EFFECT OF FERTILIZERS AND SORBENTS APPLIED TO THE SOIL ON HEAVY METAL TRANSFER FROM THE SOIL

Ján Hecl, Štefan Tóth
Plant Production Research Centre – Agroecology Research Institute in Michalovce, Slovak Republic

ABSTRACT

The research evaluates the immobility effect of some fertilizers and sorbents on heavy metal transfer from soil to soybean and on the mobile and mobilized fractions of the content of heavy metal in the soil. A pot experiment with the excessive content of toxic heavy metals was performed. Heavy metals, such as Cd, Pb, and Ni were applied to pot soil before soybean planting as a chloride salts in the following doses: Cd – 5 mg, Pb – 150 mg, and Ni – 100 mg per kg (ppm). The trial was established under two soil pH levels – natural soil pH = 6.5, and adjusted soil pH by sulphuric acid to pH = 5.5. Soil and plant (whole plants, empty pods, and seeds) samples were analyzed and the content of Cd, Pb, and Ni was established by the atomic absorption spectrometry (AAS). To analyse heavy metal content in the soil samples, the following extractants were used: 2M HNO₃ (nitric acid), EDTA (ethylenediaminetetra acetic acid), and CaCl₂ (calcium chloride). The data was statistically evaluated by ANOVA using Statgraphics. Applied fertilizers and sorbents decreased the content of the heavy metals in the soil in comparison with the untreated control. Heavy metal contents in soybean plant parts during the trial years were only a little different from the average of single plant parts. There was a statistically significant correlation between heavy metal content in the soil and soybean in Cd and Ni in the EDTA extractant. The best results from the point of heavy metal immobilization were obtained in the manured variant for the lowest heavy metal content. Soil pH range (5.5–6.5) during the trial period had no influence on the heavy metal content in soybean. The positive immobilization effect of humus on the heavy metals was observed under a lower pH value.

Key words: soybean, heavy metals immobilization, fertilizers.

INTRODUCTION

Industry development and concentration has a long-lasting negative influence on the environment. The effect of air and soil pollution negatively influences crop production. According to the report on the environmental status in 2004, the district of Michalovce was among the most polluted areas due to emissions from a fossil-fuel power plant and a chemical factory (Energetika s.r.o. Strázske and Cenon s.r.o. Strázske).

By the environmental regionalization of the Slovak Republic (from year 1997) on the basis of a complex evaluation of the status of the air, groundwater, surface water, soil, rock environment, biota and others factors, 5 categories of living environment quality were determined: 1. environment of high level, 2. suitable environment, 3. moderately disturbed environment, 4. disturbed environment, 5. strongly disturbed environment. As much as 52% of the district area belong to the 4^th category of the environmental qualitative evaluation, i.e. disturbed environment, which is 2.5 times more than the Slovak average. Even though in the last years by means of various enviro-technological activities and industry production the declining situation has been positively changed, the residues of contaminants will still be a problem.

The adjacent area of the chemical company Chemko Strázske was found to be contaminated not only by PCB, but also by heavy metals. This is the negative result of Slovak industrial development in the last century. The soil, in contrast with water and air, has the ability to eliminate contaminants by their interaction with inorganic or organic soil compounds so that their horizontal and vertical soil movement can be slowed down or eventually stopped. These natural eliminative abilities of the soil are used as an alternative method of remediation. Natural remediation includes a lot of physical, chemical, and biological processes which are able to reduce quantity, toxicity, mobility of the contaminant and its concentration in the soil without human interference. It is well known that every great soil group has its own ability to eliminate toxicity and mobility of contaminants. There are several methods of increasing natural ability of the soil to eliminate contaminants by means of manure or fertilizer, as well as sorbent application to the soil.

