Copper and cadmium content in Polish soil: Analysis of 25‐year monitoring study

Metals can accumulate in agricultural soils, presenting a serious threat to human health; therefore, it is important to analyze the quality of these soils to avoid possible harm related to their contamination in the future. Despite the importance of monitoring arable soil quality, few studies have examined the current state of Cu and Cd soil contamination through analysis of historical data and temporal trends in heavy metal content. Therefore, this study was aimed at analyzing the spatial variability of Cu and Cd content (expressed by toxicity indices) and assessing the level of contamination of arable soils in Poland over the last 25 years (1995–2020). The average Cu content in soil throughout the duration of the study was ~10 mg/kg. The average Cd content increased from 0.7 mg/kg (in 1995) to 3.4 mg/kg (in 2020). The evaluation of Cu and Cd soil contamination showed that soils contaminated with Cd and Cu constituted 1.4% and 2.3% of the total monitoring points, respectively. The geoaccumulation index and pollution index ranged for Cu from −5.23 to 3.09 (mean: −2.50) and from 0.02 to 6.40 (mean: 0.20), and for Cd from −5.23 to 6.92 (mean: −1.67) and from 0.01 to 60.58 (mean: 0.44). The soil was practically uncontaminated with Cu and Cd in 98.77%/98.92% and 93.44%/97.92% of cases, respectively. On a national scale, the contents of Cu and Cd in soils depend on soil properties (pH, C, OM, ST, and CM) to a very small extent. An assessment of the spatial distribution of Cu and Cd concentrations in Polish arable soils indicated regional differences related to the degree of industrialization/urbanization. The obtained results show the impact of human activity on the level of heavy metals present in soils.


