**3. Pollution of soils, observed by the forest ecosystem monitoring network**

The International Co‐operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests operating under the UNECE Convention on Long‐range Transboundary Air Pollution (CLRTAP), level I, has been implemented in Bulgaria since 1986, and level II—since 1998. The "Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests" (1986–2010), adopted by the Programme, is implemented in order to study the acid status, eutrophication and heavy metal content in soils. A significant part of the obtained results has been published [45–49]. The results, obtained for soils of a total of 104 soil profiles, were summarized for a 20‐year period—from 1986 until 2008 [50].

The results for Cambisols and Luvisols from the regions of western Balkan Mountains, Sredna Gora, Rhodope Mountains and Strandzha, obtained for the period 2009–2015, are presented in this book. Data on 62 level I soil profiles from the national forest ecosystem monitoring network were summarized.

#### **3.1. Soil acidification**

The implementation of the forest ecosystem monitoring in Bulgaria began in 1986—a period when soil acidification in some parts of Europe had already been proven [51–56].

**Figure 16.** pHCaCl2 in Cambisols for the periods 1998–2008 and 2009–2015.

The lack of basic information about time series data, obtained from permanent sample plots in the past, did not allow to record the impacts of regional and/or global transfer of acid atmospheric depositions on soils, as well as the subsequent restoration processes due to the measures undertaken. On the basis of the information, obtained for a 20‐year period, it was proven that soil acidity is stable over time and did not change for the period from 1986 to 2008 [50].

The trends of stability in soil acidity continued for the period 2009–2015. The absence of statistically significant differences between the values of pHCaCl2 for the periods from 1998 to 2008 and from 2009 to 2015 for Cambisols and Luvisols is presented on **Figures 16** and **17**.

**Figure 17.** pHCaCl2 in Luvisols for the periods 1998–2008 and 2009–2015.

The average pH value of Cambisols was 4.28 and 4.78 of Luvisols, respectively. The buffer range, assessed using Ulrich's concept [57], did not change and remained in the "mostly low" category. It was mainly due to proton exchange with base cations.

The analysis of the available information allows to conclude that there no impact of acid atmospheric depositions on pH of the monitored Cambisols and Luvisols for the period 1986– 2015.

#### **3.2. Soil eutrophication**

**3. Pollution of soils, observed by the forest ecosystem monitoring network**

The International Co‐operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests operating under the UNECE Convention on Long‐range Transboundary Air Pollution (CLRTAP), level I, has been implemented in Bulgaria since 1986, and level II—since 1998. The "Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests" (1986–2010), adopted by the Programme, is implemented in order to study the acid status, eutrophication and heavy metal content in soils. A significant part of the obtained results has been published [45–49]. The results, obtained for soils of a total of 104 soil profiles, were summarized for a 20‐year period—from 1986 until

The results for Cambisols and Luvisols from the regions of western Balkan Mountains, Sredna Gora, Rhodope Mountains and Strandzha, obtained for the period 2009–2015, are presented in this book. Data on 62 level I soil profiles from the national forest ecosystem monitoring

The implementation of the forest ecosystem monitoring in Bulgaria began in 1986—a period

The lack of basic information about time series data, obtained from permanent sample plots in the past, did not allow to record the impacts of regional and/or global transfer of acid atmospheric depositions on soils, as well as the subsequent restoration processes due to the measures undertaken. On the basis of the information, obtained for a 20‐year period, it was

when soil acidification in some parts of Europe had already been proven [51–56].

**Figure 16.** pHCaCl2 in Cambisols for the periods 1998–2008 and 2009–2015.

2008 [50].

network were summarized.