Heavy metal immobilization in soils by the changes of soil characteristics (soil pH, sorption colloidal complex, organic matter quantity and quality, and Fe/Mn quantity) can minimize heavy metal uptake by plants, which is a way of yield quality improvement. Regular liming and manure application decrease heavy metal availability to plants [9,36]. Willaert and Verloo [43] ascertained that fertilizers such as ammonium sulphate, ammonium nitrate, and urea increase the amount of exchangeable and water-soluble Cd in soils. Manures increase Cd adsorption in the soil [37]. Kulich et al. [27] and Dudka et al. [12] on the basis of experiments confirmed that Cd concentration in spring wheat decreases when the soil is manured. Manures and composts decrease effectively the availability of heavy metals in the soil with an average organic matter content and high concentrations of P and Fe [4]. Effective influence of manure was documented by Narwal et al. [31], who figured out that after manure application the concentration of Ni dropped down by 39%, 40%, 45%, and 35% in carrot, fenugreek, spinach, and winter wheat, respectively. Toxicity reduction of heavy metals by means of organic matter is explained by their adsorption on active surfaces. Concentration of Cd in kernel and straw of spring barley was reduced by the application of sodium humate (humic acids) to the soil [13]. The other possibility of reducing heavy metal mobility is considered to be the use of various soil sorbents and improvers – ionex, bentonite, and zeolite [18]. The use of natural zeolites in agriculture is very broad. They can also fix heavy metals due to their high sorption capacity. Chlopecka and Adriano [6] found that zeolites can reduce the accumulation of Cd, Pb, and Zn by 72%, 81%, and 41%, respectively. The other positive effect of zeolite application is summarized by Knox et al. [25]. Effective straw ploughing down into contaminated soils was researched by Tlustošs et al. [40] who found a decrease in Cd and Zn concentration in spinach by 45% and 25%, respectively.

This research is focused on the up-to-date problems with heavy metal soil contamination, as well as soybean, which is recognized as a strategic crop worldwide. The aim of the study was to evaluate the immobility effect of some fertilizers and sorbents on heavy metal transfer from soil to soybean and on the mobile and mobilized fractions of heavy metal soil content.

MATERIAL AND METHODS

The effect of Licesin, Klinofert, Zeomix – NPK, farmyard manure, and inorganic sorbent – zeolite on the availability and mobility of some heavy metals was tested in a pot experiment (plastic pots of 25 cm in diameter) with soybean, cultivar OAC Vision over 2003–2005. The used fertilizers of Slovak provenance were chosen due to their ability to immobilize heavy metals. The soil for the pots was taken from the topsoil of Gleyic Fluvisol – clayey soil and Eutric Flufisol – sandy-loam soil. The depth of pot soil was 22 cm. The bottom of the pot was covered by a drainage layer of 4 cm. Pot soil was sieved and mixed, totally homogenized before pot filling. The soil for every pot was weighted (9 kg), so that every pot contained the same amount of soil. Soil surface was 5 cm below the pot brim. Four pots were prepared in the same way for every variant. The pot trial was established at the experimental station in Milhostov on 12th May 2003, 11th May 2004, and 4th May 2005.

The heavy metals were applied in the form of chlorides in the following doses: Cd – 5 mg, Pb – 150 mg, and Ni – 100 mg. The doses were counted in mg·kg^-1 (ppm). The natural content of heavy metals in the soil before their application is summarized in Table 1. The trial was established under two soil pH levels – natural soil pH = 6.5 and adjusted soil pH by sulphuric acid to 5.5. The pots were irrigated on a daily basis with 98 ml of water per pot, that is 2 dm³ per m². A three-factor experiment was established by a block method with four repetitions. The experiment included 288 plastic pots together during the trial period. The factors and their levels are described in Table 2. Fertilizer doses were applied basing on soybean recommendations.