| INTRODUCTION
Soil pollution is a global phenomenon mainly related to the intensification of anthropogenic phenomena, such as rapid urbanization and industrial development (Khosravi et al., 2018).According to the latest data presented in the European Environment Agency's State of the Environment 2020 report, the condition of soils in Europe has deteriorated considerably in recent years, and forecasts for the coming years do not foresee improvements.Owing to the persistent threats to soils, it is necessary to take appropriate action in all EU countries, including the introduction of an effective policy for soil protection (Heuser, 2022).The accumulation of heavy metals in soil has become a serious problem, particularly in developing countries (Adimalla & Wang, 2018).Heavy metal pollution poses a serious threat to human health because of the high toxicity, environmental mobility, accumulation in the food chain, and lack of biodegradability of heavy metals (Fei et al., 2022;Szyma nska-Pulikowska et al., 2023;Wdowczyk & Szyma nska-Pulikowska, 2022).In addition, heavy metals persist long term in soil environment and can adversely affect soil fauna and flora through soil-microbial interactions and microbial processes (Kumar et al., 2019;Zhao et al., 2022).Heavy metals are a persistent stress that threatens the local ecosystem; studies have shown their impact on changes in microbial metabolic activity, population diversity, and soil microbiome abundance (Qian et al., 2023;Yang et al., 2022).
In recent years, studies on the contamination of agricultural soils with heavy metals have been conducted worldwide (Bigalke et al., 2017;De Temmerman et al., 2003;Wang et al., 2021).This problem is very important because they can have a lethal effect on soil, plant productivity, and thus ensuring food safety (Mansoor et al., 2021).
Cu and Cd are heavy metals whose presence in soil is mainly caused by anthropogenic activities closely related to agriculture, such as the application of sewage sludge, fertilizers, or compost (Wang & Xing, 2002;Wojtkowska et al., 2015;Yuan et al., 2007;Wiatkowski et al., 2010).These metals are included on the list of priority pollutants provided by the United States Environmental Protection Agency (Mulligan et al., 2001).Excess Cu in soil is highly phytotoxic, while Cd has no established physiological or biochemical role in plants and is highly phytotoxic.As research has shown that the concurrent occurrence of cadmium (Cd) and copper (Cu) stresses has been shown to be more detrimental to crop production (Kaushik et al., 2022).
In the environment, Cu is most often found in the form of Cu 2+ cations, although it can also form complex anions and cations depending on the environmental conditions (Wojtkowska et al., 2015).The origin of Cu in soil may be either the underlying bedrock or anthropogenic sources.Its presence may be associated with agricultural activities such as applying pesticides or fertilizers (Rusjan et al., 2007).For example, Cu is commonly used as a fungicide in vineyards and orchards.High Cu contents have also been recorded in soils contaminated by the activities of the mining industry and Cu metallurgy, reaching concentrations as high as hundreds of milligrams per kilogram of soil (Wojtkowska et al., 2015).Cu is an essential element for plants that accumulates mainly in their roots and shows low mobility from the roots to aboveground organs (Wdowczyk & Szyma nska-Pulikowska, 2023).
Similar to Cu, the presence of Cd in soils may be attributed to anthropogenic sources such as mining, phosphate fertilizers, combustion of fossil fuels, industrial wastewater (e.g., from electroplating processes), alloy preparation, production of plastics and paint pigments, vehicle emissions, and pesticides (Kirkham, 2006;Wang & Xing, 2002;Yuan et al., 2007).Cd is one of the most common and harmful heavy metals in soil owing to its bioavailability and high toxicity (Ciesielczuk et al., 2014;Medy nska-Juraszek et al., 2009).Excessive accumulation of Cd in soil can lead to the inhibition of plant growth and disruption of soil functions, and it poses a threat to human health (Qiu et al., 2020).
Metals such as Cu and Cd can accumulate in agricultural soils, presenting a serious threat to human health; therefore, it is important to analyze the quality of these soils to avoid possible harm related to their contamination in the future.In recent years, many studies have been conducted on the heavy metal content of soils worldwide, including assessments of the risk of contamination of meadow soils with heavy metals in Poland (Ciarkowska, 2018), the condition of urban soils in the metropolitan area of Mexico (Morton-Bermea et al., 2009), soil contamination with heavy metals from mining areas in China (Li et al., 2014), and the accumulation of Cd and U in arable soils in Switzerland (Bigalke et al., 2017).
Despite the importance of monitoring arable soil quality, few studies have examined the current state of Cu and Cd soil contamination through analysis of historical data and temporal trends in heavy metal content.Data on the scale of this problem in agricultural soils in European countries still seem insufficient.Therefore, the authors focused on a case study of a Central European country (Poland).
These studies are complementary to previous studies because, unlike previous ones, they cover a longer period of time and a larger number of samples, thanks to which they allow for a better assessment over The obtained results may be the basis for developing universal recommendations for the rational management of arable soils contaminated with copper and cadmium in areas with various types of anthropogenic pressure.In addition, they can be the basis for rational planning by local government authorities in terms of improving soil pollution and protection.The limitation of these studies is that they concern only arable soils of one country (Poland) located in a temperate climate.Although the research concerns only Poland, the results may also be useful for other countries with similar soil and climatic conditions.
The chemical properties of arable soils in Poland have been monitored since 1995 as part of the State Environmental Monitoring program (Chief Inspectorate for Environmental Protection, 2023).Soil samples were collected every 5 years at 216 measurement and control points (in an area of $140,000 km 2 ) located on arable land in Poland (the location of the points on the map of the administrative division of Poland is shown in Figure 1a).
The distribution of the sampling points considered the diversity of soil properties, exposure of soils to pollution sources, and different geographical and administrative units.Sampling points were distributed such that $50% of the samples represented typical rural areas, 23% were from rural areas located 5-20 km from industrial/ urban areas, 20% of the samples represented areas directly affected by industrial emissions (at a distance of <5 km), and 7% were directly exposed to transport emissions (Maliszewska-Kordybach et al., 2008).samples collected from 1995 to 2020.Cu and Cd were selected for the analysis due to the fact that they pose a serious threat to human health due to the possibility of accumulation in agricultural soils.

| Soil sampling
At each monitoring point, soil samples were collected from the top layer (0-20 cm) of the arable fields using a steel soil probe (Sun et al., 2022).Each point sample ($1 kg) consisted of $10 individual samples collected from an area of 100 m 2 (10 m Â 10 m).After mixing individual samples, a collective sample representative of the measurement and control points was created.After arriving at the laboratory, the soil material was air-dried for 48 h at a temperature of $20 C.
The soil was then mixed, passed through a 2-mm sieve, and stored in the dark until undergoing further analysis (Maliszewska-Kordybach et al., 2008;Siebielec, 2017).

