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Basic soil properties

The oxidisable carbon (Cox) measured in the soil samples ranged within the common values for tilled agricultural soil, which is generally<5% (Table 1)14. A weak positive correlation was observed between HNO3 extractable Cu and Cox (P0.05). Lead, Zn and Cd also showed positive correlation but to only a slight extent, whereas correlation between As and Cox was negative in both soil layers. The slight to no correlation between Cox and the extractable soil PTE proves that if there was any additional organic matter (OM) input to the soil, it did not significantly add to the topsoil contamination, albeit available PTE released by OM mineralization may be either taken up by roots of the following cultures or leached. A weak positive correlation between Cu and Cox can be expected due to the Cu affinity for organic matter. Soil pH was neutral for the majority of samples, with some ranging to the moderately alkaline spectrum15. No significant relationships were determined between soil pH and the phyto-available PTE (as a percentage of HNO3 extract).

The highest pseudo-total soil PTE concentrations were for As, followed by Zn, Cd and Pb. The sample containing the lowest As concentration (min.) exceeded the soil background level (SBL)12 1.9 times (Table 2). The SBL allows for the comparison between contaminated soils and background levels of PTE in soils. In the most contaminated sample, As exceeded the SBL 93 times. Regarding differences between soil layers, a slightly higher pseudo-total As concentration was found in the deeper B layer (possibly due to leaching); HNO3 extractable median of 54.5mgkg1 as opposed to 49.1mgkg1 in the A layer. However, the greatest maximum content of As was observed in the A layer (418mgkg1). Hork and Hejcman7 performed a large-scale characterization of pollution levels in the region north of KH. Interpolations of PTE showed that As was frequently found in the range of hundreds to thousands of mg kg1. The large number of dumps of waste rock and slag in the area surrounding the gardens contain not only primary minerals of As, but also secondary minerals. Secondary As minerals such as bukovskyite (Fe3+2(AsO4)(SO4)(OH)), pitticite (Fe3+20(AsO4,PO4,SO4)13(OH)249H2O), and scorodite (Fe3+(AsO4)2H2O) were created by weathering of arsenopyrite, and also zykaite (Fe3+4(AsO4)3(SO4)(OH)15H2O), kankite (Fe3+(AsO4)3.5H2O), and parascorodite (Fe3+(AsO4)2H2O)16,17,18,19. Arsenic is firmly bound to oxides of Fe/Al in the form of arsenite(III) or arsenate(V)8, and so can be considered largely immobile in mineral type soils; this is reflected in the relatively low phyto-available portion of this element in the studied soils (mean=2.1% and 2.4% in A and B layers respectively). However, when exposed to soil solutions containing organic anions in the dissolved organic carbon (DOC), e.g., organic acids such as oxalic acid, citric acid, and malic acid, research (including research done by Ash et al.20) has shown that As can be released into solution by various mechanisms, including the complete dissolution of the mineral oxide to which As was bound20,21. Therefore, the addition of organic residues and manures to soil is likely to enhance the mobility of As and its potential uptake by plants. At the same time however, with sufficient irrigation the released As can be leached to greater soil depth, thus eliminating the pathway of As exposure by inhalation or ingestion of contaminated soil at the surface. Another option could involve the use of Fe-oxides in order to sorb mobile As22. Despite its generally low relative availability, As was the most phyto-available element compared to the other studied PTE, although control soils had only a slightly lower percentage of availability. Congruent to observations by Xu and Thornton23, who studied As-contaminated gardens at a mining area in southern England, the phyto-available content correlates with the total As content (R=0.80 and 0.81 for A and B respectively).

Regarding Cd, more than a quarter of the data were in excess of the 1mgkg1 SBL limit. Enrichment with Cd in the KH soils is particularly evident when compared to the control soils. Cadmium is a metal that is characterised by generally higher mobility than other metals with similar valence, such as Cu, Pb, and Zn, which are associated with binding to organic matter carbonates and clays. Higher mobility of Cd usually translates into enhanced plant uptake but can also mean greater vertical leaching; in this case, little difference in total Cd contents between A and B layers was observed.