Characteristics of the used fertilizers and sorbents:

• licesin is a complete sustainable energy fertilizer, conditioner which has all the positive characteristics of an artificial fertilizer. Its application is able to decrease the accumulation of heavy metals in the soil as a result of their bonding to humic acids present in fertilizers. Important advantages of Licesin are that, due to the presence of micro-organisms, other soil-organic activities which improve the content and structure of the soil are accentuated. It is made on the basis of secondary materials from the dairy industry. By the producer Licesin the content is: C – 34%, N – 8%, P – 0.20%, K – 1.00%, Ca – 2.50%;

• klinofert: three-component mineral fertilizer based on natural zeolite – clinoptilolite. It is a type of fertilizer with slow displacement activity and assumptions of ecological effects. It fixes heavy metals in its grid so that they cannot be absorbed by plants. Klinofert content: N – 6.0%, P – 1.97%, K – 6%;

• organic sorbent: farmyard manure with nutrient contents: C_ox – 14.3%, N – 1.5%, P – 0.26 mg·kg^-1, K – 1.49 mg·kg^-1, dry mass – 22%;

• inorganic sorbent: Zeolites of the clinoptilolite type, as natural, inert and nontoxic materials are widely applicable in the contemporary notion of ecological agriculture. Their structure is ideal for sorption and ion exchange processes. Natural zeolites can also be used for improving the physical properties of soils and for the treatment of contaminated soils.

Soybean plants were harvested on 26th August 2003, 30th August 2004, and 5th September 2005. Soil and plant samples were taken at harvest every year. Soil and plant (whole plant, empty pod, and seed) samples were analyzed and the content of Cd, Pb, and Ni was established by the atomic absorption spectrometry (AAS). To analyse heavy metal content in the soil samples, the following extractants were used: 2M HNO₃ (nitric acid), EDTA (ethylenediaminetetra acetic acid), and CaCl₂ (calcium chloride). Detailed description of the chemical methods used for the analyses can be found in the work of Hecl [17]. Plant material was analyzed after its mineralization by flame or flameless AAS.

In total, 5184 pieces of data were obtained, which were then divided into two separate files, one half included the data measured in plant material and the other half in the soil. The following structure was used: 2 great soil groups x 2 pH levels x 3 heavy metals (Cd, Pb, Ni) x 6 variants of immobilization x 3 years x 3 plant parts (regarding the plant only) or 3 extractants (regarding the soil only) x 4 repetition. The data were tested by ANOVA using Statgraphics 6.0 with further factor influence evaluation according to Dubovský [11].

RESULTS AND DISCUSSION

The average content of heavy metals Cd, Pb, and Ni was 29.8 in the soils, 4.82 in soybean seed, 9.32 in the whole soybean plant, and 1.63 in empty soybean pod. Replications showed statistically nonsignificant influences under the level of significance of 0.9997 for both soil and plant samples. This shows that the experiment was performed precisely. According to literature references [5,16], the value of pH/KCl is the most important soil property that influences the mobility of metals in the soil. It is not unambiguous for all metals. After Barančíková [1], the metals were divided into two groups and then commonly judged. In one group, Cd, Cr, Pb, Zn, and Ni were given. In our work on that account metals were statistically evaluated commonly because all three metals (Cd, Pb, and Ni) at pH lower than 5.5 have maximal mobility in soil environment.

Heavy metals in the soils
The results of ANOVA for the heavy metal content in both soils are presented in Table 3. An extractant was found to be a dominant source of variability. The extractants caused almost 64% of heavy metal content variability in the soils. More than 1% of variability was caused by a great soil group. The immobilization variants caused 0.23% of variability only, but it was statistically significant. An influence of the experimental year was found to be nonsignificant. The used extractants revealed very different values of heavy metal content. The average numbers of the measured values were as follows: 0.30 in CaCl₂, 18.3 in EDTA, and 70.8 in HNO₃ (ppm). These results confirmed that a selection of extractant must be done very purposely. The extractant CaCl₂ is suitable for the mobile forms of heavy metals directly available to plants, EDTA for mobilized heavy metals in the soil, and HNO₃ for the potentially available forms of heavy metals in soil. Potentially mobilized content of heavy metals in soil represents heavy metal supplies that could be mobilized by a change in soil-ecological conditions, applied fertilizers, etc. This form is the most important from the point of biotoxicity [3].

