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UDC 632.95.024.4:546.47/.49:001.891.57
THE STUDY OF PHYTOTOXICITY OF CADMIUM IONS ON THE MODEL SYSTEMS
Basov Y.V., Kozyavina K.N., Associate Professors Orel State Agrarian University, Orel City, Russia
E-mail: [email protected]
ABSTRACT
The biological activity of technogenically polluted soil and phytotoxicity of cadmium ions have been studied on the model systems. The content of heavy metals in the soil, in the aerial and root phytomass has been determined.
KEY WORDS
Technogenically polluted soil; Biological activity; Phytotoxicity; Ions; Heavy metals; Model systems.
In natural conditions the soil and plants contain heavy metals (HMs), however, their excessive accumulation can result in the destruction of the integrity of natural complexes. Cadmium is among ten elements which are recognized as the main pollutants of the biosphere because of its toxicity and ability to be accumulated in food chains.
According to the rough estimation, at present in the world there have been accumulated (in mln t) Cu-300, Zn-200, Cr-70, Pb-20, Ni-3,5, Cd-0,6, Hg-0,5. The environment has never known such a volume of HMs on the Earth's surface. One of the most dangerous toxicants of the environment is cadmium (Cd). In the natural environment cadmium is found only in very small quantities - that is why his poisonous effect has been revealed only recently.
The sampling ecological and toxicological examination of the agricultural soils held by the FSBE "Verkhoviekhimradiologia» in 2010-2011 near the main highways and railways in the Orel region, revealed HMs pollution (Cu, Zn, Cd , Pb). Thus, in Verkhovskiy district, in the area of 2585 ha., Zalegoshchenskiy district (3667ha.), Livenskiy district (1960 ha.), Novoderevenkovskiy district (315 ha.) and Krasnozorenskiy district (1441 ha.) the significant excess of Maximum Permissible Concentrations of cadmium (1mg/kg) was revealed.
The maximum permissible daily doses (MPDD) of different HMs coming into a human body with food and water, vary widely from 0.1 mg (Hg) to 5 mg (Zn). The comparison of MPDD with the mass of HMs, which are in the biosphere, and a simple calculation suggest that these substances can have multiple poisoning potential for the humanity.
MATERIALS AND RESEARCH METHODS
The aim of this work is to study the biological soil activity in the technogenically-damaged area, depending on the growth of crops and to study the mobility of cadmium compounds in soil and ways to reduce its phytotoxicity.
Main tasks:
- to study the effect of different types of crops on the biological activity of technogenically-damaged soils,
- to identify the relationship between the content of mobile forms of HMs both in soil and plants;
- to study the absorption capacity of the soil under the influence of various factors (organic matter content, the concentration of cadmium ions, the pH of the environment);
- to study the effectiveness of various ways to reduce the phytotoxicity of cadmium in the "soil-plant "system by the bio-indication method.
The biological soil activity was determined by the application method.
The sterile thin unbleached linen cloth was sewn on the polymer film (the film segment was 10 cm wide). The length depended on the horizon tested: for the arable one it was 20-25 cm. The film was sterilized with alcohol and the cloth was ironed. The cloth was tightly pressed to the vertical side of the fresh soil cut of 25-30 cm deep, then the cloth was powdered with the soil, and the cut was covered with the soil. The top edge of the cloth must be put into the soil for 3 - 5 cm. The experiment replication is held 3- 5 times.
In a month (in 2-3 months) the cloth was carefully taken out, washed from the soil and decomposition products, dried and weighed. To define the process dynamics some clothes were put into the soil simultaneously and taken out at the certain intervals of time. The intensity of cellulose destruction process was determined by the weight losses.
The experiment was carried out on the test plot of the Orel State Agrarian University. The soil of the plot was technogenically damaged. Near the plot there was a highway, some agricultural and constructional works were being done at that time.
The following crops were planted on the test plot: two-row barley, soft wheat, common barley, durum wheat, oats, lupine, oil radish, millet and buckwheat. A plot of herbs served as a control plot, where there were not any melioration measures. The experiment was repeated 4 times.
The climatic conditions of the Orel region favor the cultivation of leguminous and grain crops. The favorable conditions for their cultivation are water (leguminous crops need water more than grain crops), heat, oxygen, nutrients (leguminous crops need more nutrients), light, adequate supply of phosphorus, potassium, calcium, magnesium, boron, molybdenum, nitrogen (for grain crops). In summer 2011 the sum of average daily temperatures higher than 10 degrees C was 2200-2500 C and the weather conditions were not favorable for leguminous and grain crops cultivation.
The estimation scale of biological soil activity in intensity of cellulose destruction was used (% decomposed cloth during the vegetation season) Zvyagintseva (1980): very low < 10, low 10-30, the average 30 - 50, high 50 - 80, very high > 80.
According to the scale it can be noted that the biological soil activity on this plot is:
- very low under the following crops - two- row barley, soft wheat, common barley, oats, lupine, oil radish;
- low - durum wheat, millet, buckwheat.
