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Ecological resistance of the soil–plant system to contamination by heavy metals
Journal of Geochemical Exploration 123 (2012) 33–40
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Ecological resistance of the soil–plant system to contamination by heavy metals T.M. Minkina a,⁎, G.V. Motuzova b, S.S. Mandzhieva a, O.G. Nazarenko c a b c
Southern Federal University, Faculty of Biology, Soil Science Department, 813, 194/1, prosp. Stachki, Rostov-on-Don, 344090, Russia Moscow State University, Department of Soil Science, Moscow, Russia State Center of Agrochemical Service, Rostov region, Russia
a r t i c l e
i n f o
Article history: Received 1 November 2011 Accepted 21 August 2012 Available online 29 August 2012 Keywords: Heavy metal compounds The soil–plant system Bioavailability Accumulation coefficient
a b s t r a c t Under consideration is the input of Cu, Zn and Pb to barley grown on ordinary chernozem and chestnut soil under conditions of a pot experiment. The application of these elements is accompanied by accumulation in plants in the following way: Zn ≫ Cu > Pb. The increase in soil contamination degree and combined application of heavy metals promote more intensive accumulation of Cu and Zn in barley straw as compared to grain. It is shown that the Cu, Zn and Pb amounts in grain and straw are highly dependent on the content of metal compounds which are not firmly bound in soils. Differences in the plant tolerance to heavy metals are reflected by an accumulation coefficient. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The study is needed to comprehend mechanisms responsible for governing the absorption of heavy metals by soil components and their potential accessibility to plant roots. It allows optimizing the plant nutrition in soils with a low content of nutrient elements and estimating dangerous excessive accumulation of heavy metals in plants on contaminated soils. The knowledge about metal compounds in soil provides a deeper insight to evaluating the actual supply of plants with these metals. The quantitative assessment should be based upon a comprehensive analysis of the interaction between the metal status in soils and the accumulation of metals in plants. Sorption and desorption processes occur simultaneously and control the bioavailability of chemical substances in soil, their content in liquid phase and mobile state in the composition of soil solid phases (Anisimova, 2008; Ovcharenko, 1996). The content of metal compounds presumably available or unavailable to plants is defined by extraction as a useful tool for solution of the metals, which are bound with soil components with different forces. The capability of soil components to metal fixation should be considered as a main mechanism responsible for ecological resistance of soils to heavy metals. This is an objective criterion to determine the suitability of contaminated soils for their use in agriculture. The plants reveal tolerance to contaminants as well. One of the resistance mechanisms is a limited input of heavy metals to the terrestrial part of plants, the reproductive organs in particular. The physiological sense of this phenomenon consists probably in reducing the metal content in those plant parts that display an active biosynthesis. ⁎ Corresponding author. Tel./fax: +7 8632 637531. E-mail address: [email protected] (T.M. Minkina). 0375-6742/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2012.08.021
The different indices are used for the evaluation of plants tolerance to heavy metals. The Bioconcentration factor (BC) is an index of the accumulation degree of chemical elements in plants. It is calculated as based upon the ratio between the content of any element in plant or in its organ and the total content in soil (Kovalevskiy, 1991). This coefficient allows judging about the biophylity extent of the chemical elements. The Translocation factor (TF) is an objective criterion to assess the quantity of metals transferred from soil to plants. It is calculated as a ratio between the metal content in plant mass and the content of its mobile compounds in soil, because the latter are most available for plants (Bruks, 1996). The main objective of the present work is to study the ecological resistance of barley to Cu, Zn and Pb at different levels of monoand polymetal pollution of chernozem and chestnut soil and to identify relationships between protective functions of soil and plant against heavy metals. Cu, Pb and Zn were taken as contaminants widely spread in Rostov region. 2. Materials and methods 2.1. Study area The Lower Don (Rostov oblast, Russian Federation) is located in two soil zones. These are steppes with ordinary and southern chernozems and dry steppes with chestnut soils. The steppe zone occupies 6.2 million ha (two thirds of the oblast area) in the most humid regions of the oblast. The dry-steppe zone occupies 3.4 million ha in the eastern part of the oblast. The natural conditions of the chernozemic zone are most favorable for agriculture. Soils were developed under continental climatic conditions with an annual precipitation
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T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
of 400–500 mm. Calcareous loess-like clays and loams are the predominant parent rocks of chernozems. The zone of chestnut soils is characterized by an arid climate with an annual precipitation of 300–350 mm. Carbonate and carbonate–sulfate loess-like loams and clays are the main soil-forming rocks of chestnut soils. Lands in the Lower Don basin have been intensively used over centuries; there are almost no soils free from anthropogenic impacts. We studied a deep, low-humus, heavy loamy, calcareous ordinary chernozem developed from the loess-like loam and a medium-deep, medium solonetzic, loamy chestnut soil developed from the loess-like loam at the Rostov oblast (Russia). 2.2. Sampling and analysis of soils The major properties of the studied soils are (1) ordinary chernozem is calcareous, thick, with the content of humus—3.9%, physical clay —53.1%, clay—32.4%, CaCO3—0.4%, pH=7.5 and exchangeable cations in mmol(+)kg : Ca2+—295, Mg2+—55, Na+—1, N\NO3—8, P2O5—32 and 248 mg/kg of K2O; (2) chestnut soil is moderate thick, mediumsolonized, with the following properties: 2.6% of humus, 47.7% of physical clay, 29.5% of clay, 0.1% of CaCO3, pH= 7.8, the composition of exchangeable cations is the following: Ca2+—202 mmol(+)kg, Mg2+—45 mmol(+)kg, Na+—24 mmol kg-1+ N\NO3—6 mg/100 g, P2O5—12 mg/100 g and K2O—380 mg/100 g. Samples from the plow (0–20 cm) layer were used in the experiment. The soil samples were contaminated with different doses of heavy metals introduced separately and in combination. The experiment was performed in 4-l plastic vessels. To ensure drainage, a 3-cm thick layer of haydite and a 3-cm thick layer of washed river sand were placed on the bottom of the vessels. Then, the vessels
were filled with 4 kg of the soil sieved through a 5-mm sieve. Dry acetic salts of Zn, Cu and Pb (Zn(CH3COO)2, Cu(CH3COO)2, and Pb(CH3COO)2) were separately and in combination added to the soil. The selected application rates of metals corresponded to the maximum permissible concentrations of Cu, Zn and Pb total content and concentration of its mobile forms (Table 1) and well agreed with the existing level of soil contamination in the Rostov region (Minkina et al., 2007). The salts of acetic acid were selected because of their good solubility and their capacity for quick and complete interaction with the soil mass. Contrary to the salts of mineral acids, acetates of heavy metals have some advantages: their hydrolysis is not accompanied by a sharp shift toward the strongly acid reaction, and acetate anions are the natural products of plant metabolism and cannot significantly change the nutrient regime of the soil. The experiments were performed in triplicate. After the soil composting for one month, a test culture—barley (Hordeum sativum distichum) (Odesskiy-100 cultivar) was sown. During the vegetation period the soil moisture was at a level of 60% from the field water capacity. Pots were used as cultivation containers in the open air for 2 years. Soil samples were collected before the experiment, 1 and 2 years later after barley harvesting in the phase of waxy maturity. 2.3. General analytical procedures The heavy metal concentration in all the soil samples decomposed by HF + HClO4 was determined by flame atomic absorption spectrophotometry (FAAS) (Scientific Buck 200 A). Sequential extraction was used to detect the content of metals that are not firmly bound to soil components and characterize the potential
Table 1 The total content, exchangeable and specifically adsorbed compounds of heavy metals (mg/kg) in the case of monoelement contamination of an ordinary chernozem and chestnut soil. Application rate, mg/kg
Zn Control (no added metal) 23a 50 75 100b 300c LSD0.95 Cu Control (no added metal) 3a 10 30 55b 100c LSD0.95 Pb Control (no added metal) 6a 25 32b 55 100c LSD0.95 a b c
Ordinary chernozem
Chestnut soil
Exchangeable
Specifically adsorbed
The 1st year
The 1st year
The 2nd year
The 2nd year
Total content
Exchangeable
Specifically adsorbed
The 1st year
The 1st year
The 1st year
The 2nd year
The 2nd year
Total content The 2nd year
The 1st year
The 2nd year
0.5
0.4
6.5
6.6
69.0
65.0
0.4
0.4
7.7
8.6
65.0
58.9
1.1 2.5 4.1 7.5 25.4 1.5
0.7 1.5 3.6 4. 0 22.5 1.9
16.3 31.9 48.4 54.5 57.6 5.6
14.8 46.2 60.6 67.2 90.7 6.7
92.9 119.0 139.9 165.0 365.1 18.5
89.4 121.0 137.0 159.1 360.0 20.0
1.9 2.9 4.4 10.8 35.1 2.1
1.2 2.1 3.9 6.7 20.6 1.5
14.5 33.1 47.0 57.1 53.2 6.6
15.4 38.2 71.2 74.6 90.7 5.5
83.0 109.1 139.0 162.2 355.0 15.9
78.0 99.9 135.0 165.1 356.0 19.1
0.3
0.3
1.8
1.9
43.9
43.7
0.3
0.2
2.4
2.3
41.5
39.4
0.4 0.8 1.1 1.3 4.4 0.9
0.4 0.5 0.7 1.9 3.1 0.8
3.3 4.2 10.6 17.0 25.1 4.5
3.2 4.6 10.8 18.6 25.0 5.8
46.1 53.2 72.5 100.3 135.0 15.0
44.0 51.5 70.5 93.4 138.7 14.1
0.5 0.9 1.1 3.1 7.9 0.7
0.4 0.8 2.6 2.3 4.9 1.1
2.6 4.5 8.5 14.4 22.3 5.4
4.2 5.6 12.8 14.2 23.1 4.8
42.3 47.0 64.0 91.8 125.0 13.0
40.1 46.2 65.4 85.7 128.6 14.5
0.6 0.7 0.8 1.4 3.4 8.2 1.2
0.6 0.6 0.8 1.1 2.2 4.5 0.8
2.4 2.2 3.6 4.8 5.9 21.7 3.3
2.3 2.3 2.3 3.4 5.6 19.1 3.7
25.0 33.1 42.0 60.0 78.2 127.0 12.8
23.5 28.3 44.0 56.0 75.9 125.0 13.5
0.5 0.4 1.3 1.8 4.6 10.8 0.7
0.4 0.7 1.0 1.7 3.4 11.3 1.4
1.9 2.7 2.6 4.5 8.1 21.9 4.3
2.0 3.0 2.5 3.8 6.4 17.6 3.3
20.0 28.2 32.0 53.0 71.1 119.0 10.7
22.0 28.3 39.0 52.5 75.0 115.0 12.5
Corresponded to the Russian maximum permissible concentration (MPC) of heavy metal exchangeable compounds. Corresponded to the Russian MPC of heavy metal total content. Corresponded to the International MPC of heavy metal total content.