138 Soil Contamination - Current Consequences and Further Solutions

**3.1. Soil acidification**

The ratio organic C/N organic layer: organic C/N mineral layer in forest soils has been accepted as the indicator for changes occurring in nitrogen cycle due to increased amounts of nitrogen depositions. It is considered that regarding soils in forest ecosystems in Europe, the values of this ratio, which are below the critical minimum (1.0), occur in areas with increased deposition of nitrogen‐containing components. Exceptions are determined in the northern parts of the continent due to causes of natural origin—harsh climatic conditions, delayed decomposition and accumulation of organic matter [22]. The changes, occurring in soils under the impact of nitrogen depositions, are towards eutrophication [58, 59]. According to ICP Forest data (2011), 61% of the soils on the continent are sensitive to this process. Under the impact of eutrophi‐ cation, nitrogen in soils shifts from a state of shortage to saturation—a process, most clearly expressed in northern and Central Europe [60].

No decrease of this ratio under the critical level, due to increased nitrogen depositions, was registered for soils in Bulgaria during the period 1998–2008 [50] (see **Table 1**).


**Table 1.** Ratio org. C/total N in litter (mull—OL and OF and moder—OL and OFH) compared to the ratio org. C/total N in 0–10 cm soil layer.

The results, obtained during the next evaluation period (2009–2015), confirmed this trend. The minimum values, specified in **Table 1**—0.43 for the period 1998–2008 and 0.80 for the period 2009–2015, were determined in spruce stands from the Rhodope Mountains at an altitude of 1400–1600 m (in the regions of Shiroka polyana locality and Progled village). The stands are located on flat terrains with northern exposure, where the accumulation of organic matter occurs. Under the influence of the cold mountain climate, the decomposition of the organic matter is delayed. Since there are other sample plots in these areas, the results of which are not below the critical limit, it can be assumed that the determined low ratios are the result of naturally occurring processes.

#### **3.3. Heavy metal content in soils in forest ecosystems**

It is considered that heavy metal content in litter represents the sum of their background concentration plus the contribution of atmospheric depositions [61]. The amounts of heavy metals in litter and soils in forest ecosystems in Bulgaria have been a subject to monitoring since 1986. The lack of previous information does not allow determining the impacts of regional and/or global transfer of pollutants. The assessment of data, collected in the period 1986–2008, reveals that in most of the cases the heavy metal content in litter was higher compared to the surface soil layer. The conducted analysis proved that litter, formed on more acidic and scarce in some element soils, contains higher concentrations than the surface soil layer of the respective soil profile. This is most clearly expressed for Cu and Mn. The results, obtained for copper, are presented on **Figures 18** and **19** [50].

It has been determined that the high soil acidity creates a large amount of easily accessible for the plants forms of heavy metals, which is one of the main ways to enrich the litter. In such cases, the high concentrations of heavy metals in litter should be considered as a function of soil acidity and not as a contamination with aerosol origin.

No decrease of this ratio under the critical level, due to increased nitrogen depositions, was

**[org. C/total N (litter)]/[org. C/total N (surface soil layer)] 1998–2008**

**2009–2015**

registered for soils in Bulgaria during the period 1998–2008 [50] (see **Table 1**).

140 Soil Contamination - Current Consequences and Further Solutions

**Layer/period Mean SD min max**

OL/0–10 cm 2.52 0.33 1.87 2.80 OF/0–10 cm 2.08 1.18 0.43 5.52

OL/0–10 cm 3.74 2.04 1.39 6.63 OFH/0–10 cm 1.70 2.04 0.80 2.38

N in 0–10 cm soil layer.

naturally occurring processes.

**3.3. Heavy metal content in soils in forest ecosystems**

copper, are presented on **Figures 18** and **19** [50].

soil acidity and not as a contamination with aerosol origin.

**Table 1.** Ratio org. C/total N in litter (mull—OL and OF and moder—OL and OFH) compared to the ratio org. C/total

The results, obtained during the next evaluation period (2009–2015), confirmed this trend. The minimum values, specified in **Table 1**—0.43 for the period 1998–2008 and 0.80 for the period 2009–2015, were determined in spruce stands from the Rhodope Mountains at an altitude of 1400–1600 m (in the regions of Shiroka polyana locality and Progled village). The stands are located on flat terrains with northern exposure, where the accumulation of organic matter occurs. Under the influence of the cold mountain climate, the decomposition of the organic matter is delayed. Since there are other sample plots in these areas, the results of which are not below the critical limit, it can be assumed that the determined low ratios are the result of