| Laboratory analysis
The Cu and Cd contents of soil samples taken from 1995 to 2005 were determined using atomic absorption spectrometry, whereas the contents of those taken since 2010 were determined using inductively coupled plasma mass spectrometry (ICP-MS) in a solution after soil mineralization with aqua regia.In 1995In , 2000In , and 2005, the AANALYST 800 atomic absorption spectrometer (PerkinElmer) was used, and in 2010, 2015, and 2020-the ICP-MS spectrometer Agilent 7850 (Agilent Technologies).The quantification threshold for copper is 2 mg/kg and for cadmium is 0.0069 mg/kg until 2015 and 0.5 mg/kg in 2020.The article adds information on this equipment as well as quantification thresholds for copper and cadmium.
Soil mineralization with aqua regia was performed in accordance with the international standard ISO-11466: Soil quality-extraction of trace metals soluble in aqua regia (Siebielec, 2017;Tomczyk et al., 2021;Tomczyk, Gałka, et al., 2022;Tomczyk, Wiatkowski, et al., 2022).The organic matter (OM) content was calculated using the following equation (Maliszewska-Kordybach et al., 2008): where C is the organic C content in each soil sample, and 1.724 is a constant.

| Evaluation of soil contamination with selected heavy metals
In the EU, no uniform threshold values for heavy metal content in soils have yet been established, with the exception of the limit on the use of sewage sludge in agricultural soils (EU Directive 86/278/EC).The problem with adopting uniform values results from the fact that the toxicity and bioavailability of heavy metals in soils depend not only on their total content in soil, but also on factors, such as OM content, climate, and land use (Lado et al., 2008).ecological or health hazard (Ballabio et al., 2018;T oth, Hermann, et al., 2016;T oth, Hermann, et al., 2016).
A detailed description of the results and discussion of the evaluation of Cu and Cd content in arable soils are described in the Supporting Information (Appendix A).

| Data treatment and statistical analysis
The

| Geoaccumulation index
Calculation of the I geo allowed for the assessment of soil quality based on a comparison of the current state of pollutant concentrations in the tested soils with the results obtained in previous years.The I geo of the soil samples was calculated using the following equation (Li et al., 2014): where C n is the measured concentration of each heavy metal in soil (mg/kg), B n is the value of the geochemical background of heavy metals present in the surface layer of soil according to IUNG guidelines (Cu, 25 mg/kg; Cd, 0.5 mg/kg), and 1.5 is a constant that accounts for potential differences in the output data (Loska et al., 2004).
The I geo PI is divided into seven classes, which are assigned

| Pollution index
Soil quality was assessed using the single PI, which allows the determination of which heavy metals pose the greatest threat to the soil environment and allows for comparison among types.PI was calculated using the following equation (Hu et al., 2014): where

| Hierarchical cluster analysis
HCA allows the division of points into clusters.The applied HCA enabled the detection of structures and the division of objectsmonitoring points representing the content of heavy metals into clusters, so that the degree of connection between objects belonging to the same cluster was as high as possible, and with objects from other clusters as small as possible.The results of HCA are presented in the form of a dendrogram, where the distance axis shows the degree of connection between the groups of objects; that is, the lower the value on the axis, the more significant the connection (Figure 2).
The data on the content of heavy metals (Cd and Cu) in soils were subjected to HCA to detect spatial similarities and differences in sampling sites (determination of spatial variability) located throughout Previous studies have also reported that HCA can be used for reliable classification of monitoring points, thereby reducing the number of samples in the monitoring network without losing the significance of the obtained results (Govender & Sivakumar, 2020;Kazi et al., 2009;Varol et al., 2012;Wolski & Tymi nski, 2020).