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
reserve of their mobile compounds (Minkina et al., 2010). Two modifications were made to this procedure. First, 1 N acetate–ammonium buffer (AAB) (CH3COONH4) at pH = 4.8 in a soil:solution ratio of 1:5 during 18 h of extraction) providing the transfer of metal exchangeable forms into solution to characterize their actual mobility. Second, acid-decomposed compounds of metals extracted by 1 N HCl (in the soil:solution ratio of 1:10 during 1 h of extraction) permitting to determine potential reserve of mobile metal compounds in soil. Based upon the previously conducted studies (Minkina et al., 2008, 2009) it was reasonable to conclude that the acid-decomposed compounds should be considered as a sum of exchangeable and specifically sorbed forms of heavy metal compounds. Thus, in this work the compounds of heavy metals extracted by 1 N HCl are regarded as mobile compounds. There plant samples were analyzed separately: the grains and straw. They were prepared for analyzing by dry combustion at 450 °C, the rest was dissolved by an acid mixture (HNO3 + HCl). The content of heavy metals in extracts from soils and plants was determined by FAAS. BCF and TF are determined in order to study heavy metal accumulation. They were calculated separately for the grain and for the straw. 3. Results and discussion The total content of Cu, Pb and Zn in initial ordinary chernozem and chestnut soil is averaged as 44, 25 and 67 mg/kg respectively (Tables 1, 2). The content of specifically sorbed forms of Cu, Pb and Zn accounts for 7–15% from the total content of metals. The content of mobile exchangeable compounds is rather low and estimated as 0.3 mg/kg of Cu, 0.6 mg/kg of Pb and 0.5 mg/kg of Zn, not exceeding 1% from the total content of Cu and Zn and 3% from the total content of Pb. By this reason, the Cu content in barley straw and grain is insufficient in the control experiment version (initial untreated soils), the Zn amount is within the lower range of its optimal concentration for plants (Table 3). The Pb content in barley grain in control version (0.5 mg/kg) is at a level of maximum permissible concentration (MPC) for foodstuff accepted in Russia (Medical–biological requirements…, 1990). It is evident that there is a lack in correspondence between modern sanitary-hygienic standards for the Pb content in agricultural crops and the actual content of this metal in plants within the area of Lower Don.
35
An identical situation is observed in other regions of Russia (Nikityuk, 1998; Zakrutkin, 2002). The data obtained by many researchers (Firsova et al., 1997; Korneeva et al., 1994) indicate that the Pb content exceeds 0.5 mg/kg in agricultural crops on unpolluted leached chernozems and podzolized soils in the forest-steppe zone of the Trans-Ural region. According to existing standards accepted in the country the plants like the soils should be classified as lead-contaminated ones, because the content of heavy metals in plants seems to be one of the main criteria for standardizing the content of heavy metals in soils. However, the natural content of this element in plants is varying from 0.1 to 10 mg/kg (Vinogradov, 1957). For instance, the Pb content in cereals varies from 0.01 to 10 mg/kg in different countries of the world, being 1.5–2.0 mg/kg as a threshold level in grain crops of the USA, England and France. As is evident from the data obtained in the given experiment on ordinary chernozem of the Rostov region (Minkina et al., 2003), the permissible lead concentration in grain is higher than 0.5 mg. So, native standards for the content of heavy metals in soils and plants prove to be incongruous with each other and require to be specified. With increasing the content of heavy metals in soil their accumulation in plants becomes higher (Table 3). However, the quality of soils and plants is differently evaluated. For example, due to Zn addition in the amount of 100 mg/kg to the soil its concentration in grain is classified as exceeding the threshold level (Table 3), but the soil is diagnosed as contaminated by mobile forms of this metal only at its addition in the amount of 300 mg/kg (Table 1). In case with the copper the situation is quite opposite: the soil is contaminated by Cu addition rate of 100 mg/kg but the plants are regarded as unpolluted ones. Plants display physiological-genetic mechanisms of tolerance to heavy metals as well. Our experiment showed that barley is sensitive to the Zn content in soil. It is manifested in capability to eliminate this metal input to grain being added even in small amounts and to accumulate it in straw (Table 1). The results of identical field experiment indicated that zinc is accumulated predominantly in barley roots: Zn concentration ratio in different plant organs (grain:stem:root) changed from 1:1:1 in control to 1:1:3 in the experiment version with Zn application at the level of 300 mg/kg (Minkina, 2006). Quite another reaction of barley in case of Cu input to soil. At all the addition rates of this metal its concentration seems higher in grain
Table 2 The total content, exchangeable and specifically adsorbed compounds of heavy metals (mg/kg) in the case of polyelement contamination of an ordinary chernozem. Application rate, mg/kg
Exchangeable
Specifically adsorbed
Total content
The 1st year
The 2nd year
The 1st year
The 2nd year
The 1st year
The 2nd year
Zn Control (no added metal) Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.5 1.9 5.9 12.9 24.3 1.4
0.4 1.1 4.2 8.5 16.9 1.4
6.5 16.5 41.3 62.9 59.3 6.3
6.6 19.5 45.9 76.5 96.6 6.7
69.0 90.4 115.7 165.0 370.0 15.9
65.0 82.4 110.0 158.0 376.0 185.2
Cu Control (no added metal) Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.3 0.7 1.4 2.1 6.6 1.1
0.3 0.6 1.0 2.7 5.6 1.2
1.8 3.7 5.8 22.7 32.3 6.5
1.9 4.1 7.5 24.2 32.1 5.3
43.9 45.2 53.7 101.1 142.7 14.0
43.7 42.9 52.6 98.0 149.5 13.9
Pb Control (no added metal) Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.6 0.6 1.0 3.3 7.5 1.5
0.6 0.6 0.7 2.2 6.8 1.4
2.4 2.9 4.7 9.5 22.7 2.7
2.3 2.8 5.9 9.4 17.4 4.5
25.0 30.0 45.0 67.2 126.0 13.2
23.5 25.4 45.6 64.0 117.0 14.0
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T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
Table 3 The contents of heavy metal in spring barley organs in the case of monoelement contamination of an ordinary chernozem and chestnut soil, mg/kg. Application rate, mg/kg
Ordinary chernozem
Chestnut soil
The 1st year
The 2nd year
The 1st year
The 2nd year
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
Zn Control 23 50 75 100 300 LSD0.95
24.5 26.5 42.2 50.5 69.9 88.4 3.9
20.4 47.4 54.4 70.0 77.9 107.9 3.3
21.2 20.2 26.4 39.8 53.4 69.5 1.2
17.3 27.7 49.9 55.1 73.5 87.6 8.2
22.4 28.3 51.7 52.6 80.8 95.5 1.5
17.6 40.2 66.8 71.4 103.6 116.1 5.3
15.9 19.3 25.1 34.7 57.9 67.3 2.1
18.5 27.1 48.2 49.1 70.6 88.7 8.4
Cu Control 3 10 30 55 100 LSD0.95
1.0 1.9 2.6 4.5 7.5 9.1 0.5
1.2 2.0 2.0 3.0 4.7 7.7 1.7
0.8 1.5 1.9 2.2 4.5 5.9 0.3
0.6 1.0 1.2 2.0 3.8 5.0 0.5
2.1 4.0 5.6 6.7 7.4 9.7 0. 4
2.0 5.8 8.2 8.8 11.9 16.0 0. 8
1.2 2.1 2.8 4.3 5.2 6.2 0.6
1.4 3.5 5.5 4.8 7.2 9.8 0.3
Pb Control 6 25 32 55 100 LSD0.95
0.5 0.5 0.7 1.7 2.1 3.9 0.3
1.2 1.0 2.2 2.7 3.8 4.7 1.0
0.3 0.3 0.4 0.6 1.5 2.1 0.3
1.0 1.0 1.7 1.7 2.4 2.9 1.1
0.6 0.8 0.6 2.3 2.6 3.9 0.4
1.0 1.3 1.4 3.3 4.3 5.8 0.3
0.3 0.5 0.2 1.8 1.5 2.1 0.3
0.7 1.0 1.1 1.8 3.0 4.8 0.8
Note: The maximum permissible concentration of Zn—50 mg/kg, Pb—0.5 mg/kg, Cu— 10 mg/kg in barley grain accepted in Russia.
as compared to straw (Table 3). It speaks about the fact that its addition rates are not critical and promote the optimal Cu concentration in grain. In unpolluted soils the Pb amount in straw is 2.4 times higher than in grain. As is seen from the results obtained in field experiments (Minkina, 2006) and other literature sources (Dmitrakov and Dmitrakova, 2006; Garmash and Garmash, 1987; Govorina et al., 2007), the major part of lead in unpolluted and polluted soils is accumulated in plant roots and only a small part of this metal is transported into stem and in grain to a lesser extent. Such distribution may be determined by protective mechanisms of plants capable to eliminate the ascending flow of Pb ions even in unpolluted soil. The metal distribution in plant organs is dependent not only on the protective functions of plants but also the composition and properties of soils. In soils with a higher buffering capacity the protective functions of plants are expressed to a lesser extent. So, the coefficient of metal accumulation in plant roots (root:overground mass ratio) is smaller by several times in chernozem than that in fine-textured soddy-podzolic soil (Vazhenin, 1984). It is also shown that the Zn and Cu content in barley straw on the chestnut soil, which reveals a smaller buffering capacity as compared to chernozem, exceeds the amount of these metals in grain. It is a result of plant protective functions in terms to heavy metals manifested at their maximum loads on soil. As seen from Table 3, the content of these metals in grain is almost identical but in barley straw it is higher by 2 times on the chestnut soil in comparison with chernozem. The given regularity for the Pb content is manifested to a lesser extent. The content of Zn, Cu and Pb in barley grain and straw decreased in the second year after their application. This is possibly connected with the decrease of the mobile forms of metals in the soils. The metal input to plant is also affected by its compound form. For instance, there is information (Mineev et al., 1982) that the barley absorption of lead added in the form of PbCO3 is lower by 3 times than that in the form of PbNO3.
