The content of Cd and Pb in undeveloped and shallow soils and plant material in the area of national park Tara

Ultramafic (serpenites) are a group of igneous or metamorphic rocks, which are characterized by high concentrations of Mg, Fe, Ni, Cr, and Co, along with low concentrations of Ca, P, and K, contain less than 45% silicon (SiO2). Тara Mountain is natural good of western Serbia where areas under serpentinites occupy 28%. Researchers agree that the flora of serpentine areas is unique and botanically very important. The specificity of the flora and the development of vegetation on serpentinites are characterized by special mechanisms of plant species adaptation to increased concentrations of some heavy metals in the soil with low content of essential elements (nutrients). This paper examines the influence of serpentinite geological substrate on the occurrence of certain plant species in the initial stages of plant community development. The soil was mainly sampled in rock crevices in the early phases of soil development (Lithosols) where the influence and origin of the metals from the rock can be considered the most obvious. Content of cadmium (Cd) and lead (Pb) in geological substratum, soil and plants biomass is analysed in order to differentiate levels and extents of natural and anthropogenic pollution and also deposition data from the EMEP program (European Monitoring and Evaluation Programme). Results show that the content of lead and cadmium in the soil and plant samples are higher than the content in the corresponding rock sample, except for two soil samples, in which the content of Cd is lower. This suggest that the origin of the increased Pb and Cd content might be from anthropogenic sources. Deposition (data from EMEP program) of Cd and Pb, which was analysed for the period from 1990-2018, indicate significant cumulative effect. The values of cumulative deposition in research area have a value of 91,51 kg/km lead, while the value of cadmium is 1665 g/km, which classifies NP Tara in above-average polluted area in R. Serbia.


Introduction
Ultramafic (serpenites) are a group of igneous or metamorphic rocks, which are characterized by high concentrations of Mg, Fe, Ni, Cr, and Co, along with low concentrations of Ca, P, and K, contain less than 45% silicon (SiO2). Soils on serpentinites are rich in chromium, nickel, manganese, zinc, cobalt, lead, etc. Determining the content of metals in the geological substratum, eg. potential natural soil pollution, as well as anthropogenic pollution of soil by heavy metals is important regarding plant species usage growing on such area. Due to intensive use of medicinal plants in modern pharmacology, but also in the traditional lifestyle, there is a need to monitor the levels of elements in the herb of these plants, 68 especially potentially toxic to health, such as mercury, lead and cadmium (Obratov-Petkovic et al., 2008).
Serpentine soils have properties that are highly unfavourable for most plants. They are often characterized by gravelly texture, low clay content, shallowness, susceptible to erosion and with a low content of organic matter (Farag, 2013). Strongly serpentinized rocks are more susceptible to mechanical breakdown and disintegration, as well as to chemical decomposition involving considerable changes in chemical and mineralogical composition (Knežević et al., 2009). High concentrations of heavy metals (nickel, chromium, lead, cobalt, cadmium...) in serpentine soils were responsible for their infertility. Vegetative cover at serpentine sites is usually scarcer than the surrounding areas and is characterized by obligate and facultative serpentinophytes. Due to high levels of metal concentrations, plants accumulate them, which require different adaptation mechanisms to reduce their harmful effect (Farag, 2013).
The main goal of this paper is to determine the degree of influence of the parent substrate and potential anthropogenic pollution on the content of non-essential toxic metals in poorly developed soils on serpentinites. In order to determine the potential transfer of these metals in the environment, the contents in the soil, aboveground and belowground biomass were analysed. In order to define "natural pollution" rates through weathering, content of microelements in the geological substratum were analysed. Atmospheric deposition data from EMEP program were analysed.

Research area and sampling method
Geological substratum, soil, and plant biomass samples were collected in the NP Tara at 3 localities.

Cadmium (Cd)
Cadmium shows a constant concentration in geological substratum, and its value is around 0.02 mg/kg.
The maximum measured concentration of cadmium in the soil is 0.18 mg/kg, the minimum -0.003, while the limit value according to the Regulation is -0.80. The concentration of 0.003 mg/kg was measured in all samples of lithosol, and slightly higher in the samples of cambisol -0.093 and 0.186.
In biomass below ground the lowest concentration (0.003 mg/kg) was measured in Thymus sp. and

Lead (Pb)
Lead concentration of 0.02 mg/kg was measured in geological substratum from Konjska reka and Popovici, while concentration of 54.72 mg/kg was measured in Zmajevacki potok.
The maximum measured concentration of lead in the soil is 86.74 mg/kg, while the limit value according to the Regulation is 85. This is the only sample where the value exceeded the limit, and it is a sample of soil under the blueberry (Vaccinium myrtillus). However, even the minimum measured concentration in soil (0.94 mg/kg), as in all other samples, is significantly higher than that measured in the corresponding rock sample, which suggests that the presence of lead is not the result of weathering but anthropogenic origin.
This assumption is supported by the constant contents of this metal in both below ground (average 1.81 mg/kg) and above ground (average 2.56 mg/kg) plant biomass which may further indicate that this accumulation process is not naturally. None of the species, which occur naturally in this area, did not show increased accumulation as a mechanism of adaptation, when the presence of lead would be the result of natural, pedogenetic processes ( Figure 3). The measured concentration of lead in geological substratum, as well as in the soil where lithosol is present, is significantly lower than some previous measurements (Oze et al., 2020;Brankovic et al., 2016;Obratov-Petkovic et al., 2008;Vasic, 2017), while in the samples of rocks and soil from The content of Pb in the plant biomass is constant and it is in accordance with the other published data (Arsenijevic et al., 2011;Kucukbay et al., 2010). According to Kabata-Pendias (2011), the Pb content in plants is in the range of 0.05-3.0 mg/kg, therefore, the tested samples do not deviate much from the average.

Deposition data trends-EMEP
According to the European Program for Monitoring and Evaluation of Deposition Values (EMEP), the cumulative values of Cd and Pb deposition for the period 1990-2018 were obtained, presented for the whole R. Serbia (Figure 4). The values of cumulative deposition in the research area has value above the 75th percentile, ie, the value of lead is 91.51 kg/km2, while the value of cadmium is 1665 g/km2.  where the values for research area are above-average polluted region in R. Serbia, which should not be because it is a protected area.
Monitoring of heavy metals in contaminated soils and plant is one of the most significant nature conservation challenges. Such studies should provide guidelines for further control of pollutant