HEAVY METAL CONCENTRATIONS IN OATS AND THEIR AVAILABILITY IN SOIL FERTILIZED WITH COMPOSTS

Krzysztof Gondek¹, Barbara Filipek-Mazur¹, Pavel Tlustoš^2
1 Department of Agricultural Chemistry, Agricultural University of Cracow, Poland
² Department of Agrochemistry and Plant Nutrition, Czech University of Agriculture in Prague, Czech Republic

ABSTRACT

The use of waste materials for agricultural purposes even after their processing still poses various hazards involving among others a supply of heavy metal load into the soil environment. Therefore research was undertaken to determine the effect of composts of various origin on these elements their availability in soil and their concentrations in oats. The quantities of heavy metals supplied to the soil with applied fertilizers were small and except for cadmium did not cause any excessive accumulation of these elements in oats top parts. Irrespective of the studied element the largest number of heavy metals accumulated in plant root system. After a three-year period of research a considerable increase in the contents of cadmium and chromium forms available to plants were detected, the contents of available soil nickel grew slightly in the fertilized treatments and the contents of available lead forms in soil changed slightly. The investigations demonstrated a progressive process of soil acidification, which undoubtedly had a significant influence on increasing some elements, particularly cadmium availability to plants.

Key words: composts, heavy metals, oat, soil.

INTRODUCTION

The mechanism of heavy metal uptake by plants is complicated and usually constitutes a resultant of such processes as: cation exchange by cell membranes, inter cellular transport and biochemical reactions occurring in the rhisosphere.

There are two basic mechanisms of metal uptake by roots: passive and active. Heavy metal absorption by plants does not prove the indispensability of these elements but results from their soil concentrations [12].

Parent rock is the natural source of heavy metals in soil. In case of agricultural lands these elements may be additionally supplied in result of fertilization, e.g. with composts. However, heavy metal supply to the soil does not mean their uptake by plants. This process is considerably influenced by soil properties, such as: soil pH, sorption capacity, organic matter contents and soil heavy metal concentrations [4,5,15,26].

There are numerous works discussing the effect of individual soil environment agents on heavy metal sorption and mobilization [2,6,8,11,21] but the results of presented experiments pertain to the soils into which heavy metals were supplied as solutions or readily soluble salts. Soils, where waste substances containing heavy metals were used as fertilizers pose another problem. Therefore presented research aimed at determining the effect of compost of various origin on availability of these elements from soil and their content in oats.

MATERIAL AND METHODS

Three-year studies were conducted as a pot experiment in a vegetation hall of the Department of Agricultural Chemistry. PVC pots contained 5.50 kg of air-dried soil material. Composts from Krakow (two different batches) based on green wastes were used – composts (A) and (B) and also compost prepared from plant materials and produced in Nitra (Slovakia) – compost (C) and compost based on straw and produced in Prague (Czech Republic) – compost (D). The characteristic of composition of the composts were given in earlier publication [13]. Heavy metal concentrations in oats from pots where the composts were applied were compared with concentrations in plants from objects fertilized with mineral salts and farmyard manure.

Determination of chemical properties of farmyard manure and composts used for the experiment
Dry mass content was assessed in the composts and farmyard manure after samples drying at 70° in a dryer with hot air flow, and total nitrogen concentrations after sample mineralization in a concentrated sulphuric acid (V) using Kjeldahl method. In dried and crushed fertilizer samples phosphorus, potassium, magnesium, calcium, sodium and trace elements (Cu, Zn, Mn, Cr, Pb, Cd and Ni) were assessed after sample dry mineralization in a muffle furnace (at 450°C for 5 h) and ash solution in nitric acid (V) (1:2). Phosphorus was determined by Beckman DU 640 spectrophotometer at the wave length 436 nm; potassium, calcium and sodium were assessed by flame photometry (FES), magnesium and trace elements by atomic absorption method (AAS) using Philips PU 9100X spectrometer [20]. Results of compost chemical composition were given in Tables 2 and 3 and discussed in detail in the other publication [13].

In the vegetative experiment doses of compost and farmyard manure were determined on the basis of their nitrogen concentrations. Nitrogen dose was 0.8 g per pot. Phosphorus and potassium were balanced in all treatments to the highest level supplied with organic fertilizers – phosphorus to 1.41 g·pot^-1 as phosphorite meal, potassium to 1.21 g·pot^-1 – as water KCl solution.
The experiment was conducted on soil material with 32% of <0.02mm particles. Detailed characteristics of soil material have been presented in Table 4.