It is worth emphasizing that the combined application of heavy metals has influence on the plant tolerance to metal due to antagonistic additive or synergic interaction. The results of the present study showed that the accumulation of heavy metals in barley assumes quite another character in the case of their combined application. In chernozem the content of Zn mobile forms reveals an increase (Table 2), zinc is highly accumulated in barley grain already at the first addition rate of heavy metals (from 26.5 to 44.6 mg/kg), being insignificantly increased later at the level of 53–55 mg/kg (Table 3). It gives evidence that the higher polymetal contamination of soils is conducive to increasing the protective ability of plants, what is expressed in appearing a barrier between straw and grain. The activity of such barriers in plant is a sequence of the high zinc biophylity (Stepanyuk, 2000). At a higher addition rate of heavy metals the ratio of Zn content between barley straw and grain becomes increased by 2.4 times as compared to the experiment version, in which heavy metals were separately added to soil. It speaks about an increased content of metal mobile compounds in soil which is unfavorable for plants (Table 2). The interaction pattern of heavy metals in soil in the case of their combined application and peculiar accumulation of zinc in barley made it possible to obtain practically unpolluted grain and highly polluted straw (Table 4). The reverse dependence between the heavy metal content in plants and soils is explained not only by physical–chemical adsorption of metals by soils. It is supposed that the plants possess specific physiological–biochemical mechanisms not only to eliminate the excessive input of chemical elements to them but also to decrease their content in living tissues at maximum concentration in soil (Kovalevskiy, 1991). Cu accumulation in barley grain reveals regularity identical to the adsorption of Zn: it is highly accumulated at the first application rate from 1 to 7.7 mg/kg and then independent on the application rate the Cu content becomes stabilized in grain at the level of 9 mg/kg (Table 4). The ratio of the heavy metal content in the plant organs (straw: grain) is increased with increasing the soil contamination (from 1.2 to 2.7) at the maximum application rate, thus resulting in the decline of the metal amount in grain and its increase in soil. Hence, due to increasing the load on the soil–plant system and combined application
Table 4 The contents of heavy metal in spring barley organs in the case of polyelement contamination of an ordinary chernozem, mg/kg. Application rate, mg/kg
The 1st year
The 2nd year
Grain
Straw
Grain
Straw
Zn Control Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
24.5 44.6 44.0 55.1 53.3 4.6
20.4 47.5 64.8 87.0 143.3 10.8
21.2 21.7 31.9 44.8 53.7 3.3
17.3 37.5 64.8 74.9 111.0 7.7
Cu Control Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
1.0 3.1 7.7 9.0 8.8 1.9
1.2 5.8 10.2 25.4 23.8 3.1
0.8 2.2 4.0 6.3 5.6 1.1
0.6 4.1 7.6 13.5 15.8 2.1
Pb Control Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.5 0.6 1.6 4.7 6.4 0.5
1.2 2.5 4.6 4.7 9.3 0.9
0.3 0.3 0.9 3.3 4.1 0.5
1.0 1.6 3.0 4.0 5.1 1.2
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
40 0 0
100
200
300
400
The total contant in soil, mg/kg Zn 160 140 120 100 80 60 40 20 0 0
25
50
75
100
The content of mobile compounds in soil, mg/kg
Cu
Straw Grain
30 25 20 15 10 5 0 0
50
100
150
The total contant in soil, mg/kg Cu 30 25 20 15 10 5 0 0
5
10 15
20
25 30 35 40
45
The content of mobile compounds in soil, mg/kg
The content in plants organ, mg/kg
80
The content in plants organ, mg/kg
120
The content in plants organ, mg/kg
The content in plants organ, mg/kg The content in plant organs, mg/kg
Zn
The content in plants organ, mg/kg
type of heavy metals are destroyed due to a higher soil contamination (Chirkova, 2002). Barley reveals different types of metal absorption. The accumulative type is characteristic of it to absorb Cu and Zn and the indicative type— to absorb Pb (Kabata-Pendia and Pendias, 2001; Kabata-Pendias, 2001; Prasad and Hagemeyer, 1999). In the case of indicative type the metal input to plants has no barrier or it is directly proportional to its content in soils. Usually the metal toxicity takes place for plants. The results of field investigations carried out on polluted chernozem showed a more significant influence of Pb on barley quality and productivity (Minkina et al., 2007). It makes possible to use barley as a crop-indicator to evaluate Pb contamination of the environment. Parallel with genetic peculiarities of plants the mobility of heavy metals in soils should be considered as an important factor determining their input to plants. Mechanisms responsible for regulating the heavy metal absorption by plants are grouped into internal and external ones. The internal mechanisms are conditioned by protective properties of plants, while external—by inactivation of metals by soil (soil barrier properties). The effect of root emission is also regarded to external mechanisms (Barsukova, 1997). The share of soil and wheat growing on it in the formation of ecological potential of the soil–plant system has been estimated by V.B. Ilyin (2004), who noticed that the buffering capacity of soil served as a basic protective function against lead, the protective properties of wheat showed an insignificant effect. Among the heavy metals under consideration the plants absorb Zn to a considerable extent (Table 5) and the BC of this metal is higher by one order as compared to Cu and Pb. The BC of Zn permits to consider zinc as an element, which is highly accumulated by plants. In case of Cu input to chernozem the ВС is increased by 3.5 times in barley grain and by 2 times in straw. The plants grown on the chestnut soil reveal an increase by 2.4 and 2.3 times respectively. At the same time, the BC for Zn and Pb is practically independent on soil contamination degree. Hence, BC doesn't show the plant reaction to the level of soil contamination. Based upon a correlation analysis it seemed reasonable to conclude that the accumulation of heavy metals in plants is closely connected with the amount of their mobile forms in soils (the correlation coefficient r = 0.70–0.90) to a more considerable extent than with the total content of heavy metals (r = 0.31–0.56).
of heavy metals the protective barriers of plants become activated. It is possible to observe the same regularity for Cu accumulation in barley straw—the Cu content is increasing in straw by 10 times at the application rate of 10 mg/kg and gets stabilized (24 mg/kg) in case of its higher amount in soil. The combined application of heavy metals to soil leads to accelerated absorption of Pb by barley. The application rates of lead at the level of 55 mg/kg and 100 mg/kg added separately and in combination with the other heavy metals cause its different content in barley grain and straw (Table 3), it being known that the ratio of its content between straw and grain is firstly increasing to be decreased later. This result well agrees with that demonstrated by Chernykh (1988), Garmash and Garmash (1987), and Smilde (1981), who showed that the toxicity of heavy metals added separately to soil is lower as compared to that manifested in case of their combined application. The relative resistance of different plant parts to heavy metal accumulation should be illustrated by a diagram compiled in the following coordinates: the metal content in plant organs—the metal content in soil with averaged values of its concentration. The more is deviated the concentration trend from any axis, the higher is resistance of the plant organ to contamination by heavy metal (Kovalevskiy, 1991). The graphic data presented in Fig. 1 confirms the theory on the barrier type of chemical element accumulation in plants at their maximum concentration in soil. It is shown that the barriers start activated between straw and grain and between root and grain in the case of Cu absorption as well as between straw and grain at Zn absorption (Fig. 2). The curves reflecting the dependence of the metal content on its amount in soil serve as an evidence of accumulation type of this metal in plants. In the first place, the accumulation type is related to the genetic factor responsible for the composition and ratio of chemical elements in plant tissues. The content of any chemical element in plant is a result of genetic and ecological factors (Ilyin, 1997). The genetic factor provides the plant resistance, on the contrary, the ecological factor destabilizes it, thus showing a great variety of this index (Ilyin, 1997; Ilyin and Stepanova, 1980). Under conditions of the background content of heavy metals in soils and a low contamination degree the genetic factor gets dominated. Mechanisms of plant resistance determining the absorption
160
37
Pb 10 8 6 4 2 0 0
50
100
150
The total content in soil, mg/kg Pb 10 8 6 4 2 0 0
10
20
30
40
The content of mobile compounds in soil, mg/kg
Fig. 1. The concentration trends of Cu, Zn and Pb content in the soil–plant system under the polymetallic pollution of ordinary chernozem.
38
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
60
40
20
0 0
50
100
10
Pb content in grain, mg/kg
Cu content in grain, mg/kg
Zn content in grain, mg/kg
80
8 6 4 2 0 0
150
Zn content in straw, mg/kg
20
7 6 5 4 3 2 1 0
40
0 1 2 3 4 5 6 7 8 9 10
Cu content in straw, mg/kg
Pb content in straw, mg/kg
Fig. 2. Dependence of the metals content in grain and straw of barley on the total content and mobile compounds of metals in contaminated chernozem.