It is considered that heavy metal content in litter represents the sum of their background concentration plus the contribution of atmospheric depositions [61]. The amounts of heavy metals in litter and soils in forest ecosystems in Bulgaria have been a subject to monitoring since 1986. The lack of previous information does not allow determining the impacts of regional and/or global transfer of pollutants. The assessment of data, collected in the period 1986–2008, reveals that in most of the cases the heavy metal content in litter was higher compared to the surface soil layer. The conducted analysis proved that litter, formed on more acidic and scarce in some element soils, contains higher concentrations than the surface soil layer of the respective soil profile. This is most clearly expressed for Cu and Mn. The results, obtained for

It has been determined that the high soil acidity creates a large amount of easily accessible for the plants forms of heavy metals, which is one of the main ways to enrich the litter. In such cases, the high concentrations of heavy metals in litter should be considered as a function of

**Figure 18.** Reaction of soil solution. pH‐a—reaction of soils where the Cu content in litter is higher than the content in the surface soil layer; pH‐b—reaction of soils where the Cu content in litter is lower than the content in the surface soil layer.

**Figure 19.** Copper content in soils with moder type of litter. (a)—copper content in soils where the concentration of copper in litter is higher than the concentration in the surface soil layer; (b)—copper content in soils where the concen‐ tration of copper in litter is lower than the concentration in the surface soil layer.

The content of Cu, Pb and Zn in soils from the regions of western Balkan Mountains, Sredna Gora, Rhodope Mountains and Strandzha remained relatively constant for the period 1986– 2008 [50]. That tendency remained over time due to the absence of statistically proven differences in the content of Cu, Pb and Zn in Cambisols and in Luvisols for the periods 1998– 2008 and 2009–2015 (see **Figures 20**–**25**).

**Figure 20.** Cu content in Cambisols in the periods 1998–2008 and 2009–2015.

**Figure 21.** Cu content in Luvisols in the periods 1998–2008 and 2009–2015.

**Figure 22.** Pb content in Cambisols in the periods 1998–2008 and 2009–2015.

The content of Cu, Pb and Zn in soils from the regions of western Balkan Mountains, Sredna Gora, Rhodope Mountains and Strandzha remained relatively constant for the period 1986– 2008 [50]. That tendency remained over time due to the absence of statistically proven differences in the content of Cu, Pb and Zn in Cambisols and in Luvisols for the periods 1998–

2008 and 2009–2015 (see **Figures 20**–**25**).

142 Soil Contamination - Current Consequences and Further Solutions

**Figure 20.** Cu content in Cambisols in the periods 1998–2008 and 2009–2015.

**Figure 21.** Cu content in Luvisols in the periods 1998–2008 and 2009–2015.

**Figure 23.** Pb content in Luvisols in the periods 1998–2008 and 2009–2015.

Pollution was determined in some areas, located near industrial enterprises. Pollution of Regosols, based on an example of the copper producing plant near the town of Pirdop, which affects mainly the surface soil layer and litter due to active absorption of copper from plants in acidic environment (pH H2O = 4.34) is presented on **Figure 26**.

**Figure 24.** Zn content in Cambisols in the periods 1998–2008 and 2009–2015.

**Figure 25.** Zn content in Luvisols in the periods 1998–2008 and 2009–2015.

Due to the lack of norms for evaluation of soil pollution with heavy metals in forest ecosystems in Bulgaria, the accumulation rate (AR) has been accepted as the criterion for its confirmation. It is calculated as the ratio between the concentration of a certain metal in the surface soil layer (0–10 cm) and the layer 60–80 or 20–40 cm, depending on the soil depth. According to some authors [62, 63] when AR >1.50, the soil is polluted and the main pollution source is the atmospheric depositions. Regarding the soils from agricultural lands in Bulgaria, these rates were differentially calculated by types of metals back in 1978 [64] and the AR values are close to 1.5.