There were important differences between the great soil groups. Each one obtained 1296 pieces of data (ppm). The average content of the heavy metals was 26.96 in clayey soil and 32.68 in sandy loam soil. 864 pieces of data (ppm) were measured in one experimental year. The average value in 2003 was 29.6, in 2004 – 29.7, and in 2005 – 30.0. The same amount of data – 864 was measured for every tested heavy metal. Cd content revealed the lowest value of 2.21 on average, Ni average content was 31.2, whilst Pb average content was the highest – 56.0. Each of the six immobilization variants received 432 pieces of data. Variant C₅ with manure application had an average value of the heavy metal content of 25.7, variant C₄ with Zeomix – 26.9, variant C₆ with zeolite – 30.4, variant C₂ with Licesin – 30.9, and variant C₁ without immobilization – 34.4. Similar positive effect of bentonite, zeolite, and compost on Pb content decrease in Ca(NO₃)₂ extractant was found by Geebelen et al. [15].

Heavy metal in soybean plant
Statistical results of heavy metal content in soybean plant are presented in Table 4. Dominant source of variability in heavy metal content was a kind of heavy metal – 72%, whilst different soybean plant parts caused 27% of variability. Two great soil groups caused 0.28% of variability, but statistically highly significant. Experimental year had no significant influence on the total variability. Based on 864 pieces of data (ppm), each of the measured soybean plant parts had a different heavy metal content. The lowest content was measured in soybean pod – 1.61 on average, in seed – 4.77 on average, and in the whole plant – 9.17 on average (Fig. 1).

There were no important differences between the great soil groups based on 1296 pieces of data (ppm) per one great soil group. The average value was 5.43 in clayey soil and 5.05 in sandy loam soil. The average heavy metal content in soybean seed on clayey soil was 5.62, in soybean pod 1.88; on sandy loam soil it was lower: 4.01 in seed and 1.41 in pod. On the contrary, average heavy metal content in the whole soybean plant was higher on sandy loam soil – 9.77, against 8.87 on clayey soil.

The influence of year, where 864 pieces of data (ppm) were obtained, was quite similar in terms of heavy metal content in soybean seed, pod, and the whole plant. The average content in 2003 was 5.08, in 2004 – 5.28, and 2005 – 5.40. The content of the heavy metals in the whole plant in the experimental years was not very different from the average value. There is not enough literature sources regarding year influence on heavy metals in plants.

Every experimental heavy metal (Cd, Ni, and Pb) obtained 864 pieces of data (ppm). The lowest content values were obtained for Pb – 1.54 on average, higher values for Cd – 1.70 on average, and the highest values for Ni – 12.5 on average.

Every variant of immobilization obtained 432 pieces of data to be evaluated (ppm). The average content values of the six variants were as follows: variant C₅ with manure application – 4.35, variant C₂ with Licesin – 4.64, variant C₆ with zeolite – 5.21, variant C₃ with Klinofert – 5.23, variant C₄ with Zeomix – 5.42, and variant C₁ without immobilization had the highest content – 6.69.

Cd, Ni, and Pb contents in the soil and plant according to the different variants
Average heavy metal contents in the soil and plant are presented in Fig. 1. The used soil extractants revealed different heavy metal contents. As can be seen from the graph, the Cd and Ni contents in the plant correlate with their contents in the soil in EDTA. The experimental results correspond with the findings of Davies et al. [10] and Soon and Bates [38], who documented close relation between heavy metal content tested in the extractant EDTA and heavy metal uptake by plants. In terms of Pb, its content in the soil in the extractant CaCl₂ correlates with its content in plants. These findings are in accordance with the sources published by Houba et al. [23], Whitten and Ritchie [42], and Horburg and Brümmer [19,20,21,22].

As can be seen in (Fig. 2), the highest average content of Cd, Ni, and Pb was found in variant C₁ without any soil activators. Similar results were obtained in plant from the same variant. Generally favoured variant from the point of heavy metal content in extracting forms of the soil and the total content in the plant was the variant C₅, where the lowest values were measured. Similar positive influence of manure on Cd content decrease in oat, maize, babley, and poppy was presented by Tlustoš et al. [39]. Also Gaj and Schung [14] ascertained that organic matter application to contaminated soils (mostly by Cd and Zn) caused a reduction in the uptake into the above-ground plant parts of soybean and winter wheat.