| Spearman's correlation matrix
The Spearman's correlation matrix allows the determination of interactions between factors along with their strengths and directions.The coefficient R was used to measure the strength of the relationship between the two variables.Two variables can be considered highly correlated when j R j ≥ 0.7, moderately correlated when 0.4 ≤ j R j < 0.7, and weakly correlated when j R j < 0.4 (Shan et al., 2013).
The results of this analysis are shown in Figure 3.
Correlation analysis is a useful statistical tool for assessing the relationship between selected soil properties and heavy metal content.The following factors were considered in the correlation analysis: pH, organic carbon (C) content, OM content (calculated using Equation 1), soil type (ST), and clay mineral (CM) content.The selection of parameters included in the matrix was based on indicators that are known to influence the content and behavior of heavy metals in soil (Ballabio et al., 2018;Li et al., 2022;Zinn et al., 2020).
All the analyzed parameters showed positive correlations (r from 0.17 to 0.91), and their strengths were not diverse.A strong correlation was observed only between C and OM contents (R = 0.91).However, weak and medium correlations were observed in the analyses of other parameters.The pH was moderately correlated with OM (R = 0.53), which is consistent with the results of other studies (Shan et al., 2013;Zhang et al., 2023).
Furthermore, past studies have shown that pH and OM are the most important factors that control the availability and influence the content of heavy metals in soil (Amini et al., 2005;Barančíková et al., 2004;Kirkham, 2006;Li et al., 2018).However, the results of the current study showed that the concentrations of Cd and Cu in the analyzed samples were weakly correlated with pH (R = 0.30 and 0.29) and moderately correlated with OM (R = 0.57 and 0.51).The coefficient of determination R 2 , also calculated, proved that the variability of the Cu and Cd content could be dictated only in about 10% by the variability of pH and in about 25% by the volatility of OM, and depended respectively in 90% and 75% on other factors.Additionally, pH and CM content were not correlated (R = 0.22), which is consistent with the findings of other studies (Turner et al., 2003;Zheng et al., 2022).
A moderate correlation coefficient was also noted between the CM content and the Cd and Cu contents (R = 0.42 and 0.60, respectively), confirming that this factor contributed to the accumulation of heavy metals in soil (Abd El-Aziz, 2021).An average correlation was also observed between Cd and Cu (R = 0.47).Furthermore, the ST also determines the content of heavy metal fractions and their behavior in soil (Li et al., 2022;Zinn et al., 2020).However, in previous studies, the Cu and Cd contents were poorly correlated with ST.In addition, the interaction of all parameters can be modified by various factors, such as anthropogenic pressures or chemical reactions, which can affect the variability and ability to migrate in the environment (Tomczyk, Gałka, et al., 2022;Tomczyk, Wiatkowski, et al., 2022).
To a very small extent, the Cu and Cd contents in soils depend on the tested soil properties (pH, C, OM, ST, and CM).The main factors affecting the heavy metal content of soil in the study area (apart from natural variability and physical processes) were intensified mining, metallurgical, and industrial activities.
The presented research results have limitations related to the geographical location of the country and temperate climate conditions.Therefore, the transfer of our conclusions to other climatic zones (e.g., tropical, arctic, and Mediterranean climate) is limited.

| Evaluation of soil contamination using the I geo
Due to its high level of accuracy, I geo is one of the most frequently used indicators for assessing soil contamination (Yan et al., 2018).
Figure 4 shows the I geo values (calculated using Equation 3) for Cu (Figure 4a) and Cd (Figure 4b).To assess contamination using I geo , data were first divided into groups using HCA (Figure 4).
The Cu I geo for most of the selected groups (Figure 4a) remained low (I geo < 0) throughout the study period , indicating a lack of Cu soil contamination.Similar to the Cu I geo levels, the Cd I geo levels in most of the selected groups remained low (I geo < 0, uncontaminated) throughout the study period .In group 2AIIa, the I geo remained in class 1 (0 ≤ I geo < 1, uncontaminated to moderately contaminated) for the duration of the study period.In group 2AI, the I geo was in class 3 (2 ≤ I geo < 3, moderately to heavily contaminated) from the beginning of the study, but since 2010, it showed an increased value in class 4 (3 ≤ I geo < 4, heavily contaminated).In group 1, the I geo values were in class 6 (I geo ≥ 5, extremely contaminated) from the beginning of the study period, and the points in this group remained the most polluted with Cd.
The level of heavy metal soil contamination is influenced by the intensity and manner of land use (Zemełka et al., 2019).The I geo value has been commonly used to assess the degree of heavy metal contamination of agricultural soils.For example, I geo was used in studies assessing soil contamination in an agricultural valley in Mexico, where the average values for the analyzed elements (Cu I geo , 0.2; Cd I geo , 0.

