Availability of Selected ( Pollutant ) Elements and their Influence on Soil Composition in Urban Area

Interest in growing fruits and vegetables in private gardens is rising due to nowadays ecological awareness. Avoiding artificial fertilizers and plant protection products does not guarantee the absence of toxic substances, especially heavy metals in the soil and thus in the fruits harvested. Due to either geological bedrock weathering or environmental pollution, garden soils may be rich in certain potentially toxic elements. In the present study ten garden soils from central Croatia have been analysed by the BCR method for the contents and bioavailability of aluminium, cadmium, chromium, cobalt, copper, lead, manganese, nickel and zinc. The total amounts of the elements are in the concentration range as reported for agricultural soils in different geographical regions. Only two soils of the capital Zagreb have higher concentrations of pollutant metals, such as chromium, cobalt, copper, lead, manganese and zinc. Regarding nutrients, all soils have met the needs of common garden plants.


INTRODUCTION
The importance of home gardening is rising in the last years due to the elevated ecological awareness.Fertilizers and plant protection products are considered to lower food quality by their specific toxic properties.Organic farming represents growing plants without synthetic fertilizers and pesticides or genetically modified organisms; however, the inorganic composition of the soils is not regulated. 1Only general limits of levels in soils used for agricultural purposes are given by different jurisdictions. 2,3In general, plants are useful indicators of environmental heavy metal contamination and can be used to monitor pollution across both spatial and temporal scales. 4Fruits produced by organic agriculture in private gardens may also contain toxic elements, such as cadmium, lead, and arsenic due to contamination via air or by uptake from polluted soils.
The favorite products of home gardening are vegetables, like tomatoes and lettuce, and fruits, especially many types of berries, e.g.strawberries, blackberries, raspberries and currants.
Each of these plants has special requirements regarding micro-nutrients and trace elements, i.e. vitamins or minerals needed in small amounts for normal function of the organism.That a micro-nutrient is essential is indicated by its presence in healthy tissue and that its uptake and distribution are homeostatic control.Only small amounts of such nutrients are needed and any deficiency can cause diseases or ill-health, retarded growth and poor harvest.Plants require the following elements: nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg), manganese (Mn), iron (Fe), boron (B), copper (Cu), zinc (Zn), and aluminium (Al). 5lants tend to selectively uptake and accumulate certain metals as ions; the level of essential elements depends where the plant is grown, i.e. it is affected by the geochemical characteristics of a soil.The resulting elemental pattern is thus determined by the bioavailability of the elements present in soil and the selectivity of their accumulation.In plants, the major uptake occurs Croat.Chem.Acta 88 (2015) 23.
via their roots.Additional sources of these elements are environmental parameters, such as rainfall, atmospheric dusts (dry deposition), plant protection agents and fertilizers that can be absorbed through their leaves. 6Soil analysis is important since the chemical composition of soil reflects both the soil bedrock and the anthropogenic inputs such as pollutants from industrial processes.Information on the mobility or bioavailability of metals however is scant, and special extractions steps are needed to determine the bioavailability of the elements in the plant. 7The most frequently applied method is recommended by the European Commission, Community Bureau of Reference (BCR) for sediments and soils, due to its simplicity. 8he aim of the present study was the quantitative determination of the elemental content of ten private garden soils from central Croatia.Toxic as well as essential elements were selected, namely Al, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Ni, Pb, and Zn.Their availability was studied by the BCR method.

Soil Samples
The garden soil samples from the A-horizon were collected in September 2009 at ten sampling sites in Zagreb and its surroundings (see Figure 1).The bedrock geology of the sampling sites is shown in Table 1.After collection the soil samples were dried for 24 hours at 105 °C, ground with a metal-free device and stored at a cool place.

Chemicals and Glassware
Nitric acid (HNO 3 ; 65 % w/w p.a.) and the ICP Multielement Standard IV, both from Merck (Darmstadt, Germany), were used to prepare the standard calibration solutions as well as for the digestion step.The reagents for the extraction steps, i.e. acetic acid (CH 3 COOH), hydroxylamine hydrochloride (NH 2 OH×HCl), hydrogen peroxide (H 2 O 2 ), ammonium acetate (NH 4 C 2 H 3 O 2 ) and hydrochloric acid (HCl) were analytical grade and purchased from Kemika (Zagreb, Croatia).Before use all glassware was soaked in 7 mol/L nitric acid for 24 hours, and then rinsed with supra-pure water.

