

Fig. 4
figure 4

Spearman correlations between the studied properties in the organic horizon O

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Fig. 5
figure 5

Spearman correlations between the studied properties in the humus mineral horizon A

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Fig. 6
figure 6

The projection of variables on a plane of the first and second PCA factor in organic horizon O

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Fig. 7
figure 7

The projection of variables on a plane of the first and second PCA factor in humus mineral horizon A

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4 Discussion

Our results confirm that the introduction of shrubs in pine monocultures affects soil properties and, consequently, heavy metal accumulation. The addition of shrubs, especially European hazelnut, increased soil pH and the quality of SOM compared to pine monocultures alone. Soil horizons from the European hazelnut stands had lower C/N ratios than alder buckthorn stands or pine monocultures. The C/N ratio is a useful tool for assessing SOM quality (Amorim et al., 2022). Additionally, serves as an environmental indicator to link C and N cycles and provides information on the composition, stability, and breakdown of SOM (Averill & Waring, 2017). Understanding the determinants of soil organic carbon (SOC), total nitrogen, and C/N ratios is critical for predicting how changes in land use affect SOC storage across terrestrial ecosystems (Cotrufo et al., 2019). The lower C/N value confirms more efficient decomposition of detritus delivered to the forest floor. Previously conducted research confirmed the acidifying effect of pine monocultures and the slowing down of the decomposition processes of the supplied organic matter (Błońska et al., 2018; Błońska et al., 2021; Lasota et al., 2021).

Soil properties, including organic matter content, significantly influence the bioavailability of heavy metals (Xu et al., 2022). Our study revealed a close relationship between SOM content, pH, and heavy metal content. Pine stands with shrub admixtures (alder buckthorn and European hazelnut) had lower C and N contents than pine monocultures, indicating that organic matter decomposes faster in pine stands with shrub admixtures than pine monocultures. Reduced organic matter accumulation increases the mobility of heavy metals, allowing them to move deeper into the soil profile. This relationship is also influenced by soil pH. Pine monocultures accumulate heavy metals in their needles (Pająk et al., 2015), which are transformed into organic matter after fallout. Humus, which is difficult to decompose and acidifies the soil, increases ion sorption (Maciaszek, 1986). The high content of raw organic matter in the O horizon increases the binding capacity of heavy metals, while at the same time hindering the movement of heavy metals to lower soil horizons (Mazurek et al., 2017), as seen in the pine monoculture we studied. The changes in soil properties likely changed the forms of the heavy metals studied and increased their mobility. Alder buckthorn, although a deciduous shrub and expected to have an analogous effect on heavy metals, did not result in the same soil changes as European hazelnut. Alder buckthorn affected soil properties and changed the mobility of heavy metals but a lesser extent than European hazelnut.

According to Kabata-Pendias (2011), the permissible levels of the studied heavy metals are 1 mg·kg-1 for Cd, 30 mg·kg-1 for Cu, 50 mg·kg-1 for Cr, 50 mg·kg-1 for Pb, and 100 mg·kg-1 for Zn. In our study, the soil exceeded the permissible levels for heavy metals, especially for organic horizon in the pine monoculture and alder buckthorn stand. In our study, exceeded the permissible levels for Cd and Ni content concern the organic horizon in the alder buckthorn stand, as well as Cu and Zn for the alder bucthron stand and pine monoculture. The highest exceedances in the tested soils concerned lead. The Pb content was more than double the permissible level for the organic horizons of the all experimental variants in all horizons, except for horizon A in the soils of pine stands. Exceeded metal contents are related to the chemical properties of the tested soils. The heavy metal excesses in the organic horizons of the alder buckthorn stand and pine monoculture resulted from the degree of decomposition of organic matter and the acidification of the organic horizon. All the heavy metal contents studied fell within the permissible levels for both soil horizons in the European hazelnut stand, which is directly related to pH and SOM quality. Previous research suggests that the distribution of Pb, Cd, and Zn in forest soils is determined by the decomposition of organic matter (Egli et al., 1999). The type of humus in forest soils influences heavy metal accumulation, with variability in SOM and pH playing a large role in heavy metal mobility and bioavailability (Lasota et al., 2020).

In the European hazelnut stand, heavy metal content was similar in the organic and mineral horizons, which is likely the result of bioavailability caused by soil chemical properties. Heavy metal bioavailability is influenced by physical, chemical, and biological factors, given how biological factors can modify metal availability by releasing oxygen, protons, and organic acids, and by associations with mycorrhizal fungi (Ernst, 1996). The contents of Cd, Mn, and Zn differed significantly between the organic and mineral horizons of of the European hazelnut stand. These differences between horizons were more pronounced for the alder buckthorn stand and pine monoculture than the European hazelnut stand, which is a consequence of their chemical properties that may have increased heavy metal mobility in the soil profile. Soil pH significantly affects heavy metal mobility, in turn altering the activity and environmental risks associated of heavy metals. Soil pH are critical determinants of heavy metal mobility in soil environments. Generally, increased soil pH decreases heavy metal mobility (De Matos et al., 2001; Kim et al., 2015). Specifically, Kim et al. (2016) observed that the use of biochar (with a pH of 10.3) as a soil amendment led to a rise in the immobilization of Cd and Pb, a process enhanced by the higher pH.

5 Conclusion

The presence of heavy metals was associated with soil properties, including pH, N and C contents, the C/N ratio, and base cation concentrations. Shrub admixtures ,particularly the European hazelnut, in pine-dominated forests influenced heavy metal accumulation, which we attribute to the impact of these shrubs on soil organic matter quality. Soils from European hazelnut shrubs had significantly reduced heavy metal contents compared to pine monocultures and alder buckthorn stands. Our findings suggest that integrating shrubs into pine forests can beneficially alter the organic matter properties of the soil and reduce heavy metal accumulation. The inclusion of European hazelnut positively impacted the tested soil properties, underscoring its utility in forest management and soil quality improvement, particularly in regions prone to heavy pollutant deposition.