**Figure 26.** Cu content in Regosols (mg kg−1). OL—unaltered dead remains of plants; OFH—fragmented partly decom‐ posed and well‐decomposed organic matter.

**Figure 24.** Zn content in Cambisols in the periods 1998–2008 and 2009–2015.

144 Soil Contamination - Current Consequences and Further Solutions

**Figure 25.** Zn content in Luvisols in the periods 1998–2008 and 2009–2015.

Due to the lack of norms for evaluation of soil pollution with heavy metals in forest ecosystems in Bulgaria, the accumulation rate (AR) has been accepted as the criterion for its confirmation. It is calculated as the ratio between the concentration of a certain metal in the surface soil layer (0–10 cm) and the layer 60–80 or 20–40 cm, depending on the soil depth. According to some authors [62, 63] when AR >1.50, the soil is polluted and the main pollution source is the The forest ecosystem soils are characterized by biogenic‐accumulative processes, which are part of the forest soil‐forming process. Under its influence, the rates increase regardless of the presence or absence of exchangeable acidity [50] and repeatedly exceed the value of 1.50. These processes should be taken into consideration when assessing heavy metal content in soils and should not be considered as pollution. Average and maximum values of AR were determined for the soils from the regions of western Balkan Mountain, Sredna Gora, Rhodope Mountain and Strandzha, calculated on the basis of data, collected in the period from 1986 to 2015, from sites located away from industrial emission sources.

The maximum accumulation rates as the result of the natural heavy metal content in surface soil layers are presented in **Table 2**.


**Table 2.** Ratio org. C/total N in litter (mull—OL and OF, and moder—OL and OFH) compared to the ratio org. C/total N in 0–10 cm soil layer.

Higher values should be determined in order to prove pollution.

#### **3.4. Nutrient and heavy metal content in Devnya industrial zone**

Soil in the territory of Devnia industrial region is of the type Haplic kastanozems, with pH 7.3 and well supplied with basic nutrients. The humus content varied between 2.00 and 3.56%, total nitrogen was in the range of 0.135–0.344% [65]. The mean values of nutrients and heavy metals determined in surface soil layers in the open and under the plantations with *Frainus americana* and *Celtis australis* for 10‐year period (1996–2005) are reported in **Table 3**.


**Table 3.** Ten year (1996–2005) mean values of nutrients and heavy metals in soil in the open and under plantations of *Fraxinus americana* L. and *Celtis australis* L. in Devnya industrial region: Polluted area and Control—at 500 and 15,000 m from the point source of pollution, respectively.

The data showed a higher content of all analysed elements in the polluted area. In the open, at 500 m to the emission sources, the level of Ca (4.7 times more than the control) and P (2 times above the control) was particularly increased, while the content of K and Mg increased with 15 and 32%, respectively. The surface soil layers of the industrial area contained 2.8 times more copper, 2 times more lead and 1.9 times more zinc than the remote area. Remarkable accumu‐ lation of calcium was found under the plantations with Fraxinus *americana*—7, 5 times more than under the control plantation, while under the plantations with *Celtis australis,* this accumulation was 3.6 times more than the control. The accumulations of the other macroele‐ ments in the surface soil under the two plantations were approximately the same. This accumulation is due to dust and aerosol deposition entering the soil from industrial production and transport. This is especially true for calcium, phosphorus and copper. Potassium, phos‐ phorus and magnesium had higher values under the plantation with *Celtis australis*. The mean concentrations of heavy metal in the polluted soil ranged from 44.3 to 78.6 mg/kg for Cu, from 51.2 to 80 mg/kg for Zn and from 38.5 to 41.3 mg/kg for Pb. The highest content of copper was established in the soil under *Fr. americana* and of zinc—under *C. australis*. The lead content in the polluted soil was almost the same in the open and under of the two plantations. Most elements in the polluted zone, with the exception of calcium and copper, were accumulated in largest quantities in the soil under the plantation with *C. australis*. This can be used in the selection of species for afforestation in such areas. As the metals have a different mobility, they are transported from roots to shoots in different manner. Zn is more mobile than Cu and Pb [66], and the accumulation of Zn in the aboveground parts of the trees could be expected to be more intensive. The observed levels of Zn and Pb in the studied soil were within the range of the maximum tolerable levels. The soil content of Cu in the open and under the plantation with *Fr. americana* slightly exceeded the maximum tolerable level [17]. Results showed that under the impact of the local industrial emissions the soils in Devnya region were contaminated with heavy metals.