In calculating the TF the amount of metal mobile compounds is assessed by the content of this metal in acetate–ammonium or in another identical extract. But it is very difficult to compare the
investigation results obtained by different researchers and to take into consideration the ability of mobile compounds to transfer from soil into the soil solution and to be uptaken by plants. Having
Table 5 Bioconcentration factor (BC) and translocation factor (TF) of heavy metals by spring barley organs. Application rate, mg/kg
Ordinary chernozem Zn Control 23 50 75 100 300 Cu Control 3 10 30 55 100 Pb Control 6 25 32 55 100 Application rate, mg/kg
Chestnut soil Zn Control 3 10 30 55 100 Cu Control 6 25 32 55 100 Pb Control 23 50 75 100 300
BC
TF (exchangeable compounds)
TF (mobile compounds)
The 1st year
The 2nd year
The 1st year
The 2nd year
The 1st year
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
0.36 0.28 0.35 0.36 0.42 0.24
0.30 0.51 0.46 0.50 0.47 0.30
0.33 0.23 0.22 0.29 0.34 0.19
0.27 0.31 0.41 0.40 0.46 0.24
49.00 24.09 16.88 12.32 9.32 3.48
40.80 43.09 21.76 17.07 10.39 4.25
53.00 28.86 17.60 11.06 13.35 3.09
43.25 39.57 33.27 15.31 18.38 3.89
3.5 2.1 1.4 1.0 1.2 0.9
2.9 2.3 1.6 1.3 1.4 1.3
3.0 1.3 0.6 0.6 0.7 0.8
2.5 1.8 1.0 0.9 1.0 1.0
0.02 0.04 0.05 0.06 0.07 0.07
0.03 0.04 0.04 0.04 0.05 0.06
0.02 0.03 0.04 0.03 0.05 0.04
0.01 0.02 0.02 0.03 0.04 0.04
3.33 4.75 3.25 4.09 5.77 2.07
4.00 5.00 2.50 2.73 3.62 1.75
2.67 3.75 3.80 3.14 2.37 1.90
2.00 2.50 2.40 2.86 2.00 1.61
0.5 0.5 0.5 0.4 0.4 0.3
0.6 0.5 0.4 0.3 0.3 0.3
0.4 0.4 0.4 0.2 0.2 0.2
0.3 0.3 0.2 0.2 0.2 0.2
0.02 0.02 0.02 0.03 0.03 0.03
0.05 0.03 0.05 0.05 0.05 0.04
0.01 0.01 0.01 0.01 0.02 0.02
0.04 0.04 0.04 0.03 0.03 0.02
0.83 0.71 0.88 1.21 0.62 0.48
2.00 1.43 2.75 1.93 1.12 0.57
0.50 0.50 0.50 0.55 0.68 0.47
1.67 1.67 2.13 1.55 1.09 0.64
0.2 0.2 0.2 0.3 0.2 0.1
0.4 0.3 0.5 0.4 0.4 0.2
0.1 0.1 0.1 0.1 0.2 0.1
0.3 0.3 0.6 0.4 0.3 0.1
BC
The 2nd year
TF (exchangeable compounds)
TF (Mobile compounds)
The 1st year
The 2nd year
The 1st year
The 2nd year
The 1st year
Grain
Straw
Grain
Grain
Straw
Grain
Grain
Straw
Grain
Grain
Straw
Grain
0.34 0.34 0.47 0.38 0.50 0.27
0.27 0.48 0.61 0.51 0.64 0.33
0.27 0.25 0.25 0.26 0.35 0.19
0.31 0.35 0.48 0.36 0.43 0.25
56.00 14.89 17.83 11.95 7.48 2.72
44.00 21.16 23.03 16.23 9.59 3.31
39.75 16.08 11.95 8.90 8.64 3.27
46.25 22.58 22.95 12.59 10.54 4.31
2.77 1.73 1.44 1.02 1.19 1.08
2.17 2.45 1.86 1.39 1.53 1.31
0.24 0.23 0.23 0.25 0.35 0.18
2.06 1.63 1.20 0.65 0.87 0.80
0.05 0.09 0.12 0.10 0.08 0.08
0.05 0.14 0.17 0.14 0.13 0.13
0.03 0.05 0.06 0.07 0.06 0.05
0.04 0.09 0.12 0.07 0.08 0.08
7.00 8.00 6.22 6.09 2.39 1.23
6.67 11.60 9.11 8.00 3.84 2.03
6.00 5.25 3.50 1.65 2.26 1.27
7.00 8.75 6.88 1.85 3.13 2.00
0.78 1.29 1.04 0.70 0.42 0.32
0.74 1.87 1.52 0.92 0.68 0.53
0.48 0.46 0.44 0.28 0.32 0.22
0.56 0.76 0.86 0.31 0.44 0.35
0.03 0.03 0.02 0.04 0.04 0.03
0.05 0.05 0.04 0.06 0.06 0.05
0.01 0.02 0.01 0.03 0.02 0.02
0.03 0.04 0.03 0.03 0.04 0.04
1.20 2.00 0.46 1.28 0.57 0.36
2.00 3.25 1.08 1.83 0.93 0.54
0.75 0.71 0.20 1.06 0.44 0.19
1.75 1.43 1.10 1.06 0.88 0.42
0.25 0.26 0.15 0.37 0.20 0.12
0.42 0.42 0.36 0.52 0.34 0.18
0.13 0.02 0.01 0.03 0.02 0.02
0.29 0.27 0.31 0.33 0.31 0.17
The 1st year
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
Cu
3,0 2,0 1,0 0,0 Control
Cu 3 + Pb 6 + Cu 10 + Cu 55 + Pb Cu 100 + Pb Zn 23 Pb25+ Zn 50 32 + Zn 100 100 + Zn 300
The application rates, mg/kg
Pb 0,9
1,6 1,2 0,8 0,4 0,0 Control
Cu 3 + Pb Cu 10 + Cu 55 + Pb Cu 100 + 6 + Zn 23 Pb25+ Zn 32 + Zn Pb 100 + 50 100 Zn 300
The application rates, mg/kg Grain
Translocation factor
Translocation factor
Translocation factor
Zn 4,0
39
0,6 0,3 0,0 Control
Cu 3 + Pb 6 + Cu 10 + Cu 55 + Pb Cu 100 + Pb Zn 23 Pb25+ Zn 50 32 + Zn 100 100 + Zn 300
The application rates, mg/kg
Straw
Fig. 3. Translocation factor (TF) of Zn, Cu, and Pb in barley grain under different application rates of metals in the case of polymetallic pollution.
calculated the ratio between the Zn content in plants and its mobile forms in soils, it was taken as a sum of exchangeable forms and those extracted by sodium acetate at pH= 5 (Perelomov and Pinskiy, 2005). Because the exchangeable forms are not always available for plants it is more expedient to calculate the coefficient of accumulation by means of the heavy metal content extracted by 1 N HCl, i.e. according to the total content of compounds which are not firmly bound. The TF is dependent on the level of metal load (Table 5). Already after the first Zn addition rate the TF of this metal in grain becomes declined by 2 times being decreased later with increasing the metal load on soil. The intensive Zn accumulation in barley straw decreases to a lesser extent. In the case of Cu input to soil the plants continue to uptake this metal, the TF of this metal in plant remains unchanged for a long period of time and starts declining only in the course of increasing the application rates of this metal. A high ability of plants to absorb zinc provides its sufficient content in straw and grain of barley grown on control unpolluted soils. Even small addition rates of this metal reduce its availability by plants. The supply of barley with copper is not sufficient because of its little biophylity and low mobility in soil. That's why even small Cu amounts added to soil promote an optimal concentration of this metal in plants. With increasing the addition rates the protective mechanisms of plants start to activate. As regards the Pb uptake by plants it doesn't change in case of increasing its addition rates in soil. The TF of this metal is lower by 2 times in grain than in straw in all the experiment versions. Under conditions of enhanced polymetal contamination it is possible to observe a significant differentiation of Cu and Zn accumulation intensity in plant organs as compared to monometal contamination: the metal accumulation in barley grain gets decreased as compared to straw, this is testified by the value of TF. Pb accumulation reveals an opposite regularity (Fig. 3). Thus, two indices allow judging about the plant tolerance to heavy metals: (1) intensive accumulation of heavy metals in plants when the metal accumulation in plant is behind increasing the content of its mobile forms in soil, (2) the metal redistribution in plant organs when its accumulation in grain is behind that in straw. One should notice that the values of TF serve as evidence of protective possibilities of the soil–plant system, because they demonstrate changes in metal mobility in soil, on the one hand, and, on the other hand, the reaction of plants to these changes.
4. Conclusion In the present study a concept has been formulated to show ecological resistance of the soil–plant system to pollutants. Ecological resistance of soils to pollutants including heavy metals is a soil ability to eliminate direct adverse effects of excessive amounts of chemical substances on soil living organisms as well as indirect effects exerted by these chemical substances on water, air and the other
environmental conditions. This is explained by the ability of soil components to pollutant fixation. Ecological resistance of plants to pollutants is the plant's ability to eliminate their input to generative organs. The plants are capable to accumulate pollutants in the composition of their organs which are not so valuable for plant homeostasis. The results of this study have clearly demonstrated peculiar ecological resistance of the soil–plant system to heavy metals in the steppe zone of Rostov region. In unpolluted ordinary chernozem and chestnut soil Zn is intensively absorbed by barley, Cu—to a lesser extent. Pb is characteristic of a low absorption by plants. With increasing the soil contamination by heavy metals the total content of these metals in soil is also increased. The amount of their mobile compounds becomes higher as well what speaks about decreasing the ecological resistance of soils to heavy metals; the protective functions of soils to pollutants start to weaken. The plant resistance to pollutants is increased. The barley as a test crop reveals different resistance to a higher content of Cu, Zn and Pb in soil. Their resistance is testified by limiting the input of metals to plant generative organs due to declining accumulation intensity of metals in plants and activating barrier mechanisms. The plant tolerance to heavy metals may be expressed in the following way: Zn > Cu > Pb. The plants are tolerant to Zn and Cu when the latter are in mobile forms and to Pb in case within a low range of its concentration. Hence, apart from the soil buffering capacity to heavy metals the mechanisms responsible for self-regulation of plant organs are activated in order to absorb metals from soil and to decrease their accumulation in generative organs. This capacity of plants is especially manifested in terms of biophyle elements. The buffering capacity of soils and protective mechanisms of plants allow decreasing the toxic effects of heavy metals in case of increasing their content in the environment. From the obtained results it is concluded that TF may be used as a quantitative measure of protective function of the soil–plant system based upon the ratio between the amount of any chemical element in the dry mass of plant or its organ and the content of mobile compounds in soil. TF is promising to determine the regional threshold of maximum heavy metal concentration that is allowed in plants. Acknowledgments This work was supported by the Russian Foundation of Fundamental Research (project no. 11-05-90351-RBU_а) and the Ministry of Science of the Russian Federation (project nos. РН 5.5349.2011, 16.740.11.0232, 16.740.11.0528 and 16.740.11.0054). References Anisimova, L.N., 2008. Co, Cu and Zn accumulation by barley depending on the content and forms of these metals in a soddy-podzolic soil. Journal Agrochemistry 10, 62–68.