For Pb, concentrations exceeding the SBL were detected in approximately one quarter of the samples from both layers. However, the maximum pseudo-total Pb concentration was observed in deeper (1530cm) soil samples; this may reflect the smelting practices that took place in past centuries. Lead sulphides were added to the smelter to decrease the melting temperature of silver24; because smelting activities ceased long before the establishment of the vegetable gardens, it is likely that the most enriched soils have been buried by imported topsoil or newly developed surface soil layers. Lead isotope analysis would be necessary to confirm the Pb source. Independent t-test confirms the higher content of Pb in the B layer samples; nevertheless, both layers A and B contained considerably more Pb than in the control soil.

Besides As, Zn was the only PTE whose median concentration in KH soils was above the SBL. However, while excess Zn is phytotoxic, it is generally considered relatively nontoxic for animal and humans, and concentrations must be highly excessive for symptoms of toxicity to manifest in humans25. Furthermore, Zn is a micronutrient element in plants, and so concentrations at or near baseline or recommended guideline levels are not a concern26.

The plant samples (Table 3) were contaminated with higher concentrations of As, Cd, Pb and Zn than the allowable quantity (AQ) and maximum allowable quantity (MAQ) set by the Ministry of Health in the Czech Republic (Decree No. 53/2002)9. The plants samples also exceeded the maximum permitted concentrations of Cd (0.020.1mgkg1) and Pb (0.1mgkg1) set by the EU directive (Decree No 1881/2006)27.

Higher As concentrations occurred in cucumbers, onions, garlic, potato tubers, and peppers (max values reaching 5.09, 3.01, 3.73, 1.04 and 1.22mgkg1, respectively). Higher As concentrations in some plant parts could be explained by fractions of bioavailable As in soils, deposition of dusts on plants (that may contaminate the stomatal chambers) with above-ground edible biomass, longer planting periods and different garden plots and soils in the area26. Cadmium concentrations in edible plant parts were highest in several of the potato tubers and pepper plants, reaching concentrations up to 0.30 and 0.68mgkg1, respectively. Cadmium can be observed to being efficiently stored by root and leaf systems, depicting the bioavailability of Cd in soils (up to 5%), indicating a relationship between Cd in plants and Cd in the growth medium26.

Several factors that affect the concentrations of Pb in a plant are pollution and accumulation abilities of plants, with atmospheric deposition of Pb on above ground biomass being an important source of Pb contamination in plants26,28. The plant samples with the highest Pb concentrations were peppers, potato tubers, and tomatoes (max concentrations of 4.42, 3.65 and 2.06mgkg1, respectively).

Soluble Zn is readily available for plant uptake, however, rate of uptake is controlled by plant species and cultivars26. With regards to our results, Zn concentrations in the plants were up to 15 times higher than the AQ in the case of zucchini (Table 3). The high Zn concentrations in the edible plant parts correlated to the high concentrations in the soils, reaching up to 759mgkg1 in some samples.