Periodical chemical analyses on plant and soil material
Oat was the test plant each year (Dragon c.v. in the first year and Kasztan c.v. in the second and third year). Plant density, supplementary fertilization rates and the length of growing period are given in Table 1. The plants were harvested at full maturity and the obtained biomass yield was separated into grain, straw and roots. Yields of individual parts were dried in a dryer with hot air flow (at. 70°C), weighed and dry matter contents were assessed separately for grain, straw and roots. Results of oats yielding were discussed in an earlier publication [13]. Dried and crushed plant material samples were dry mineralized in a muffle furnace (at 450°C for 5 h) [20]. Obtained ash was dissolved in nitric acid (1:2) and still hot replaced to volumetric flaks. In such prepared samples Cd, Cr, Ni and Pb contents were assessed by ICP-AES method in JY 238 Ultrace apparatus. In the soil material (sampled separately from each pot) pH was assessed in a suspension of soil and water, and in soil and 1 mol·dm^-3 KCl suspension, where soil to solution ratio was 1:2.5. Organic carbon was determined after sample mineralization in potassium dichromate by Tiurin method. Selected heavy metals (Cd, Cr, Pb and Ni) were extracted (for one hour) from soil with 1 mol·dm^-3 HCl solution at soil to solution ratio 1:10 [20].

Data analysis
Analyses of plant and soil material from the experiment were conducted in four simultaneous replications whereas on organic materials and initial soil in two replications. A sample of laboratory materials with known parameters was added to each series of the analyzed material and the result was considered reliable if relative standard deviation (RSD) did not exceed 5%.

The obtained results were elaborated statistically using one factor ANOVA and differences estimation by Duncan test at significance level α < 0.05 [24]. Standard deviation (SD) and variability coefficient (V%) were computed for the analyzed parameters.

RESULTS AND DISCUSSION

Results concerning chemical composition of composts, particularly heavy metal concentrations indicate a potential use of these materials as fertilizers (Tables 2 and 3). Therefore vegetative studies were undertaken to determine the effect of compost supplement to the soil on heavy metal availability to plants and their concentrations in oat.

Oat yield
Yields of oat grain in the first year of research were the largest on the object fertilized with mineral salt. The increase was statistically significant in comparison with yields from all treatments. From among the applied composts, those from Nitra (C) and Prague (D) (Fig. 1) markedly better affected the grain yield than the composts produced in Krakow. In the second year of the experiment the biggest grain yields were obtained on the object where  mineral salts and composts from Krakow (B) were used as fertilizers. On the other treatments grain yields were significantly smaller at non-significant differentiation between objects. In the third year of research comparable yields of oat grains were registered on treatments with mineral salts, farmyard manure and compost (A). Summary yield of oat grains for three years was the largest, like in individual years, on mineral salt treatment (102.74 g·pot ^-1) (Table 5). On the objects where composts were used for fertilization summary oat grain yield was comparable with registered on farmyard manure treatment (80.66 g·pot ^-1).

The yield of oat straw in the first year of research, like the yield of grain was significantly the largest on mineral salt treatment. Fertilization with farmyard manure and composts from Krakow (A) and (B) shaped oat yield on a level similar to determined on the control (Fig. 2). In the second year of investigations the largest straw yield was obtained on the object receiving compost from Krakow (B) (29.82 g·pot ^-1). The yields of straw from the other treatments did not differ significantly. A similar relationship was found in the third year of the experiment. Summary yield of oat straw was the largest on mineral treatment, whereas on the other compost treatments (except the compost B treatment) it was similar to obtained on farmyard manure (Table 5).

Oat root biomass after the first year of the experiment was not significantly different (Fig. 3), except the mineral salt treatment. In the second year markedly more oat roots (in relation to mineral salt treatment) was noted on all objects receiving organic fertilization, except the object where the compost from Nitra (C) was used. In the third year of the experiment significantly less roots were found only on the object fertilized with farmyard manure, whereas on the other treatments the differences were not statistically significant. The summary amount of root biomass  was the largest on mineral salt treatment, while compost fertilization shaped the root biomass yield on a level similar to registered on farmyard manure treatment (Table 5).