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Barsukova, V.S., 1997. Physiological–genetic aspects of plant resistance to heavy metals, Novosibirsk. Bruks, R.R., 1996. Biological methods employed in search of minerals, Moscow. Chernykh, N.A., 1988. Impact of different Zn, Pb, Cd content in soil on the composition and quality of plants, Cand. Thesis. Chirkova, T.V., 2002. Physiological bases for plant resistance, St-Petersburg. Dmitrakov, L.M., Dmitrakova, L.K., 2006. Translocation of Pb in oats plant. Journal Agrochemistry 2, 71–77. Firsova, V.P., Pavlova, T.S., Toshchev, V.V., Prokopovich, Ye.V., 1997. Comparative study of the heavy metal content in forest, meadow and arable soils within the foreststeppe zone of Trans-Ural region. Journal of Ecology 2, 96–101. Garmash, G.F., Garmash, N.Yu., 1987. Distribution of heavy metals in organs of cultural crops. Journal Agrochemistry 5, 40–46. Govorina, V.V., Rakipov, N.G., Lin, Keo Sonhiak, Fedorenkova, N.K., 2007. The content and distribution of Cd, Pb and Ni in summer wheat depending on the level of mineral nutrition and contamination by heavy metals. Journal Agrochemistry 3, 61–67. Ilyin, V.B., 1997. Buffering capacity of soils and permissible level of their contamination by heavy metals. Journal Agrochemistry 8, 65–70. Ilyin, V.B., 2004. Assessment of protective possibilities of the soil–plant system under conditions of soil contamination by lead (results of vegetable experiment). Journal Agrochemistry 4, 52–57. Ilyin, B.V., Stepanova, M.D., 1980. Protective possibilities of the soil–plant system under conditions of soil contamination by heavy metals. Heavy Metals in the Environment, p. 8085. Moscow. Kabata-Pendia, A., Pendias, H., 2001. Trace Elements in Soils and Plants. CRC Press, Boca Raton, Fl. Kabata-Pendias, A., 2001. Phytoindication as a tool for studying the environment. Sibirian Ecological Journal 2, 125–130. Korneeva, S.F., Bednyagin, G.V., Penezhina, G.V., Fokina, Z.P., Nigamatullin, Ye.Kh, 1994. Complex characteristics of radioactive and chemical contamination of soils and plants at the territory of Chelyabinsk region. Transact.: Radiation, Ecology, Health, pp. 153–159. Ekaterinburg, part 1. Kovalevskiy, A.L., 1991. Biogeochemistry of plants, Novosibirsk. Medical-biological requirements and sanitary standards for determining the quality of foodstuffs, Moscow, 1990. Mineev, V.G., Alexeev, A.A., Trishina, T.A., 1982. Heavy metals and the environment under conditions of modern chemization. Journal Agrochemistry 9, 126–139.
Minkina, T.M., 2006. Translocation of Zn and Pb in technogenically contaminated soils. Journal Newsletter, Southern Research Centre of Russian Academy of Sciences 2 (4), 60–66. Minkina, T.M., Kryshchenko, V.S., Fedoseenko, S.V., 2003. Grain quality of barley under conditions of polluted ordinary chernozem. Journal Scientific Thought of Caucasus 2, 119–123. Minkina, T.M., Motuzova, G.V., Nazarenko, O.G., Samokhin, A.P., Kryshchenko, V.S., Mandzhaeva, S.S., 2007. Effects of different amendments on Zn and Pb mobility in contaminated chernozem. Journal Agrochemistry 10, 67–75. Minkina, T.M., Motuzova, G.V., Nazarenko, O.G., Kryshchenko, V.S., Mandzhieva, S.S., 2008. Forms of heavy metal compounds in soils of the steppe zone. Eurasian Soil Science 41 (7), 708–716. Minkina, T.M., Motuzova, G.V., Nazarenko, O.G., Mandzhieva, S.S., 2009. Group composition of heavy metal compounds in the soils contaminated by emissions from the Novocherkassk Power Station. Eurasian Soil Science 42 (13), 1–10. Minkina, T.M., Motusova, G.V., Nazarenko, O.G., Mandzhieva, S.S., 2010. Heavy Metal Compounds in Soil: Transformation upon Soil Pollution and Ecological Significance. Nova Science Publishers, Inc. 188 pp. Nikityuk, N.V., 1998. Mobility of heavy metals in carbonate chernozems and ways to assess it, Cand. Thesis, Krasnodar. Ovcharenko, M.M., 1996. Mobility of heavy metals in soil and their availability to plants. Journal Agrarian Science 3, 39–41. Perelomov, L.V., Pinskiy, D.L., 2005. Immobilization of Zn-bearing water-soluble salts in soil. Journal Pochvovedenie 7, 60–72. Prasad, M.N.V., Hagemeyer, J. (Eds.), 1999. Heavy Metal Stress in Plants. Springer, Berlin. Smilde, K.W., 1981. Heavy metal accumulation in crops grown of sewage sludge amended with soils. Journal Plant and Soil 62, 3–14. Stepanyuk, V.V., 2000. Effects of heavy metal compounds on crop yields and their input to plants. Journal Agrochemistry 1, 74–80. Vazhenin, I.G., 1984. Plant roots as bioindicators of soil contamination by toxic elements. Journal Agrochemistry 2, 73–77. Vinogradov, A.P., 1957. Geochemistry of Rare and Dispersed Chemical Elements in Soils, Moscow. Zakrutkin, V.E., 2002. Landscape geochemistry and technogenesis, Postov-on-Don.
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Ecological resistance of the soil–plant system to contamination by heavy metals T.M. Minkina a,⁎, G.V. Motuzova b, S.S. Mandzhieva a, O.G. Nazarenko c a b c
Southern Federal University, Faculty of Biology, Soil Science Department, 813, 194/1, prosp. Stachki, Rostov-on-Don, 344090, Russia Moscow State University, Department of Soil Science, Moscow, Russia State Center of Agrochemical Service, Rostov region, Russia
a r t i c l e
i n f o
Article history: Received 1 November 2011 Accepted 21 August 2012 Available online 29 August 2012 Keywords: Heavy metal compounds The soil–plant system Bioavailability Accumulation coefficient
a b s t r a c t Under consideration is the input of Cu, Zn and Pb to barley grown on ordinary chernozem and chestnut soil under conditions of a pot experiment. The application of these elements is accompanied by accumulation in plants in the following way: Zn ≫ Cu > Pb. The increase in soil contamination degree and combined application of heavy metals promote more intensive accumulation of Cu and Zn in barley straw as compared to grain. It is shown that the Cu, Zn and Pb amounts in grain and straw are highly dependent on the content of metal compounds which are not firmly bound in soils. Differences in the plant tolerance to heavy metals are reflected by an accumulation coefficient. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The study is needed to comprehend mechanisms responsible for governing the absorption of heavy metals by soil components and their potential accessibility to plant roots. It allows optimizing the plant nutrition in soils with a low content of nutrient elements and estimating dangerous excessive accumulation of heavy metals in plants on contaminated soils. The knowledge about metal compounds in soil provides a deeper insight to evaluating the actual supply of plants with these metals. The quantitative assessment should be based upon a comprehensive analysis of the interaction between the metal status in soils and the accumulation of metals in plants. Sorption and desorption processes occur simultaneously and control the bioavailability of chemical substances in soil, their content in liquid phase and mobile state in the composition of soil solid phases (Anisimova, 2008; Ovcharenko, 1996). The content of metal compounds presumably available or unavailable to plants is defined by extraction as a useful tool for solution of the metals, which are bound with soil components with different forces. The capability of soil components to metal fixation should be considered as a main mechanism responsible for ecological resistance of soils to heavy metals. This is an objective criterion to determine the suitability of contaminated soils for their use in agriculture. The plants reveal tolerance to contaminants as well. One of the resistance mechanisms is a limited input of heavy metals to the terrestrial part of plants, the reproductive organs in particular. The physiological sense of this phenomenon consists probably in reducing the metal content in those plant parts that display an active biosynthesis. ⁎ Corresponding author. Tel./fax: +7 8632 637531. E-mail address: [email protected] (T.M. Minkina). 0375-6742/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gexplo.2012.08.021
The different indices are used for the evaluation of plants tolerance to heavy metals. The Bioconcentration factor (BC) is an index of the accumulation degree of chemical elements in plants. It is calculated as based upon the ratio between the content of any element in plant or in its organ and the total content in soil (Kovalevskiy, 1991). This coefficient allows judging about the biophylity extent of the chemical elements. The Translocation factor (TF) is an objective criterion to assess the quantity of metals transferred from soil to plants. It is calculated as a ratio between the metal content in plant mass and the content of its mobile compounds in soil, because the latter are most available for plants (Bruks, 1996). The main objective of the present work is to study the ecological resistance of barley to Cu, Zn and Pb at different levels of monoand polymetal pollution of chernozem and chestnut soil and to identify relationships between protective functions of soil and plant against heavy metals. Cu, Pb and Zn were taken as contaminants widely spread in Rostov region. 2. Materials and methods 2.1. Study area The Lower Don (Rostov oblast, Russian Federation) is located in two soil zones. These are steppes with ordinary and southern chernozems and dry steppes with chestnut soils. The steppe zone occupies 6.2 million ha (two thirds of the oblast area) in the most humid regions of the oblast. The dry-steppe zone occupies 3.4 million ha in the eastern part of the oblast. The natural conditions of the chernozemic zone are most favorable for agriculture. Soils were developed under continental climatic conditions with an annual precipitation
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T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
of 400–500 mm. Calcareous loess-like clays and loams are the predominant parent rocks of chernozems. The zone of chestnut soils is characterized by an arid climate with an annual precipitation of 300–350 mm. Carbonate and carbonate–sulfate loess-like loams and clays are the main soil-forming rocks of chestnut soils. Lands in the Lower Don basin have been intensively used over centuries; there are almost no soils free from anthropogenic impacts. We studied a deep, low-humus, heavy loamy, calcareous ordinary chernozem developed from the loess-like loam and a medium-deep, medium solonetzic, loamy chestnut soil developed from the loess-like loam at the Rostov oblast (Russia). 2.2. Sampling and analysis of soils The major properties of the studied soils are (1) ordinary chernozem is calcareous, thick, with the content of humus—3.9%, physical clay —53.1%, clay—32.4%, CaCO3—0.4%, pH=7.5 and exchangeable cations in mmol(+)kg : Ca2+—295, Mg2+—55, Na+—1, N\NO3—8, P2O5—32 and 248 mg/kg of K2O; (2) chestnut soil is moderate thick, mediumsolonized, with the following properties: 2.6% of humus, 47.7% of physical clay, 29.5% of clay, 0.1% of CaCO3, pH= 7.8, the composition of exchangeable cations is the following: Ca2+—202 mmol(+)kg, Mg2+—45 mmol(+)kg, Na+—24 mmol kg-1+ N\NO3—6 mg/100 g, P2O5—12 mg/100 g and K2O—380 mg/100 g. Samples from the plow (0–20 cm) layer were used in the experiment. The soil samples were contaminated with different doses of heavy metals introduced separately and in combination. The experiment was performed in 4-l plastic vessels. To ensure drainage, a 3-cm thick layer of haydite and a 3-cm thick layer of washed river sand were placed on the bottom of the vessels. Then, the vessels