Jolly et al.29 investigated transfer factors of PTE into different vegetables that were grown on soil with elevated PTE concentrations. They also observed a relative abundance of As, Cd, Pb and Zn in the edible parts of plants, with highest concentrations in Amaranthus and elevated concentrations also in tomatoes, radish, spinach and beans. Tremlov et al.6 found As concentrations ranging from 1.6 to 64mgkg1 in dried plant edible tissues grown on contaminated KH soils with limited plant available As in soils with highest concentrations in parsnip and black radish and lowest concentrations in savoy cabbage and lettuce. The study by Tremlov et al.6 presents results similar to this study, where we found low plant available As in soils, however, plant samples still surpassed As guideline values. Another study by Tremlov et al.30 found both low and high As concentrations in different plant species ranging from 0.02 to 39.30mgkg1 with arsenite and arsenate being the predominate As compounds. A study conducted by Krlov et al.31 on soils contaminated by mining activities in KH showed low plant available concentrations for As and Pb (not exceeding 0.5% of pseudototal) and relatively high plant available concentrations for Cd and Zn (47 and 60%, respectively). In the aboveground biomass of the plants studied by Krlov et al.31, low As concentrations were found (ranging from 0.36 to 3.64mgkg1) in the plant species, indicating a low translocation rate. In our study, As concentrations in our plant samples were up to 5.09mgkg1, therefore concentrations were much lower than results presented by Tremlov et al.6,30 but similar to Krlov et al.31. Cadmium in the study by Krlov et al.31, was more readily translocated in the plant tissues, with concentrations in edible plant parts between 0.02 and 2.58mgkg1. Our Cd concentrations in the plants went up to 0.68mgkg1 and was found in peppers. Therefore, Cd was not as easily translocated into the aboveground plant parts, which could have been due to soil type and plant species/cultivars. The Cd values in our study and the study by Krlov et al.31 in majority of cases surpassed both the limits set by the Ministry of Health in the Czech Republic9 and the European directive27. In the case of Zn, high concentrations found by Krlov et al.31 ranged between 21 and 228mgkg1 were similar to the results from this experiment (11.83 to 153mgkg1), were concluded as not phtotoxic. Concentrations of Pb ranged between 0.04 and 1.03mgkg1 in the study by Krlov et al.31, while in our results, Pb concentrations were significantly higher (0.43 to 4.42mgkg1). Our results exceeded the MAQ and the European directive, which states the limit of Pb in foodstuff as 0.10mgkg1. Therefore, PTE concentrations in plants are highly influenced by the plant species and the soil physio-chemical properties. Despite the low plant availability of PTE, concentrations in plants studied in this experiment still exceeded the guideline values set for edible plants, as shown in Table 3.

Potentially toxic elements in soil can transfer to humans in a number of ways, including the direct consumption of contaminated soil particles with unwashed vegetables, on unwashed hands, through soil ingestion by children, infants, and pets, by inhalation of dust, or through uptake into edible vegetables32. A further exposure to soil PTE is by its inadvertent transport to the inside of houses from the garden; Laidlaw et al.33 showed that the source of interior Pb dust was primarily from soil in two out of three houses. Izquierdo et al.34 performed a comprehensive risk assessment for PTE bioaccessible in urban gardens. Their conclusions highlighted a combined exposure for children; soil ingestion due to play, and consumption of vegetables grown on contaminated soil. Drahota et al.3 found health risks, especially related to As, associated with ingestion of mine waste materials and contaminated urban soils. In several localities surrounding KH, mine waste slags were re-cultivated into gardens and fields35, therefore posing a risk to humans.

Soil PTE levels vary and are difficult to predict in city vegetable gardens due to the heterogeneous nature of urban pollution and past land uses. Nonetheless, many affordable and feasible (for households) remediation techniques exist that can help decrease the plant available fractions. Such remediation techniques involve the incorporation of clays, compost, biochar, clean top-soils, or by providing a crop-cover, and by growing ornamental plants rather than edible ones. Such remediation techniques have been considerably studied with promising results35,36,37,38,39,40,41. However, when implementing amendment measures, several factors must be taken into account. Soil properties (eg. pH, soil organic matter, Cox, etc.) as well as the type of contamination and the main contaminants present are the most important factors. Implementing amendment for As contamination widely differ from amendments that would work for Zn or Pb, for example.

The plants with the highest overall PTE concentrations were peppers, potato tubers, tomatoes and cucumbers, therefore the gardeners are recommended to avoid planting these plant species in their gardens or to use different cultivars that could possibly accumulate less PTE in the edible plant parts. The plant with the lowest uptake of As and Cd into the edible plant parts were apples, therefore, planting fruit trees rather than vegetables, could be a solution. While growing of ornamental plants instead of edible ones is a tactical way to combat plant to human transfer of risk elements in the garden soils surround KH, another possibility is the plantation of trees. Trees have the ability to retain risk elements bound in soils, albeit the uptake ability of trees can be relatively low and depends on the level of soil contamination42,43. The chosen amendment would differ greatly from garden to garden depending on the plants cultivars, the soil type and the highest PTE present in the soils and plants.

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Assessment of potential exposure to As, Cd, Pb and Zn in vegetable garden soils and vegetables in a mining region | Scientific Reports - Nature.com