In comparison with the control plot the highest soil activity was observed under buckwheat and millet.
The average data of the biological soil activity under agricultural crops are shown in Table 1.
Table 1 - The indicators of biological soils activity
Crop %
Two-row barley 9,97
Soft wheat 6,9
Common barley 8,05
Durrum wheat 14,7
Оats 3,7
Lupine 4,45
Oil radish 4,25
Millet 23,15
Buckwheat 29,3
Control 16,55
Having summarized the results of the research one can conclude that the biological soil activity is being formed slowly, because of its technogenical damage. Ecosystem stability is connected with the soil stability , i.e. the ability of the soil to perform its ecological functions under the conditions of technogenesis. Heavy metals (HMs) are considered to be the most dangerous among numerous pollutants of the environment.
Accumulation of HMs in phytocenoses cannot be infinitely large, and reaches certain concentrations, which have the upper and lower limits. Beyond the threshold concentrations the plants show both physiological and morphological changes.
The toxicant ions content in the soil and their translocation into plants is a complex process, which depends on many factors. To understand the mechanism of action of each of them, one should study the influence of individual factors on the phytotoxic action of TMs under the experimental conditions.
The soil samples from the test plot were taken along the diagonal of the field before sowing and then analyzed. The results of the analysis of the soil samples from the test plot of Agroecology and Environment Protection Department of the 17.04.2010 are shown in Tables 2 and 3.
Table 2 - The results of soil samples analysis from the test plot
№ № soil samples The results of the analysis
Zn, mg/kg Cu, mg/kg Pb, mg/kg Cd, mg/kg
1 1 0,32 1,0 1,32 0,032
2 2 0,97 0,85 2,13 0,057
3 3 0,43 0,63 3,16 0,151
4 4 0,26 0,83 1,18 0,134
5 5 0,11 0,88 2,08 0,028
6 6 Less than 0,0025 0,92 1,44 0,118
7 7 0,12 0,94 2,10 0,037
8 8 Less than 0,0025 0,80 2,25 0,173
9 9 Less than 0,0025 093 2,86 0,043
Table 3 - The results of soil samples analysis from the test plot
№ № soil samples P2O5, mg/100r of the soil K2O, mg/100g of the soil Humus, % pH
1 1 12,0 5,0 2,97 4,9
2 2 13,2 7,7 2,54 5,1
3 3 12,3 8,2 1,95 5,1
4 4 11,3 6,8 2,26 5,0
5 5 11,7 6,3 2,15 5,0
6 6 14,2 7,3 1,95 5,2
7 7 13,4 6,8 2,02 5,2
8 8 12,7 6,8 2,40 5,2
9 9 16,6 8,2 1,95 5,5
10 10 14,8 10,4 2,02 5,6
The selection and agrochemical research of the soil samples were carried out in accordance with the generally accepted and recommended techniques (Workshop on agricultural chemistry, ed. Mineeva V.G.,2001), and «Methodological guidelines for integrated agrochemical examination of the agricultural soils» and «Methodological guidelines for conducting field and laboratory research under the control of environmental pollution by metals».
HMs content in the soil, aerial and root phytomass were determined according to the following methods:
- method of taking the measurements of mass concentrations of the gross forms of TMs in powder samples of the soils by the x-ray fluorescence method on the spectrometer «SPECTROSCAN» (MVI-05-97) Spectron SPU».Petersburg,1997.
- method of measurement of mass content of mobile forms of metals (copper, lead, zinc, nickel, cadmium, cobalt, chromium, manganese) in the soil samples by means of the atomic absorption analysis. PD 52.18.289-99. The State Committee of the USSR of Hydrometeorology. Moscow. 1990. The plant samples analysis was conducted by the inversion voltammetry method on the polarograph AKB-07 MK.
The mathematical treatment of the research results was done on the computer with the software package «Polar 4.1». The research was conducted by the recommendations of
B.A.Dospekhov (1985), V.I.Peregudov (1978), B.A.Yagodin (1982). Record of the aerial and root biomass was conducted by the methods of G.S. Posypanov (1991).
The experiment was carried out on the test plot of the Department «Agro-ecology and Environmental Protection» of the Orel SAU and in the protected ground of the Orlovsky SSE ASRI of Leguminous and Cereal Crops of the RAA in 2010-2012. In the plot experiment cadmium translocation was studied in agrocoenoses in the sowing of oats (a variety «BORETS»), buckwheat (a variety of «DEVYATKA») and white mustard. The leached black earth soil of the Livensky district of the Orel region was used in the model systems. According to the agrochemical indicators, it is characterized by the high content of the organic matter (humus), which is of great importance for soil fertility and plant nutrition.
pHsalt 6,5; humus content 6.2%; amount of absorbed bases 46,0mg.-EQ./100g; hydrolytic acidity 0,26mg.-EQ./100g; exchange acidity 0,01mg.-EQ./100r.The content of the gross forms of cadmium (Cd) is 0,29 mg/kg, the mobile ones is 0,08mg/kg extracted by the acetate-buffer solution.