| Evaluation of soil contamination using the PI
The univariate contamination index, or PI, is commonly used to assess the level of soil contamination by a specific pollutant (Ren et al., 2018).Figure 5 shows the PI values (calculated using Equation 2) of Cu (Figure 5a) and Cd (Figure 5b).Before assessing contamination using the PI, data were divided into groups based on HCA (Figure 2).
The Cu PI for the majority of selected groups (Figure 5a) remained at the level of PI ≤ 1.0 throughout the research period , indicating a low level of soil contamination.In the case of group 2AI, moderate soil contamination was noted (1.0 < PI ≤ 3.0).
Only group 1 showed high Cu soil contamination (PI > 3.0) from the beginning of the study period, and its level increased further from 2010 forward.
Substantially higher PI values were achieved for Cd than for Cu.In the case of groups 2A1 and 1, the Cd PI remained at the level of PI > 3.0, indicating a high level of soil contamination.
Cd had the highest average PI, indicating that Cd was the main pollutant and posed the higher than Cu ecological risk.Also, Cu, a much smaller percentage of samples showed a higher degree of contamination (Wieczorek & Baran, 2022).
Many researchers have used the toxicity index to assess soils with heavy metals in Poland, for example, during research in industrialized areas (Hołtra & Zamorska-Wojdyła, 2020), urbanized areas (Hołtra & Zamorska-Wojdyła, 2018, 2023;Zgłobicki et al., 2021), along roads (Radziemska & Fronczyk, 2015), or covering soils in valuable in nature, including in National Parks (Łyszczarz et al., 2020;Mazurek et al., 2017).It is recommended to introduce corrective actions at all monitoring points with I geo values greater than 0 (contaminated), that is, for Cd from 10 to 17 points (respectively: 1995 and 2010), and for Cufrom 2 to 4 points (1995, 2000, 2005, and 2020) out of 216 monitored.In these points, it is proposed to increase the frequency of Cu and Cd concentration tests, as well as in-depth case studies on the reasons for exceeding the limit values.As a result, it will allow for more effective soil protection and determination of necessary remediation actions (e.g., immobilization of pollutants in the solid phase of soil or removal of pollutants from the soil).

ACKNOWLEDGMENTS
The APC is co-financed by the Wrocław University of Environmental and Life Sciences, the University of Opole, and the Institute of Environmental Engineering Polish Academy of Sciences in Zabrze.
time and to obtain more representative results.So far, no studies have been carried out taking into account the use of toxicity indices for Polish arable soils on such a large spatial-time scale, which gives grounds for formulating recommendations for rational soil use.This study aimed to: 1. analyze the spatial variability in the Cu and Cd content of arable soils in Poland, 2. identify the current status of Cu and Cd contamination of arable soils in Poland, 3. analyze temporal trends in the heavy metal content of agricultural soils in Poland over the last 25 years (1995-2020) on a national scale, 4. assess the level of contamination of agricultural soils in Poland, and 5. analyze the variability of toxicity indices in terms of space and time.