were filled with 4 kg of the soil sieved through a 5-mm sieve. Dry acetic salts of Zn, Cu and Pb (Zn(CH3COO)2, Cu(CH3COO)2, and Pb(CH3COO)2) were separately and in combination added to the soil. The selected application rates of metals corresponded to the maximum permissible concentrations of Cu, Zn and Pb total content and concentration of its mobile forms (Table 1) and well agreed with the existing level of soil contamination in the Rostov region (Minkina et al., 2007). The salts of acetic acid were selected because of their good solubility and their capacity for quick and complete interaction with the soil mass. Contrary to the salts of mineral acids, acetates of heavy metals have some advantages: their hydrolysis is not accompanied by a sharp shift toward the strongly acid reaction, and acetate anions are the natural products of plant metabolism and cannot significantly change the nutrient regime of the soil. The experiments were performed in triplicate. After the soil composting for one month, a test culture—barley (Hordeum sativum distichum) (Odesskiy-100 cultivar) was sown. During the vegetation period the soil moisture was at a level of 60% from the field water capacity. Pots were used as cultivation containers in the open air for 2 years. Soil samples were collected before the experiment, 1 and 2 years later after barley harvesting in the phase of waxy maturity. 2.3. General analytical procedures The heavy metal concentration in all the soil samples decomposed by HF + HClO4 was determined by flame atomic absorption spectrophotometry (FAAS) (Scientific Buck 200 A). Sequential extraction was used to detect the content of metals that are not firmly bound to soil components and characterize the potential
Table 1 The total content, exchangeable and specifically adsorbed compounds of heavy metals (mg/kg) in the case of monoelement contamination of an ordinary chernozem and chestnut soil. Application rate, mg/kg
Zn Control (no added metal) 23a 50 75 100b 300c LSD0.95 Cu Control (no added metal) 3a 10 30 55b 100c LSD0.95 Pb Control (no added metal) 6a 25 32b 55 100c LSD0.95 a b c
Ordinary chernozem
Chestnut soil
Exchangeable
Specifically adsorbed
The 1st year
The 1st year
The 2nd year
The 2nd year
Total content
Exchangeable
Specifically adsorbed
The 1st year
The 1st year
The 1st year
The 2nd year
The 2nd year
Total content The 2nd year
The 1st year
The 2nd year
0.5
0.4
6.5
6.6
69.0
65.0
0.4
0.4
7.7
8.6
65.0
58.9
1.1 2.5 4.1 7.5 25.4 1.5
0.7 1.5 3.6 4. 0 22.5 1.9
16.3 31.9 48.4 54.5 57.6 5.6
14.8 46.2 60.6 67.2 90.7 6.7
92.9 119.0 139.9 165.0 365.1 18.5
89.4 121.0 137.0 159.1 360.0 20.0
1.9 2.9 4.4 10.8 35.1 2.1
1.2 2.1 3.9 6.7 20.6 1.5
14.5 33.1 47.0 57.1 53.2 6.6
15.4 38.2 71.2 74.6 90.7 5.5
83.0 109.1 139.0 162.2 355.0 15.9
78.0 99.9 135.0 165.1 356.0 19.1
0.3
0.3
1.8
1.9
43.9
43.7
0.3
0.2
2.4
2.3
41.5
39.4
0.4 0.8 1.1 1.3 4.4 0.9
0.4 0.5 0.7 1.9 3.1 0.8
3.3 4.2 10.6 17.0 25.1 4.5
3.2 4.6 10.8 18.6 25.0 5.8
46.1 53.2 72.5 100.3 135.0 15.0
44.0 51.5 70.5 93.4 138.7 14.1
0.5 0.9 1.1 3.1 7.9 0.7
0.4 0.8 2.6 2.3 4.9 1.1
2.6 4.5 8.5 14.4 22.3 5.4
4.2 5.6 12.8 14.2 23.1 4.8
42.3 47.0 64.0 91.8 125.0 13.0
40.1 46.2 65.4 85.7 128.6 14.5
0.6 0.7 0.8 1.4 3.4 8.2 1.2
0.6 0.6 0.8 1.1 2.2 4.5 0.8
2.4 2.2 3.6 4.8 5.9 21.7 3.3
2.3 2.3 2.3 3.4 5.6 19.1 3.7
25.0 33.1 42.0 60.0 78.2 127.0 12.8
23.5 28.3 44.0 56.0 75.9 125.0 13.5
0.5 0.4 1.3 1.8 4.6 10.8 0.7
0.4 0.7 1.0 1.7 3.4 11.3 1.4
1.9 2.7 2.6 4.5 8.1 21.9 4.3
2.0 3.0 2.5 3.8 6.4 17.6 3.3
20.0 28.2 32.0 53.0 71.1 119.0 10.7
22.0 28.3 39.0 52.5 75.0 115.0 12.5
Corresponded to the Russian maximum permissible concentration (MPC) of heavy metal exchangeable compounds. Corresponded to the Russian MPC of heavy metal total content. Corresponded to the International MPC of heavy metal total content.
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
reserve of their mobile compounds (Minkina et al., 2010). Two modifications were made to this procedure. First, 1 N acetate–ammonium buffer (AAB) (CH3COONH4) at pH = 4.8 in a soil:solution ratio of 1:5 during 18 h of extraction) providing the transfer of metal exchangeable forms into solution to characterize their actual mobility. Second, acid-decomposed compounds of metals extracted by 1 N HCl (in the soil:solution ratio of 1:10 during 1 h of extraction) permitting to determine potential reserve of mobile metal compounds in soil. Based upon the previously conducted studies (Minkina et al., 2008, 2009) it was reasonable to conclude that the acid-decomposed compounds should be considered as a sum of exchangeable and specifically sorbed forms of heavy metal compounds. Thus, in this work the compounds of heavy metals extracted by 1 N HCl are regarded as mobile compounds. There plant samples were analyzed separately: the grains and straw. They were prepared for analyzing by dry combustion at 450 °C, the rest was dissolved by an acid mixture (HNO3 + HCl). The content of heavy metals in extracts from soils and plants was determined by FAAS. BCF and TF are determined in order to study heavy metal accumulation. They were calculated separately for the grain and for the straw. 3. Results and discussion The total content of Cu, Pb and Zn in initial ordinary chernozem and chestnut soil is averaged as 44, 25 and 67 mg/kg respectively (Tables 1, 2). The content of specifically sorbed forms of Cu, Pb and Zn accounts for 7–15% from the total content of metals. The content of mobile exchangeable compounds is rather low and estimated as 0.3 mg/kg of Cu, 0.6 mg/kg of Pb and 0.5 mg/kg of Zn, not exceeding 1% from the total content of Cu and Zn and 3% from the total content of Pb. By this reason, the Cu content in barley straw and grain is insufficient in the control experiment version (initial untreated soils), the Zn amount is within the lower range of its optimal concentration for plants (Table 3). The Pb content in barley grain in control version (0.5 mg/kg) is at a level of maximum permissible concentration (MPC) for foodstuff accepted in Russia (Medical–biological requirements…, 1990). It is evident that there is a lack in correspondence between modern sanitary-hygienic standards for the Pb content in agricultural crops and the actual content of this metal in plants within the area of Lower Don.
35
An identical situation is observed in other regions of Russia (Nikityuk, 1998; Zakrutkin, 2002). The data obtained by many researchers (Firsova et al., 1997; Korneeva et al., 1994) indicate that the Pb content exceeds 0.5 mg/kg in agricultural crops on unpolluted leached chernozems and podzolized soils in the forest-steppe zone of the Trans-Ural region. According to existing standards accepted in the country the plants like the soils should be classified as lead-contaminated ones, because the content of heavy metals in plants seems to be one of the main criteria for standardizing the content of heavy metals in soils. However, the natural content of this element in plants is varying from 0.1 to 10 mg/kg (Vinogradov, 1957). For instance, the Pb content in cereals varies from 0.01 to 10 mg/kg in different countries of the world, being 1.5–2.0 mg/kg as a threshold level in grain crops of the USA, England and France. As is evident from the data obtained in the given experiment on ordinary chernozem of the Rostov region (Minkina et al., 2003), the permissible lead concentration in grain is higher than 0.5 mg. So, native standards for the content of heavy metals in soils and plants prove to be incongruous with each other and require to be specified. With increasing the content of heavy metals in soil their accumulation in plants becomes higher (Table 3). However, the quality of soils and plants is differently evaluated. For example, due to Zn addition in the amount of 100 mg/kg to the soil its concentration in grain is classified as exceeding the threshold level (Table 3), but the soil is diagnosed as contaminated by mobile forms of this metal only at its addition in the amount of 300 mg/kg (Table 1). In case with the copper the situation is quite opposite: the soil is contaminated by Cu addition rate of 100 mg/kg but the plants are regarded as unpolluted ones. Plants display physiological-genetic mechanisms of tolerance to heavy metals as well. Our experiment showed that barley is sensitive to the Zn content in soil. It is manifested in capability to eliminate this metal input to grain being added even in small amounts and to accumulate it in straw (Table 1). The results of identical field experiment indicated that zinc is accumulated predominantly in barley roots: Zn concentration ratio in different plant organs (grain:stem:root) changed from 1:1:1 in control to 1:1:3 in the experiment version with Zn application at the level of 300 mg/kg (Minkina, 2006). Quite another reaction of barley in case of Cu input to soil. At all the addition rates of this metal its concentration seems higher in grain
Table 2 The total content, exchangeable and specifically adsorbed compounds of heavy metals (mg/kg) in the case of polyelement contamination of an ordinary chernozem. Application rate, mg/kg
Exchangeable
Specifically adsorbed
Total content
The 1st year
The 2nd year
The 1st year
The 2nd year
The 1st year
The 2nd year
Zn Control (no added metal) Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.5 1.9 5.9 12.9 24.3 1.4
0.4 1.1 4.2 8.5 16.9 1.4
6.5 16.5 41.3 62.9 59.3 6.3
6.6 19.5 45.9 76.5 96.6 6.7
69.0 90.4 115.7 165.0 370.0 15.9
65.0 82.4 110.0 158.0 376.0 185.2
Cu Control (no added metal) Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.3 0.7 1.4 2.1 6.6 1.1
0.3 0.6 1.0 2.7 5.6 1.2
1.8 3.7 5.8 22.7 32.3 6.5
1.9 4.1 7.5 24.2 32.1 5.3
43.9 45.2 53.7 101.1 142.7 14.0
43.7 42.9 52.6 98.0 149.5 13.9
Pb Control (no added metal) Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.6 0.6 1.0 3.3 7.5 1.5
0.6 0.6 0.7 2.2 6.8 1.4
2.4 2.9 4.7 9.5 22.7 2.7
2.3 2.8 5.9 9.4 17.4 4.5
25.0 30.0 45.0 67.2 126.0 13.2
23.5 25.4 45.6 64.0 117.0 14.0
36
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
Table 3 The contents of heavy metal in spring barley organs in the case of monoelement contamination of an ordinary chernozem and chestnut soil, mg/kg. Application rate, mg/kg
Ordinary chernozem
Chestnut soil
The 1st year
The 2nd year
The 1st year
The 2nd year
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
Zn Control 23 50 75 100 300 LSD0.95
24.5 26.5 42.2 50.5 69.9 88.4 3.9
20.4 47.4 54.4 70.0 77.9 107.9 3.3
21.2 20.2 26.4 39.8 53.4 69.5 1.2
17.3 27.7 49.9 55.1 73.5 87.6 8.2
22.4 28.3 51.7 52.6 80.8 95.5 1.5
17.6 40.2 66.8 71.4 103.6 116.1 5.3
15.9 19.3 25.1 34.7 57.9 67.3 2.1
18.5 27.1 48.2 49.1 70.6 88.7 8.4
Cu Control 3 10 30 55 100 LSD0.95
1.0 1.9 2.6 4.5 7.5 9.1 0.5
1.2 2.0 2.0 3.0 4.7 7.7 1.7
0.8 1.5 1.9 2.2 4.5 5.9 0.3
0.6 1.0 1.2 2.0 3.8 5.0 0.5
2.1 4.0 5.6 6.7 7.4 9.7 0. 4
2.0 5.8 8.2 8.8 11.9 16.0 0. 8
1.2 2.1 2.8 4.3 5.2 6.2 0.6
1.4 3.5 5.5 4.8 7.2 9.8 0.3
Pb Control 6 25 32 55 100 LSD0.95
0.5 0.5 0.7 1.7 2.1 3.9 0.3
1.2 1.0 2.2 2.7 3.8 4.7 1.0
0.3 0.3 0.4 0.6 1.5 2.1 0.3
1.0 1.0 1.7 1.7 2.4 2.9 1.1
0.6 0.8 0.6 2.3 2.6 3.9 0.4
1.0 1.3 1.4 3.3 4.3 5.8 0.3
0.3 0.5 0.2 1.8 1.5 2.1 0.3
0.7 1.0 1.1 1.8 3.0 4.8 0.8
Note: The maximum permissible concentration of Zn—50 mg/kg, Pb—0.5 mg/kg, Cu— 10 mg/kg in barley grain accepted in Russia.