Extraction Procedure
Sequential extraction was based on a modified BCR protocol. 9The acid-soluble and exchangeable fraction in step 1 was prepared by adding 40 mL of acetic acid (0.11 mol/L) per approx. 1 g of dry soil sample and shaking for 16 hours at room temperature by an orbital shaker at 250 rpm.The residue was treated with 40 mL of 0.5 mol/L hydroxylamine HCl (pH = 2) to obtain the reducible fraction (step 2).The extraction condi- tions were as in step 1. Step 3 (the oxidisable fraction) was prepared by adding of 10 mL of 8.8 mol/L hydrogen peroxide and 50 mL of 1.0 mol/L ammonium acetate (pH = 2) to the residue from step 2 and by using again the same extraction conditions as above.Finally, the residue from step 3 was leached with aqua regia.The extracts from each step were separated from the solid by centrifugation (3000 rpm for 20 min), decanted into polyethylene bottles and stored at 4°C until analysed.

ICP-AES Measurements
The instrument used was a Prodigy High Dispersive ICP spectrometer operating in simultaneous mode; at settings listed in Table 2.All measurements were carried out in triplicate at the emission lines shown in Table 3.
For all analytes and all types of samples the repeatability and precision of the method were determined.The sensitivity of the method was estimated from the slope of the calibration curve.The accuracy was estimated from the recovery of the analytes from the extraction solutions spiked at 0.5 and 2 mg/L.Standard reference materials supplied by the International Atomic Energy Agency (Vienna, Austria) were treated and measured like the samples: IAEA-SL1 (lake sediment), IAEA-Soil 7, IAEA 405 (stream sediments) and IAEA SL 3 (lake sediment).

ICP-AES Validation
The accuracy of the method was evaluated by determining the recoveries by the analysis of CRMs and by spiking experiments of all analytes in the extracts of all four extraction steps.The mean recoveries of both concentrations range from 90-112 % and are listed in Table 3 along with their limit of detection (LOD).Regarding the tested CRMs the recoveries ranged from 87 to 110 % for the certified elements.Except for a few elements in single steps all LODs are < 3 μg/L.The precision for all elements ranged from 0.5 to 2.4 % in the four extraction steps.The repeatability was < 1.3 %, the intermediate precision < 5.7 %, the day-to-day reproducibility < 6.8 % and the overall uncertainty of measurement was estimated to be 4-8.5 % for all elements analysed. 10The validation of analytical procedure optimized was found to be acceptable for environmental analyses. 11

Metal Content and Extractability
The results (expressed in µg/g soil) for each extraction step together with the percentage extracted per step are Sample uptake delay 30 s listed for all elements investigated in Table 4 a-l.Each element is discussed separately.The elemental soil composition (Figure 2) shows the median total amounts of the metals analysed.The extraction behavior of all elements is compared by cluster analysis (Figure 3).Aluminium and chromium (three valent ions) are grouped together in all steps except in step 3.Each extraction step showed a different pattern.

Aluminium
Aluminium, one of the most important non-essential elements in soils, can affect the development of plant roots and reduce the plant's growth rate. 12The total amounts of aluminium ranged from 6.5 to 10.7 mg/g dry soil, while 73-89 % were not extracted.This can be attributed to the high binding affinity of Al to humic substances. 12Only less than 1 % was leached by acetic acid.In step 2, 7-11 % were extracted except from soils G7 and G8 with 14 % and 19 %, respectively.In the oxidisable fraction 2.7 up to 12 % were extracted, with a median of 4.7 %.Similar extraction behaviour was also described by Li et al., 13 who analysed soils in mining and smelting areas in England.There the total Al amounts in soil were mainly higher, approx.20 g/kg (range: 7.6-35.4mg/g).No limit concentrations were found for Al in agricultural soils. 14

Cadmium
Cd in soil occurs either naturally or is derived from human activities. 15In the analysed soils cadmium levels  16 in the Incesu-Kayseri region (Turkey) 1-4 µg/g, 17 from < 0.1 µg/g to 1.5 µg/g for horticultural soils in New Zealand, 18 a study on agricultural soils in USA led to Cd concentrations of < 0.01 µg/g to 2.0 µg/g. 19In orchard soils in China total Cd amounts of 0.7 up to 1.8 µg/g were found by Li et al. 20 In Algeria in a study on agricultural soils Cd level of 0.30 µg/g (median) was obtained. 21Cadmium levels in horticultural and agricultural soils depend on the continued use of phosphate-fertilizers and zinc containing agrichemicals, where cadmium is present as impurity. 15he total amounts of cadmium in garden soils were expected to be lower those reported for plants grown commercially.Cadmium contamination limits agricultural use of soil, thus critical soil Cd concentration has been proposed in various countries, e.g.0.3 µg/g in China. 3According to French regulation maximum Cd soil level is 2 µg/g. 14In Canada the limit concentration is 1.4 µg/g. 2 Regarding the extractability of cadmium, 30-60 % were extracted in the first two steps (10-20 % and 20-40 %, resp.) and the remaining 40-70 % were found in the residue (step 4).