General trend of favouring variants C₂–C₆ before variant C₁, an untreated control, was kept for all the observed heavy metals. Despite a higher content of Pb in the soil, the soybean plant content of Pb was the lowest, as presented in Table 5. If the measured values of Pb content in the different extractants were averaged and the number (63.6) was set as 100%, the content in soybean seed was 0.35 ppm (0.55%) only, 1.02 ppm (1.61%) in soybean pod, and 6.14 ppm (9.65%) in the whole soybean plant. Low Pb accumulation in the above-ground plant parts is connected, according to Chreneková [7], with its minimum translocation abilities, and this corresponds with the findings of Chreneková et al. [8] when it was ascertained in the trial with spring barley that the dosed of Pb from 5 to 100 mg had no phytotoxic effect. Our findings regarding heavy metal contents in soybean plant correspond with a sequence of heavy metal contents in radish, triticale, and spinach according to transfer coefficient Cd > Ni > Pb described by Němeček and Podlešáková [32]. There are significant differences among plant species regarding heavy metal content – Turan and Esringü [41].

The average content value for Ni was 37.1, 39.9% for soybean seed, 11.6 for soybean pod, and 73.1% for the whole plant. It can be concluded that Ni is taken up more by soybean than Pb. The average content value for Cd, from Table 5, is 2.52, 77.9% for seed, 72.8% for pod, and 140% for the whole plant. It can be stated that Cd has a twice as high rate of accumulation as Ni in soybean. High accumulation of Cd by plants and a close relationship between its content in the soil and plant was confirmed by Krauss [26], Passdar [33] and Matúšková [28]; soil and Cd can be accumulated in plant at a high rate.

Influence of year and soil properties
Based on correlation analyses between year influence and heavy metal content in the soil and plant, it can be concluded that year has a negligible or no influence on heavy metal content in the soil and a slight influence on heavy metal content in plant. Increasing sum of precipitation, as well as the experimental time, increase heavy metal content in plant (r = 0.11 seed, r = 0.06 plant, r = 0.07 empty pod). In the case of increasing temperatures, opposite effect was observed. The higher the temperature during the experimental time, the lower the heavy metal content in plant (r = – 0.11 seed, r = – 0.05 plant, r = – 0.07 empty pod).

Based on the influence of the experimental time, sum of precipitation, and average temperature, the usefulness of variants C₂–C₆ for decreasing heavy metal content was confirmed in each of the experimental years.

The range of pH values was too narrow to get a noticeable influence. The influence of pH values is shown in Fig. 3. The highest influence of the two different pH values was observed for Ni soil content. However, the two tested pH values had no significant influence on the content of Ni in plant. Mühlbachová et al. [29] studied various relations among pH, microbial biomass, and heavy metals. They found a few significant relationships between heavy metal content and pH or microbial biomass only.

During the experimental time, it was confirmed that increasing humus content decreases heavy metal content (average of all extracted forms used) in the soil (r = – 0.07) and also in the whole plant (r = – 0.06); on the contrary, in the case of pod (r = 0.05) and seed content (r = 0.08), increasing humus content increased heavy metal content. This trend was observed during the whole experimental period and for every variant of decontamination, as well as the interaction variant x heavy metal, where the correlation was intermediate and strong. It is interesting that the positive influence of humus was observed under the lower pH value of 5.50, under the higher pH value of 6.50 humus did not influence heavy metal content so positively. All functions of humus are not well known, as is case of the pH value. Johnson and Jones [24] showed that the amount of organic carbon is a more important parameter than the pH value in acid soils, which is different from the other authors – Pardo and Guadalix [34], and Naidu et al. [30], who confirmed that Cd sorption is highly dependent on the pH value. Barančíková [2] stated that her research did not find any correlation between organic carbon in the soil and Cd sorption.

Humus functions were more positive in clayey soil in comparison with sandy loam soil. Different humus effect on heavy metal content in the soil can be explained by the use of different extractants. Heavy metal content in the soil was decreased the most significantly in extractant CaCl₂ (r = – 0.23), less significantly in HNO₃ (r = – 0.15), and the least significantly in EDTA (r = – 0.04).