Figure
Figure 1b shows the location of measurement points taking into account different types of land use in Poland.Several parameters were analyzed in the collected soil samples, including the physicochemical properties, content of macro-and microelements, structure, radioactivity, and content of polycyclic aromatic hydrocarbons and heavy metals.However, this study focused on analyzing the content of two heavy metals (Cu and Cd) in soil European countries use several methods to determine the level of risk associated with different heavy metal contents in soil.Many countries have established their own quality standards for some heavy metals, and in Poland, the criteria contained in the guidelines of the Institute of Soil Science and Plant Cultivation (IUNG) are used to assess soils(Kabata-Pendias et al., 1993).Test results for Cu and Cd soil contents were analyzed based on the heavy metal content assessment scale developed by IUNG.The scale distinguishes six classes with different degrees of soil contamination (0, I, II, III, IV, and V): 0, uncontaminated soils with natural heavy metal content (Cu: 0-25 mg/kg; Cd: 0-0.5 mg/kg); I, soils with increased heavy metal content (Cu: 25-50 mg/kg; Cd: 0.5-1.5 mg/kg); II, slightly polluted soils (Cu: 50-80 mg/kg; Cd: 1.5-3 mg/kg); III, moderately polluted soils (Cu: 80-100 mg/kg; Cd: 3-5 mg/kg); IV, highly polluted soils (Cu: 100-500 mg/kg; Cd: 5-10 mg/kg); and V, very heavily polluted soils (Cu: >500 mg/kg; Cd: >10 mg/kg).Since the values proposed by Finnish regulations are some of the most frequently cited guidelines and threshold values for heavy metals in the literature, the results of the analyses were additionally compared with the standards set out in Finnish legislation for contaminated soil (Finland Ministry of the Environment, 2007).These values are commonly used because they are a good approximation of the average values of various national systems in Europe and have also been used in an international context for agricultural soils(Ballabio et al., 2018; T oth, Hermann, et al., 2016; T oth, Hermann, et al., 2016).According to the guidelines of the Ministry of the Environment in Finland, the threshold values for Cu and Cd are 100 and 1 mg/kg, and the guide values are 150 and 10 mg/kg, respectively (Finland Ministry of the Environment, 2007).The threshold value indicates the need for further assessment of the site, whereas the guide value indicates an results of the Cu and Cd tests in arable land in Poland were analyzed using OriginPro 2022b software (OriginLab Corporation), SPSS Statistics 26 (IBM), and Statistica 13.1 (StatSoft Polska, StatSoft, Inc.).The land use map was developed on the basis of the LULC Map (Esri 2022)-the map is derived from ESA Sentinel-2 imagery at 10 m resolution.The results of the analyses of selected heavy metals (Cu and Cd) were characterized using Spearman's rank correlation coefficient and hierarchical cluster analysis (HCA).In this study, HCA was performed on a normalized data set using the Ward method, with squared Euclidean distances as a measure of similarity.The results are presented as a dendrogram (Figure2).The cutoff line was set such that the number of groups ranged from 5 to 10.The data were further analyzed, by calculating the geoaccumulation index (I geo ) and the pollution index (PI), as described below.
Dendrogram of the content of heavy metals in the analyzed soils: (a) Cu and (b) Cd.Constructed based on HCA.HCA, hierarchical cluster analysis.[Colour figure can be viewed at wileyonlinelibrary.com] PI is the pollution evaluation score corresponding to each sample, Ci is the measured concentration of the examined metals in the soils, and Si is the secondary standard value of Cu or Cd according to IUNG guidelines (Cu, 50 mg/kg; Cd, 1.5 mg/kg).The PI value of each metal was calculated and classified as: low contamination (PI ≤ 1.0), moderate contamination (1.0 < PI ≤ 3.0), or high contamination (PI > 3.0;Chen et al., 2005;Hu et al., 2014;Taghipour et al., 2013).
Poland.A dendrogram (Figure2) was used to group the 216 sampling points into two clusters (1 and 2).Clusters were distinguished based on differences in the average heavy metal (Cd and Cu) contents recorded at the analyzed points.Performing HCA of the Cu content in soil (Figure2a) resulted in eight distinct groups.Cluster 1 consisted of four groups: 1AI, 1AII, 1BI, and 1BII, covering a total of 32 monitoring points.Cluster 2 consisted of four groups: 2AI, 2AII, 2BI, and 2BII, covering a total of 184 monitoring points.In contrast, the HCA analysis of the Cd content in soil (Figure2b) resulted in seven distinct groups.Cluster 1 consisted of one group: 1AI, covering one monitoring point, and Cluster 2 consisted of six groups: 2AI, 2AIIa, 2AIIb1, 2AIIb2, 2BI, and 2BII, covering 215 points.We found that HCA was an appropriate method for classifying the degree of Cu and Cd contamination in soils throughout the country.Taking into account the data on the spatiotemporal variability of Cu and Cd concentrations at 216 soil sampling points created as part of the monitoring network, it was possible to select those where the limit values of the geochemical background were exceeded to the greatest extent.This resulted in faster assessment of soil quality and more accurate analysis of the degree of soil contamination using I geo and PI indices, focusing only on points with high Cu and Cd contamination.
Higher I geo values were only recorded in two groups throughout the duration of the study.In group 1AII, the I geo values were in class 2 (1 ≤ I geo < 2, moderately contaminated), and in group 1AI, they were in class 3 (2 ≤ I geo < 3, moderately to heavily contaminated).In 2020, in group 1B1, the I geo value was in class 2 (1 ≤ I geo < 2, moderately contaminated), whereas in previous years of analysis, Cu soil contamination was not observed in this group.
3) indicated uncontaminated or moderately contaminated soils (classes 0-1) (Alvarado-Zambrano & Green-Ruiz, 2019).The I geo has also been used to study the degree of soil contamination in areas with longstanding citrus fruit cultivation in Greece, where Cu-containing fungicides are widely used.The I geo value for Cu in soils of older crops F I G U R E 3 Spearman's correlation matrix for the content of Cu and Cd in the tested soils.C, organic carbon; CM, clay mineral; OM, organic matter; ST, soil type (* significant at p ≤ 0.05).[Colour figure can be viewed at wileyonlinelibrary.com] reached 2.0 (classes 2-3, moderately to heavily contaminated; Triantafyllidis et al., 2020).Higher values were recorded during research in a reclaimed area in China, where the I geo for Cd in aquaculture ponds reached a maximum value of 3.57 (class 4, heavily contaminated; Yan et al., 2018).These results demonstrate the effect of human activities on the level of heavy metals present in soils.