Heavy metal concentrations in oats
Mean cadmium content in oat dry matter (from the three years of the experiment) depended on plant organ and applied fertilization (Table 6). The highest cadmium content was detected in the roots. Oat grain and straw on individual treatments contained comparable amounts of this element but there was a significant diversification among treatments. In comparison with  the value of 0.15 mg·kg^-1 dry matter [15] assumed as permissible for grain destined for consumption, a considerably exceeded content was registered in plants from all treatments except the control. In view of fodder utilization, cadmium concentrations in the studied biomass did not raise objections. Significantly largest cadmium quantities, irrespective of plant organ, were assessed in plants fertilized with mineral salts. Presented results are in agreement with these reported by Logan and Chaney [18] and Gondek and Filipek-Mazur [12], who demonstrated a more intensive cadmium uptake in a pot experiment conditions than in field cultivation. According to Chaney [8], cadmium is the element which is not affected by so called soil-plant barrier, which means that plants tolerated in their organs (and do not reveal any toxicity symptoms) the amounts of cadmium which are normally noxious for animals consuming the plants. The fact mentioned above explains why no decline in plant yield was noted on treatment receiving mineral salts where this element concentration was the highest. On the other hand Kabata-Pendias and Pendias [16] report that at its increased uptake by plants, cadmium is mostly accumulated in roots and probably forms so called phytochelatines with sulfhydrol groups and proteins.

Chromium concentrations in oat grain and straw did not exceed 0.60 mg·kg^-1dry matter (Table 6). These were contents within normal [15]. Despite slight changes in absolute concentrations of this element both in grain (Coefficient of variation – V% =13) and straw (V% =11), the differences proved statistically important. In oats roots this metal content was several times higher than assessed in the top parts. A greater diversification was also detected among treatments (V%=32). Chromium concentrations in plants from mineral treatments may be explained by an effect of dilution in a bigger oat yield. Chromium concentrations in top parts of plants, which may be used for animal fodder (grain and straw) remained on a deficient level [14]. It was caused by a passive uptake of this element from soil depending on its availability to plants. Fast chromium reduction in the soil environment, according to Kabata-Pendias and Pendias [16] and Czekała [10] causes that it is usually hardly available to plants. The above opinion is only partially right, because as demonstrated by Authors' own investigations, the element was absorbed but its main portion was retained in plant roots. The mechanism of this process might have resulted from trivalent chromium affinity to form complexes and chelates with cell wall components. In the opinion of Czekała [10], the process limits chromium penetration into the cell and translocation to the top organs, which would explain its low concentrations in grain and straw.

Lead concentrations in oat grain did not exceed 0.25 mg·kg^-1dry matter and slight diversification among treatments was observed (Table 6). This amount did not raise any objections in view of grain destination for consumption or forage [14]. Straw lead concentrations proved between 2 and 5 times higher and the largest quantities were registered in the biomass from the object receiving mineral fertilizers and compost from Prague (D). The largest amounts of lead were assessed in oats roots and significant diversification among treatments was noted. This metal concentrations in oats roots depended on its availability in the substrate and plant organ. Like cadmium and chromium, lead was retained in oats roots which resulted from forming in the root system of sparingly soluble lead forms.

Nickel content in oat depended primarily on the analyzed organ (Table 6). This element concentrations may be ranked in the following order: straw < grain < roots. Both for oat grains and straw small, significant diversification among treatments was registered and the assessed values did not raise objections as to fodder usefulness of studied biomass [15]. Nickel root concentrations were on the average more than twice higher than in grain and over 8 times higher than in straw. Nickel concentration in plants, according to many authors [1,3] depends on total content of this element in soil, although higher values of correlation coefficients are obtained between the contents of soluble nickel forms in soil and their concentrations in plants. Nickel content in oats grain did not exclude this biomass in respect to its usefulness for consumption or the more so for fodder utilization [15]. On the basis of obtained results, no excessive accumulation of this element in organs of plants fertilized with composts was detected, which resulted from its small concentration in composts.