**3.4. Nutrient and heavy metal content in Devnya industrial zone**

146 Soil Contamination - Current Consequences and Further Solutions

Soil in the territory of Devnia industrial region is of the type Haplic kastanozems, with pH 7.3 and well supplied with basic nutrients. The humus content varied between 2.00 and 3.56%, total nitrogen was in the range of 0.135–0.344% [65]. The mean values of nutrients and heavy metals determined in surface soil layers in the open and under the plantations with *Frainus*

**Polluted area Control Polluted area Control Polluted area Control**

*americana* and *Celtis australis* for 10‐year period (1996–2005) are reported in **Table 3**.

**Element In the open** *Fr. americana* **L.** *Celtis australis* **L.**

P (mg/100 g) 114.0 58.0 81.2 40 143.2 75.6 K (mg/100 g) 800.3 698.6 595.6 405.8 984.3 807.8 Ca (mg/100 g) 3984.7 848.3 4450.6 596.5 4000.5 1103.2 Mg (mg/100 g) 421.6 318.3 400.5 255.8 469.8 350.8 Cu (mg/kg) 74.6 26.9 78.6 17.5 44.3 18.8 Zn (mg/kg) 65.8 34.3 51.2 30.8 80.0 39.6 Pb (mg/kg) 40.0 19.6 38.5 17.2 41.3 24.1

**Table 3.** Ten year (1996–2005) mean values of nutrients and heavy metals in soil in the open and under plantations of *Fraxinus americana* L. and *Celtis australis* L. in Devnya industrial region: Polluted area and Control—at 500 and 15,000 m

The data showed a higher content of all analysed elements in the polluted area. In the open, at 500 m to the emission sources, the level of Ca (4.7 times more than the control) and P (2 times above the control) was particularly increased, while the content of K and Mg increased with 15 and 32%, respectively. The surface soil layers of the industrial area contained 2.8 times more copper, 2 times more lead and 1.9 times more zinc than the remote area. Remarkable accumu‐ lation of calcium was found under the plantations with Fraxinus *americana*—7, 5 times more than under the control plantation, while under the plantations with *Celtis australis,* this accumulation was 3.6 times more than the control. The accumulations of the other macroele‐ ments in the surface soil under the two plantations were approximately the same. This accumulation is due to dust and aerosol deposition entering the soil from industrial production and transport. This is especially true for calcium, phosphorus and copper. Potassium, phos‐ phorus and magnesium had higher values under the plantation with *Celtis australis*. The mean concentrations of heavy metal in the polluted soil ranged from 44.3 to 78.6 mg/kg for Cu, from 51.2 to 80 mg/kg for Zn and from 38.5 to 41.3 mg/kg for Pb. The highest content of copper was established in the soil under *Fr. americana* and of zinc—under *C. australis*. The lead content in the polluted soil was almost the same in the open and under of the two plantations. Most elements in the polluted zone, with the exception of calcium and copper, were accumulated in largest quantities in the soil under the plantation with *C. australis*. This can be used in the selection of species for afforestation in such areas. As the metals have a different mobility, they are transported from roots to shoots in different manner. Zn is more mobile than Cu and Pb

from the point source of pollution, respectively.

#### **3.5. Nutrient and heavy metal content in leaves of tree species in Devnya industrial region**

According to the data for the leaf chemical composition of *Frainus americana* L. and *Celtis australis* L., grown in Devnya industrial zone, there were well‐pronounced differences between polluted and control trees in relation to leaf nutrient concentrations (**Table 4**).