as compared to straw (Table 3). It speaks about the fact that its addition rates are not critical and promote the optimal Cu concentration in grain. In unpolluted soils the Pb amount in straw is 2.4 times higher than in grain. As is seen from the results obtained in field experiments (Minkina, 2006) and other literature sources (Dmitrakov and Dmitrakova, 2006; Garmash and Garmash, 1987; Govorina et al., 2007), the major part of lead in unpolluted and polluted soils is accumulated in plant roots and only a small part of this metal is transported into stem and in grain to a lesser extent. Such distribution may be determined by protective mechanisms of plants capable to eliminate the ascending flow of Pb ions even in unpolluted soil. The metal distribution in plant organs is dependent not only on the protective functions of plants but also the composition and properties of soils. In soils with a higher buffering capacity the protective functions of plants are expressed to a lesser extent. So, the coefficient of metal accumulation in plant roots (root:overground mass ratio) is smaller by several times in chernozem than that in fine-textured soddy-podzolic soil (Vazhenin, 1984). It is also shown that the Zn and Cu content in barley straw on the chestnut soil, which reveals a smaller buffering capacity as compared to chernozem, exceeds the amount of these metals in grain. It is a result of plant protective functions in terms to heavy metals manifested at their maximum loads on soil. As seen from Table 3, the content of these metals in grain is almost identical but in barley straw it is higher by 2 times on the chestnut soil in comparison with chernozem. The given regularity for the Pb content is manifested to a lesser extent. The content of Zn, Cu and Pb in barley grain and straw decreased in the second year after their application. This is possibly connected with the decrease of the mobile forms of metals in the soils. The metal input to plant is also affected by its compound form. For instance, there is information (Mineev et al., 1982) that the barley absorption of lead added in the form of PbCO3 is lower by 3 times than that in the form of PbNO3.
It is worth emphasizing that the combined application of heavy metals has influence on the plant tolerance to metal due to antagonistic additive or synergic interaction. The results of the present study showed that the accumulation of heavy metals in barley assumes quite another character in the case of their combined application. In chernozem the content of Zn mobile forms reveals an increase (Table 2), zinc is highly accumulated in barley grain already at the first addition rate of heavy metals (from 26.5 to 44.6 mg/kg), being insignificantly increased later at the level of 53–55 mg/kg (Table 3). It gives evidence that the higher polymetal contamination of soils is conducive to increasing the protective ability of plants, what is expressed in appearing a barrier between straw and grain. The activity of such barriers in plant is a sequence of the high zinc biophylity (Stepanyuk, 2000). At a higher addition rate of heavy metals the ratio of Zn content between barley straw and grain becomes increased by 2.4 times as compared to the experiment version, in which heavy metals were separately added to soil. It speaks about an increased content of metal mobile compounds in soil which is unfavorable for plants (Table 2). The interaction pattern of heavy metals in soil in the case of their combined application and peculiar accumulation of zinc in barley made it possible to obtain practically unpolluted grain and highly polluted straw (Table 4). The reverse dependence between the heavy metal content in plants and soils is explained not only by physical–chemical adsorption of metals by soils. It is supposed that the plants possess specific physiological–biochemical mechanisms not only to eliminate the excessive input of chemical elements to them but also to decrease their content in living tissues at maximum concentration in soil (Kovalevskiy, 1991). Cu accumulation in barley grain reveals regularity identical to the adsorption of Zn: it is highly accumulated at the first application rate from 1 to 7.7 mg/kg and then independent on the application rate the Cu content becomes stabilized in grain at the level of 9 mg/kg (Table 4). The ratio of the heavy metal content in the plant organs (straw: grain) is increased with increasing the soil contamination (from 1.2 to 2.7) at the maximum application rate, thus resulting in the decline of the metal amount in grain and its increase in soil. Hence, due to increasing the load on the soil–plant system and combined application
Table 4 The contents of heavy metal in spring barley organs in the case of polyelement contamination of an ordinary chernozem, mg/kg. Application rate, mg/kg
The 1st year
The 2nd year
Grain
Straw
Grain
Straw
Zn Control Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
24.5 44.6 44.0 55.1 53.3 4.6
20.4 47.5 64.8 87.0 143.3 10.8
21.2 21.7 31.9 44.8 53.7 3.3
17.3 37.5 64.8 74.9 111.0 7.7
Cu Control Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
1.0 3.1 7.7 9.0 8.8 1.9
1.2 5.8 10.2 25.4 23.8 3.1
0.8 2.2 4.0 6.3 5.6 1.1
0.6 4.1 7.6 13.5 15.8 2.1
Pb Control Cu 3 + Pb 6 + Zn 23 Cu 10 + Pb25 + Zn 50 Cu 55 + Pb 32 + Zn 100 Cu 100 + Pb 100 + Zn 300 LSD0.95
0.5 0.6 1.6 4.7 6.4 0.5
1.2 2.5 4.6 4.7 9.3 0.9
0.3 0.3 0.9 3.3 4.1 0.5
1.0 1.6 3.0 4.0 5.1 1.2
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
40 0 0
100
200
300
400
The total contant in soil, mg/kg Zn 160 140 120 100 80 60 40 20 0 0
25
50
75
100
The content of mobile compounds in soil, mg/kg
Cu
Straw Grain
30 25 20 15 10 5 0 0
50
100
150
The total contant in soil, mg/kg Cu 30 25 20 15 10 5 0 0
5
10 15
20
25 30 35 40
45
The content of mobile compounds in soil, mg/kg
The content in plants organ, mg/kg
80
The content in plants organ, mg/kg
120
The content in plants organ, mg/kg
The content in plants organ, mg/kg The content in plant organs, mg/kg
Zn
The content in plants organ, mg/kg
type of heavy metals are destroyed due to a higher soil contamination (Chirkova, 2002). Barley reveals different types of metal absorption. The accumulative type is characteristic of it to absorb Cu and Zn and the indicative type— to absorb Pb (Kabata-Pendia and Pendias, 2001; Kabata-Pendias, 2001; Prasad and Hagemeyer, 1999). In the case of indicative type the metal input to plants has no barrier or it is directly proportional to its content in soils. Usually the metal toxicity takes place for plants. The results of field investigations carried out on polluted chernozem showed a more significant influence of Pb on barley quality and productivity (Minkina et al., 2007). It makes possible to use barley as a crop-indicator to evaluate Pb contamination of the environment. Parallel with genetic peculiarities of plants the mobility of heavy metals in soils should be considered as an important factor determining their input to plants. Mechanisms responsible for regulating the heavy metal absorption by plants are grouped into internal and external ones. The internal mechanisms are conditioned by protective properties of plants, while external—by inactivation of metals by soil (soil barrier properties). The effect of root emission is also regarded to external mechanisms (Barsukova, 1997). The share of soil and wheat growing on it in the formation of ecological potential of the soil–plant system has been estimated by V.B. Ilyin (2004), who noticed that the buffering capacity of soil served as a basic protective function against lead, the protective properties of wheat showed an insignificant effect. Among the heavy metals under consideration the plants absorb Zn to a considerable extent (Table 5) and the BC of this metal is higher by one order as compared to Cu and Pb. The BC of Zn permits to consider zinc as an element, which is highly accumulated by plants. In case of Cu input to chernozem the ВС is increased by 3.5 times in barley grain and by 2 times in straw. The plants grown on the chestnut soil reveal an increase by 2.4 and 2.3 times respectively. At the same time, the BC for Zn and Pb is practically independent on soil contamination degree. Hence, BC doesn't show the plant reaction to the level of soil contamination. Based upon a correlation analysis it seemed reasonable to conclude that the accumulation of heavy metals in plants is closely connected with the amount of their mobile forms in soils (the correlation coefficient r = 0.70–0.90) to a more considerable extent than with the total content of heavy metals (r = 0.31–0.56).