F
I G U R E 4 Geoaccumulation index (I geo ) for groups designated through hierarchical cluster analysis (HCA) for heavy metals: (a) Cu and (b) Cd. [Colour figure can be viewed at wileyonlinelibrary.com]The contamination of arable land in Poland with selected heavy metals (Cu and Cd) at most monitoring points was low and did not change markedly during the study period (25 years).An assessment of the spatial distribution of Cu and Cd concentrations in Polish arable soils indicated regional differences related to the degree of industrialization/urbanization.Arable soils with increased Cu content were found in the impact zones of copper mines and smelters.On the other hand, increased Cd content was recorded in areas where intensified mining, metallurgical, and industrial activities were carried out.These results demonstrate the effect of human activities on the level of heavy metals present in soils.1.The average Cu content in soil throughout the duration of the study was $10 mg/kg.The average Cd content increased from 0.7 mg/kg (in 1995) to 3.4 mg/kg (in 2020).2.The evaluation of Cu and Cd soil contamination, based on the Polish system of agricultural soil classification (IUNG), showed that soils contaminated with Cd and Cu constituted 1.4% and 2.3% of the total monitoring points, respectively.3. On a national scale, the contents of Cu and Cd in soils depend on soil properties (pH, C, OM, ST, and CM) to a very small extent.All the analyzed parameters showed positive correlations, and their strengths were not diverse.In most cases, weak or medium correlations were observed between the parameters.4. Two contamination indices, PI and I geo , were used to determine the Cu and Cd contamination in arable soils.The I geo and PI ranged for Cu from À5.23 to 3.09 (mean: À2.50) and from 0.02 to 6.40 (mean: 0.20), and for Cd from À5.23 to 6.92 (mean: À1.67) and from 0.01 to 60.58 (mean: 0.44).The soil was practically uncontaminated with Cu and Cd in 98.77%/98.92% and 93.44%/97.92% of cases, respectively.Average I geo Cu values over the years have ranged from À2.87% (2020) to À2.42% (2000, 2005, and 2015), with maximum difference of 18.22%;I geo Cd: from À2.12 (2015) to À1.32 (2020), with maximum difference = 61.15%,respectively.Additionally, the average PI Cu over the years has changed from 0.19 (2020) to 0.21 (2015), maximum difference of 11.12%; PI Cd: from 0.37 (2010) to 0.52 (2000); maximum difference = 39.31%,respectively.Both indices produced a similar assessment of soil quality in the study area, indicating no Cu or Cd contamination at most monitoring points.

F
I G U R E 5 Pollution index (PI) for groups designated through hierarchical cluster analysis (HCA) for heavy metals: (a) Cu and (b) Cd. [Colour figure can be viewed at wileyonlinelibrary.com] 5.The presented research results can be the basis for rational planning by local government authorities of investment activities in the area of environmental protection, in the field of improving soil pollution and protection.In addition, these studies may be useful in planning practice, where the knowledge of the spatial differentiation of soil contamination with heavy metals Cu and Cd against the functional and spatial structure is important information in the development of planning documentation.