**Table 4.** Nutrients and metals content (M ± SD, N = 3) in the leaves of *Fraxinus americana* L. and *Celtis australis* L. growing in the polluted and control area and ratio polluted versus control.

A misbalance was observed in some nutrients in the damaged trees. Total nitrogen increases in damaged *Fr. americana* trees and decreases in polluted leaves of *C. australis*. The higher total nitrogen content in damaged leaves mainly was due to the presence of nitrogen oxides in polluted air masses, coming from the emission sources in this area. Trees take up nitrogen from the soil and air. The highest level of total nitrogen was found in the damaged leaves of *Fr. americana*, while the damaged leaves of *C. australis* had relatively poor nitrogen supply. Total phosphorus showed a severe decrease in damaged leaves of *Fr. americana*. In two of the tree species, polluted leaves had extremely lowered content of potassium. Decreased levels of total phosphorus and potassium may cause alteration in nutrient uptake because of their less efficient retranslocation in polluted stands [67]. Due to the high level of calcium in the soil, the leaves in both control and damaged trees had a great amount of calcium. A more pronounced tendency for calcium and magnesium accumulation in polluted region was found in the leaves of *C. australis*, despite of the antagonistic effect of calcium on magnesium uptake. Among the elements, the greatest accumulation was established for calcium (from 3.5 to 7 times higher than the control) and phosphorus (on average 2 times over the control). The higher magnesium level in damaged leaves of *C. australis* could be explained with an increased exchange of magnesium in polluted soils. The lower nutrients content in polluted leaves, especially of potassium and phosphorus, was due to the inhibition of total functional activity in damaged trees. The decreased concentration of potassium, known to play an important role in water regime regulation, might be regarded as an indicator for a water misbalance in polluted leaves [68]. Some specificity was found in the accumulation of separate micronutrients and heavy metals among the species. The most pronounced difference between damaged and control trees were found in copper, manganese, zinc and lead concentrations. Remarkable copper accumulation was observed in the leaves of *C. australis*. Severe manganese accumulation was found in polluted leaves both of *Fr. americana* and *C. australis*. According to some authors, manganese toxicity might be a significant constraint for the health of forests on disturbed soils [69]. The accumulation of zinc was higher in polluted leaves of *Fr. americana*. Cadmium was accumulated mostly in the leaves of afflicted*C. australis* trees and exceeded the levels of toxicity [22]. The greater amount of soluble manganese is favourable to iron availability. In polluted stands, iron was accumulated extremely by *Fr. americana* and moderately by the leaves of *C. australis*. Complex changes in chemical composition, disturbed balance of nutrient elements and increase in the content of heavy metals accompanied decline processes [68]. An uptake of heavy metals by plants occurs together with nutrients through the roots or directly through leaves. The entry of elements through the leaves is more significant for the pollution ones. The slightly alkaline reaction of soil in Devnya region does not create a large amount of easily accessible for the plants forms of heavy metals. Therefore, the accumulation of heavy metals in the leaves might be mainly due to the deposition of air pollutants. Zinc, being an essential element to the plant metalloenzymes, is translocated extensively and its uptake is dependent on metal concentration in extractable fraction in soil as well [70, 71]. The response of vegetation to pollutants depends on the degree of pollutant loading. At low pollutant loads, vegetation can act as a sink for pollutants, and no or minimal physiological alteration occurs [39]. In our study, such role may play *C. australis*. The content of copper, cadmium and especially lead in the leaves of *C. australis* exceeded the excessive values for tree vegetation and can be regarded as damaging [17]. Although the heavy metals are mostly below the critical levels of decreased growth, they may threaten tree vegetation in the region. Hence, the area studied was with slight to moderate heavy metal contamination. The accumulation levels obtained are air and soil orientated [72, 73]. The examined species accumulated mainly lead, copper, zinc and manganese.

In conclusion, each of these pollutants can be suggested as an indicator for the influence of industrial emission on the soil of the region. Changes in foliar element concentrations, howev‐ er, can take place long before pollution‐mediated plant injuries, and foliar element content is commonly used as biomonitor to investigate the distribution of air pollution.