27 vessels were prepared for the experiment. Oats were selected as a test culture (variety «BORETS»), as it can respond quickly to the absorption and accumulation of HMs.
The experiment scheme: 1. Control (Background); 2. Soil +5 APC Cd; 3. Soil + 5 APC Cd+humus; 4. Soil + 5 APC Cd+lime; 5. Soil + 5 APC Cd+ sodium acetate; 6. Soil + 50 APC Cd; 7. Soil + 50 APC Cd +humus; 8. Soil + 50 APC Cd + lime; 9.Soil + 50 APC Cd + sodium acetate.
The experiment was carried out three times. According to the approximate permissible concentrations (APC) of chemical substances in the soil (SN 2.1.7.2042-06) the gross content of Cd in the soils close to neutral, neutral (loamy and clay) when pHkcl>5,5, the APC value (taking into account the background) is -2,0 mg/kg.
RESULTS AND DISCUSSION
The solutions of TMs salts of CdSO4(54%Cd) were applied into the soil in concentrations of 5 and 50 APC, that was for Cd -10 mg/kg and 100 mg/kg of soil , taking into account the background respectively. The process of liming reduced the content of HMs mobile forms in the soil and contributed to their detoxification. The high content of the water-soluble organic compounds in the soil solution did not lead to the increase in migration ability of metals due to the formation of resistant organic-mineral complexes. Using sodium acetate resulted in the increasing pH of the environment, and under the conditions of alkalinization metal ions became very active, thus decreasing the total amount of humus.
After growing the bioculture for 30 days the soil was put to the chemical analysis for the content of gross and mobile forms of Cd (II) by the method of atomic-absorption spectroscopy to determine the percentage of TMs absorption. The results are shown in the Diagrams 1 and 2.
30 25 20 15 10
5 0
1 2 3 4 5
MODEL
H 5 OAK H 50 OAK H pH Figure 1 - The content of gross forms CD (II) in the soil, mg/kg
The results obtained indicate that with the increase in pH of the environment the gross amount of cadmium decreases. When adding Cd2+ in the soil solution in quantities multiple of 5 and 50 APC, the largest quantity of metal is absorbed by the soil in case of biohumus and lime application, as well as in the clean soil. The least amount of cadmium is absorbed by the soil when using sodium acetate.
The behaviour of Cd (II) differs significantly from the behaviour of other heavy metals. Its mobility is high in all environments, even when applying lime. The increased mobility of cadmium and as a result less safety of a plant organism from the excess ions of this element is one of the reasons for its strong toxicity.
Biohumus (Mog.3, Fig.2) dramatically reduces the quantity of mobile forms of cadmium in the soil and, thereby their toxicity. Lime when applied into the soil in less quantities than biohumus , binds ions of Cd2+ and reduces their phytotoxicity.
12 10 8
6 4 2 0
Model
H5 0AK H 50 OAK U pH
Figure 2 - The content of mobile forms of Cd (II) in the soil, mg/kg
In the variants of the experiment the exchange acidity remained constant and made -
0,01 mmol/100g. The value of saturation degree with the bases is taken into account when liming the soil. In our experiment, the value of the amount of the absorbed bases in different variants varies from 44.7 to 45,8 mmol/100g in comparison to the control, that shows a minor need for liming (Table 1).
During the studies the sprouts in variants 2 and 3 enriched with Cd2+ and biohumus looked much better. They differed by the longer leaves and branching root system. On their background control sprouts also looked rather good, and plants with sodium acetate were the most undersized. The experiment data are shown in Figures 3 and 4. The intensive growth of oats in the control soil, as well as in the vessels with lime and biohumus with the cadmium concentration of 5 APC were observed on the 15th experiment day.
Plants with sodium acetate (var.5 and 9)- turn yellow and wither despite the systematic watering and care. The oats look the most oppressed in the soil with the content of ions of Cd2+ 50 APC.
When heavy metals and CH3COONa (to create a more acidic environment) are found simultaneously in the soil, the growth of the root system and top system of a plant visually stops.
On the 28th day in the general background the plants of the control plot looked much better. The culture grown in the “soil - heavy metals - lime” system turns yellow and withers dramatically. The oats grown in the “soil - heavy metals - sodium acetate” system (var.9) die on the 30th day of the experiment. The partial death of plants is observed in the experiments with heavy metal salts.
High concentration of heavy metals in the “soil - heavy metals - acetate” system caused a sharp oppression of plant growth that leads to the formation of extremely low products performance or complete death of plants.
The most number of mobile forms with cadmium were determined for this model system. Thus, the bio-indication method has allowed to evaluate the effectiveness of various ways to reduce the phytotoxicity of TMs ions in the «soil - plant» system.
Conclusions: Biohumus dramatically reduces the number of mobile forms of Cd(II) and thereby their phytotoxicity. Lime when applied into the soil in less quantities than biohumus , binds ions of Cd2+ and reduces their phytotoxicity.
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