Having evaluated the effect of humus on heavy metal content (average of all extracted forms used) decrease globally, it can be stated that the more the humus in the soil, the lower the heavy metal content in the following order: Pb (r = – 0.11), Ni (r = – 0.08), and Cd (r = – 0.05). On the contrary, the more the humus in the soil, the greater the heavy metal content in soybean seed in the following order: Cd (r = 0.57), Ni (r = 0.36), and Pb (r = 0.20). It can be concluded basing on the results that the effect of humus (as one of the immobilization factors) is not as definite as expected. The other factor that must be taken into account is the mobility of the tested heavy metals.

From the evaluated influence of variants C₁–C_6, only positive effect of humus on heavy metal content in soybean seed (Table 6) was found in variant C₂ at Pb (r = – 0.21). This finding was confirmed by Zaujec [44]. Stable bond between organic matter and Pb²⁺ is caused by a high value of stability constant with organic chelatized groups. This finding did not correspond with Geebelen et al. [15] because in his experiment with sorbents such as bentonite, zeolite, compost, lime, power plant powder, and slug, he did not find, with the exception of zeolite, Pb decrease in lettuce. It can be said that an influence of humus is still dubitable. The decrease in heavy metal content in soybean seed results not only from the influence of humus, but also from other soil parameters. Besides humus, the influence of the nutrients P, K, and Mg was tested in our experiment. While the variants with P and Mg influenced the content of Cd, Ni, and Pb in soil or plant similarly, in case of K the situation was opposite. P and Mg with the content of Cd, Ni, and Pb in the soil correlated negatively; conversely, a positive correlation was found between P and Mg and heavy metal content in soybean seed. At the values of total correlation, K decreased the content of Ni in soybean seed (r = – 0.37) significantly, less significantly the content of Cd (r = – 0.34), and the least significantly the content of Pb (r = – 0.24). The character of total correlation between K and the content of Cd in the soil (r = 0.00), Ni in the soil (r = 0.05), and Pb in the soil (r = 0.05) was quite similar. The positive effect of K on the content of Cd, Ni, and Pb in soybean seed was found to be more significant towards the stronger correlation. From the evaluation of the effect of K on the content of Cd, Ni, and Pb in soybean seed in variants C₁–C₆ it resulted that a more positive effect of the observed variants in a stronger correlation in comparison with higher noted values of the total correlation was determined.

The balance of opposite influences of humus and K is, in our opinion, the key to the global positive effect of manure on heavy metal content in the soil and plant. Positive influence of the tested decontamination variants C₂–C₆ in comparison with variant C₁ – the untreated control can be confirmed. Trials with organic materials aimed at the immobilization of soil risky compounds show their different effect on soil processes, microelement mobility in the soil, and their transfer in the soil. These effects depend on specific qualities of the applied materials, and microelement bonds in the soil and plants [35].

CONCLUSIONS

1. Different extracts caused almost 64% of the variability of the obtained values for heavy metals. The effect of year and soil type was not significant.

2. Applied fertilizers and sorbents decreased the content of the heavy metals in the soil in comparison with an untreated control.

3. The contents of heavy metals in soybean plant parts during the trial years were only a little different from the average of single plant parts.

4. There was a statistically significant correlation between heavy metal content in the soil and in soybean between Cd and Ni extracted by EDTA.

5. The best results from the point of heavy metal immobilization were obtained in the manured variant where the measured heavy metal content was the lowest. It can be concluded that the influence of year was unimportant on heavy metal content in the soil and slightly important on the content in soybean plant parts. Soil pH range (5.5–6.5) during the trial period had no influence on heavy metal content in soybean. The positive immobilization effect of humus on the heavy metals was observed at a lower pH value; under a higher pH value the effect was not observed.

6. Positive immobilization effect of the tested sorbents and fertilizers, as well as farmyard manure used altogether was found as a kind of balance towards the single effects of organic matter, potassium, phosphorus, and magnesium.

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Accepted for print: 5.06.2009

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