Chromium
Chromium is ubiquitous in the environment, like soil, water and air, and occurs in two forms, as Cr(III) and Cr(VI).The latter is more toxic and, in plants affects negatively leaf and root growth, inhibits certain enzymes and may cause mutagenesis. 22High chromium levels in soil are caused by anthropomorphic activities.The amounts found in the analysed soils samples ranged from 10 up to 63 µg/g with 70 % between 19 and 30 µg/g.The exceptions are G6 11 µg/g, G7 41 µg/g and G8 63 µg/g.The entire range is reported by Adriano 4 as normal chromium soil level, namely 10 to 50 µg/g depending on the bedrock concentration.In Algeria (Anaba region) the median total chromium level in agricultural soil was found to be 28.3 µg/g (Ref.21) and in the Incesu-Kayseri region (Turkey) 0.5-38 µg/g. 17Even the outlier G8 is below the permissible concentration of toxic elements for agricultural soils of China (200 µg/g), 3 French regulatory limit of 150 µg/g (Ref.14) and the Canadian limit of 64 µg/g for total chromium. 2Regarding the extractability of chromium from soil it was found that 1 % was extracted in step 1 and up to 10 % in steps 2 and 3.As expected, the highest amounts were found in samples G7 and G8.

Cobalt
The French regulations' limit of cobalt in soil is 30 µg/g (Ref.14) and in Canada 40 µg/g. 2 All soils analysed contain Co in lower concentration.In the investigated soil samples six of ten had cobalt levels between 4 and 5 µg/g.Higher levels were found in G9 (7.2 µg/g) and in G7 and G8 (approx.14 µg/g).G2 has less Co, namely 3.4 µg/g.Three Turkish studies came to similar results.In the soil samples from Kayseri region the concentration of cobalt is found to be less than 3 µg/g, 23 in the Incesu-Kayseri region 0.5-38 µg/g (Ref.17) and in the Yozgat region 4-8 µg/g. 16The China National Environmental Monitoring Centre reported cobalt levels of 15 µg/g and 26 µg/g. 24In plants cobalt is required by the urease enzyme.Cobalt has only a low affinity to humic substances, 12 and it is adsorbed to clay minerals in soil and geological origin by specific reactions. 25This is reflected in its extraction behaviour: 50-70 % of Co is extracted in step 2, approx.10 % in step 1 and up to 30 % in step 3.Only 3 % of the total cobalt were detected in the residue (step 4).

Copper
Copper is essential for plants but is toxic at higher levels.The total copper levels in the soil samples analysed ranged from 10 to 25 µg/g, except for G10 containing 156 µg/g.Similar levels were reported for Turkish soil samples, namely 12-27.5 µg/g from Kayseri region 23 and ranging from 16-26 µg/g in Incesu-Kayseri region, 7 and even slightly higher than in samples from the Yozgat region (4-14 µg/g), 16 and below those reported for agricultural soils (40 µg/g in market garden and 58 µg/g orchard), 18 what can be explained by the minor use of copper-based fungicides in private gardens than in horticulture.In agricultural soils in the USA levels from 0.3 up to 495 µg/g were found with a median of 18.5 µg/g. 19Algerian agricultural soils had a median copper level of 23.8 µg/g, 21 while higher copper levels were found in Italian vineyard soils (945 µg/g). 26Chinese and French regulations stipulate maximum copper level in soil of 100 µg/g. 3,14The Canadian Ministry of Environment limits Cu in agricultural soil to 63 µg/g. 2 Copper plays a role in plants' growth.Common garden plants, like tomatoes and strawberries need Cu up to 20 µg/g, thus all soils of this study met these needs. 5egarding the extractability it was found that copper was leached mainly from soil in step 2 (reducible fraction).A similar finding was reported by Whalley and Grant. 27In step 1, only extracts of G10 was the copper level above the LOD, which could be a result of the high total concentration in the sample.Cu has a high affinity for humic substances. 12

Lead
The total lead content found ranged from 16-50 µg/g.A wide range of its concentrations are reported for soils in New Zealand, 15 namely 11-251 µg/g and in the Kayseri region (Turkey) 16-83 µg/g (Ref.23) and in the Incesu-Kayseri region (Turkey) 13-34 µg/g. 16Elevated lead levels are attributed to the usage of lead arsenate as insecticides especially in orchards.Similar levels were

Table 1 .
Bedrock geology of the sampling area for the investigated soil samples

Table 3 .
Recoveries and limits of detection for the elements in the extract solution of all four sequential extraction steps Figure 2. Median total soil concentrations of elements analysed (soil treated with aqua regia).Croat.Chem.Acta 88 (2015) 23.