Selected soil properties
Soil fertilization with composts led to significant increase in its pH (determined in H₂O and KCl), particularly after the first year of the experiment, in comparison to the soil from unfertilized object and mineral treatment (Fig. 4 and 5). Statistically proved deacidifying effect was demonstrated for composts from Krakow (A) and (B). Progressively decreasing pH value was registered in the subsequent years, irrespective of the fertilization applied and the highest rate of soil acidification was noted on mineral treatment and compost from Nitra (C). An increase in pH value under the influence of compost application was also found by Lekan and Kacperek [17], Pinamonti [22] and Szulc et al [25]. Positive effect of composts, mainly those from Krakow, on the soil pH resulted primarily from higher concentrations of calcium in comparison with quantities assessed in farmyard manure and higher in comparison with the other composts. A relatively transitory positive effect of compost fertilization on soil pH (observed only in the first year) was due to the dose of these materials, as has been corroborated by results of research conducted by Szulc et al [25], who revealed increasing pH value with increasing compost dose.

Fertilization applied caused a raise of organic carbon content, however the increase was not statistically significant in comparison with the control and mineral treatment (except for the objects fertilized with composts A and B from Krakow) (Fig. 6). According to Szulc et al [25] application of composts based on urban wastes influenced organic carbon content and the increase in this component content in soil was proportional to applied compost dose. Authors' own investigations revealed a decline in organic C content in soil of each treatment and the content of this element in soil fertilized with composts from Nitra (C) and Prague (D) proved the most stabile.

Concentrations of heavy metals in soil
Contents of cadmium forms available to plants in soil of treatments increased in the subsequent years of the experiment and after the third year was on an average 12.3% higher in relation to the contents assessed after the first year (Table 7). Beside soil fertilized with compost (D) from Prague, after the first year of the experiment this element concentrations did not reveal any significant diversification. In the second and third year a greater diversification among objects was found for cadmium and the largest amounts of this element were extracted from soil fertilized with minerals.

Concentrations of available chromium forms did not reveal any greater diversification among treatments, irrespective of the year of investigations (Table 7). In comparison with the two first years of investigations, in the third year the content of this element available forms increased on an average by over 0.5 mg·kg^-1 dry matter of fertilized object soil. A lesser chromium availability (in each year of the experiment) was characteristic for soil fertilized with farmyard manure.

The content of lead forms available to plants did not grow significantly (in the 1^st and 2^nd year) in soil where composts were applied, except for soil fertilized with compost (D) from Prague (Table 7). In the third year of the research the largest quantities of lead extracted with HCl solution were assessed in soil receiving mineral fertilizers and compost (A) from Krakow.

The contents of nickel forms available to plants was increasing successively over the subsequent years of the experiment and was on average larger (in relation to the amount  assessed in soil after the first year) in the second year by 4.5% and in the third year by 13.5% (Table 7). A relatively small diversification among treatments was registered for this element content within years. The most of available nickel forms were noted in soil fertilized with farmyard manure (in each year of the research).

According to Rutkowska et al [23] compost supplement to soil in the first place caused a release of elements, which are bound in the compost in largest amounts in exchangeable and carbonate fractions. On the basis of  investigations conducted a significant increase in the content of available cadmium and chromium forms was found and slightly increased content of available nickel forms in soil of fertilized objects, whereas the content of available lead forms in soil changed only slightly after 3 year period of the experiment.

It should be emphasized that a raise in soil heavy metals available to plants is not the same for all elements. Heavy metal passing into soil solution depends on soil properties and the element itself [15]. The investigations revealed a proceeding process of soil acidification, which undoubtedly greatly influenced increased availability of some elements, mainly cadmium, to plants [9,19]. After three years of the experiment organic carbon content decreased visibly, which might have also contributed to better availability of some heavy metals.

CONCLUSIONS

1. The quantities of heavy metals supplied to the soil with applied fertilizers were small and except for cadmium did not cause any excessive accumulation of these elements in oats top parts.

2. Irrespective of the studied element the largest number of heavy metals accumulated in plant root system.

3. After a three-year period of research a considerable increase in the contents of cadmium and chromium forms available to plants were detected, the contents of available soil nickel grew slightly in the fertilized treatments and the contents of available lead forms in soil changed slightly.

4. The investigations demonstrated a progressive process of soil acidification, which undoubtedly had a significant influence on increasing some elements, particularly cadmium availability to plants.

REFERENCES

Accepted for print: 10.12.2008

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