of heavy metals the protective barriers of plants become activated. It is possible to observe the same regularity for Cu accumulation in barley straw—the Cu content is increasing in straw by 10 times at the application rate of 10 mg/kg and gets stabilized (24 mg/kg) in case of its higher amount in soil. The combined application of heavy metals to soil leads to accelerated absorption of Pb by barley. The application rates of lead at the level of 55 mg/kg and 100 mg/kg added separately and in combination with the other heavy metals cause its different content in barley grain and straw (Table 3), it being known that the ratio of its content between straw and grain is firstly increasing to be decreased later. This result well agrees with that demonstrated by Chernykh (1988), Garmash and Garmash (1987), and Smilde (1981), who showed that the toxicity of heavy metals added separately to soil is lower as compared to that manifested in case of their combined application. The relative resistance of different plant parts to heavy metal accumulation should be illustrated by a diagram compiled in the following coordinates: the metal content in plant organs—the metal content in soil with averaged values of its concentration. The more is deviated the concentration trend from any axis, the higher is resistance of the plant organ to contamination by heavy metal (Kovalevskiy, 1991). The graphic data presented in Fig. 1 confirms the theory on the barrier type of chemical element accumulation in plants at their maximum concentration in soil. It is shown that the barriers start activated between straw and grain and between root and grain in the case of Cu absorption as well as between straw and grain at Zn absorption (Fig. 2). The curves reflecting the dependence of the metal content on its amount in soil serve as an evidence of accumulation type of this metal in plants. In the first place, the accumulation type is related to the genetic factor responsible for the composition and ratio of chemical elements in plant tissues. The content of any chemical element in plant is a result of genetic and ecological factors (Ilyin, 1997). The genetic factor provides the plant resistance, on the contrary, the ecological factor destabilizes it, thus showing a great variety of this index (Ilyin, 1997; Ilyin and Stepanova, 1980). Under conditions of the background content of heavy metals in soils and a low contamination degree the genetic factor gets dominated. Mechanisms of plant resistance determining the absorption
160
37
Pb 10 8 6 4 2 0 0
50
100
150
The total content in soil, mg/kg Pb 10 8 6 4 2 0 0
10
20
30
40
The content of mobile compounds in soil, mg/kg
Fig. 1. The concentration trends of Cu, Zn and Pb content in the soil–plant system under the polymetallic pollution of ordinary chernozem.
38
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
60
40
20
0 0
50
100
10
Pb content in grain, mg/kg
Cu content in grain, mg/kg
Zn content in grain, mg/kg
80
8 6 4 2 0 0
150
Zn content in straw, mg/kg
20
7 6 5 4 3 2 1 0
40
0 1 2 3 4 5 6 7 8 9 10
Cu content in straw, mg/kg
Pb content in straw, mg/kg
Fig. 2. Dependence of the metals content in grain and straw of barley on the total content and mobile compounds of metals in contaminated chernozem.
In calculating the TF the amount of metal mobile compounds is assessed by the content of this metal in acetate–ammonium or in another identical extract. But it is very difficult to compare the
investigation results obtained by different researchers and to take into consideration the ability of mobile compounds to transfer from soil into the soil solution and to be uptaken by plants. Having
Table 5 Bioconcentration factor (BC) and translocation factor (TF) of heavy metals by spring barley organs. Application rate, mg/kg
Ordinary chernozem Zn Control 23 50 75 100 300 Cu Control 3 10 30 55 100 Pb Control 6 25 32 55 100 Application rate, mg/kg
Chestnut soil Zn Control 3 10 30 55 100 Cu Control 6 25 32 55 100 Pb Control 23 50 75 100 300
BC
TF (exchangeable compounds)
TF (mobile compounds)
The 1st year
The 2nd year
The 1st year
The 2nd year
The 1st year
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
Grain
Straw
0.36 0.28 0.35 0.36 0.42 0.24
0.30 0.51 0.46 0.50 0.47 0.30
0.33 0.23 0.22 0.29 0.34 0.19
0.27 0.31 0.41 0.40 0.46 0.24
49.00 24.09 16.88 12.32 9.32 3.48
40.80 43.09 21.76 17.07 10.39 4.25
53.00 28.86 17.60 11.06 13.35 3.09
43.25 39.57 33.27 15.31 18.38 3.89
3.5 2.1 1.4 1.0 1.2 0.9
2.9 2.3 1.6 1.3 1.4 1.3
3.0 1.3 0.6 0.6 0.7 0.8
2.5 1.8 1.0 0.9 1.0 1.0
0.02 0.04 0.05 0.06 0.07 0.07
0.03 0.04 0.04 0.04 0.05 0.06
0.02 0.03 0.04 0.03 0.05 0.04
0.01 0.02 0.02 0.03 0.04 0.04
3.33 4.75 3.25 4.09 5.77 2.07
4.00 5.00 2.50 2.73 3.62 1.75
2.67 3.75 3.80 3.14 2.37 1.90
2.00 2.50 2.40 2.86 2.00 1.61
0.5 0.5 0.5 0.4 0.4 0.3
0.6 0.5 0.4 0.3 0.3 0.3
0.4 0.4 0.4 0.2 0.2 0.2
0.3 0.3 0.2 0.2 0.2 0.2
0.02 0.02 0.02 0.03 0.03 0.03
0.05 0.03 0.05 0.05 0.05 0.04
0.01 0.01 0.01 0.01 0.02 0.02
0.04 0.04 0.04 0.03 0.03 0.02
0.83 0.71 0.88 1.21 0.62 0.48
2.00 1.43 2.75 1.93 1.12 0.57
0.50 0.50 0.50 0.55 0.68 0.47
1.67 1.67 2.13 1.55 1.09 0.64
0.2 0.2 0.2 0.3 0.2 0.1
0.4 0.3 0.5 0.4 0.4 0.2
0.1 0.1 0.1 0.1 0.2 0.1
0.3 0.3 0.6 0.4 0.3 0.1
BC
The 2nd year
TF (exchangeable compounds)
TF (Mobile compounds)
The 1st year
The 2nd year
The 1st year
The 2nd year
The 1st year
Grain
Straw
Grain
Grain
Straw
Grain
Grain
Straw
Grain
Grain
Straw
Grain
0.34 0.34 0.47 0.38 0.50 0.27
0.27 0.48 0.61 0.51 0.64 0.33
0.27 0.25 0.25 0.26 0.35 0.19
0.31 0.35 0.48 0.36 0.43 0.25
56.00 14.89 17.83 11.95 7.48 2.72
44.00 21.16 23.03 16.23 9.59 3.31
39.75 16.08 11.95 8.90 8.64 3.27
46.25 22.58 22.95 12.59 10.54 4.31
2.77 1.73 1.44 1.02 1.19 1.08
2.17 2.45 1.86 1.39 1.53 1.31
0.24 0.23 0.23 0.25 0.35 0.18
2.06 1.63 1.20 0.65 0.87 0.80
0.05 0.09 0.12 0.10 0.08 0.08
0.05 0.14 0.17 0.14 0.13 0.13
0.03 0.05 0.06 0.07 0.06 0.05
0.04 0.09 0.12 0.07 0.08 0.08
7.00 8.00 6.22 6.09 2.39 1.23
6.67 11.60 9.11 8.00 3.84 2.03
6.00 5.25 3.50 1.65 2.26 1.27
7.00 8.75 6.88 1.85 3.13 2.00
0.78 1.29 1.04 0.70 0.42 0.32
0.74 1.87 1.52 0.92 0.68 0.53
0.48 0.46 0.44 0.28 0.32 0.22
0.56 0.76 0.86 0.31 0.44 0.35
0.03 0.03 0.02 0.04 0.04 0.03
0.05 0.05 0.04 0.06 0.06 0.05
0.01 0.02 0.01 0.03 0.02 0.02
0.03 0.04 0.03 0.03 0.04 0.04
1.20 2.00 0.46 1.28 0.57 0.36
2.00 3.25 1.08 1.83 0.93 0.54
0.75 0.71 0.20 1.06 0.44 0.19
1.75 1.43 1.10 1.06 0.88 0.42
0.25 0.26 0.15 0.37 0.20 0.12
0.42 0.42 0.36 0.52 0.34 0.18
0.13 0.02 0.01 0.03 0.02 0.02
0.29 0.27 0.31 0.33 0.31 0.17
The 1st year
T.M. Minkina et al. / Journal of Geochemical Exploration 123 (2012) 33–40
Cu
3,0 2,0 1,0 0,0 Control
Cu 3 + Pb 6 + Cu 10 + Cu 55 + Pb Cu 100 + Pb Zn 23 Pb25+ Zn 50 32 + Zn 100 100 + Zn 300
The application rates, mg/kg
Pb 0,9
1,6 1,2 0,8 0,4 0,0 Control
Cu 3 + Pb Cu 10 + Cu 55 + Pb Cu 100 + 6 + Zn 23 Pb25+ Zn 32 + Zn Pb 100 + 50 100 Zn 300
The application rates, mg/kg Grain
Translocation factor
Translocation factor
Translocation factor
Zn 4,0
39
0,6 0,3 0,0 Control
Cu 3 + Pb 6 + Cu 10 + Cu 55 + Pb Cu 100 + Pb Zn 23 Pb25+ Zn 50 32 + Zn 100 100 + Zn 300
The application rates, mg/kg
Straw
Fig. 3. Translocation factor (TF) of Zn, Cu, and Pb in barley grain under different application rates of metals in the case of polymetallic pollution.
calculated the ratio between the Zn content in plants and its mobile forms in soils, it was taken as a sum of exchangeable forms and those extracted by sodium acetate at pH= 5 (Perelomov and Pinskiy, 2005). Because the exchangeable forms are not always available for plants it is more expedient to calculate the coefficient of accumulation by means of the heavy metal content extracted by 1 N HCl, i.e. according to the total content of compounds which are not firmly bound. The TF is dependent on the level of metal load (Table 5). Already after the first Zn addition rate the TF of this metal in grain becomes declined by 2 times being decreased later with increasing the metal load on soil. The intensive Zn accumulation in barley straw decreases to a lesser extent. In the case of Cu input to soil the plants continue to uptake this metal, the TF of this metal in plant remains unchanged for a long period of time and starts declining only in the course of increasing the application rates of this metal. A high ability of plants to absorb zinc provides its sufficient content in straw and grain of barley grown on control unpolluted soils. Even small addition rates of this metal reduce its availability by plants. The supply of barley with copper is not sufficient because of its little biophylity and low mobility in soil. That's why even small Cu amounts added to soil promote an optimal concentration of this metal in plants. With increasing the addition rates the protective mechanisms of plants start to activate. As regards the Pb uptake by plants it doesn't change in case of increasing its addition rates in soil. The TF of this metal is lower by 2 times in grain than in straw in all the experiment versions. Under conditions of enhanced polymetal contamination it is possible to observe a significant differentiation of Cu and Zn accumulation intensity in plant organs as compared to monometal contamination: the metal accumulation in barley grain gets decreased as compared to straw, this is testified by the value of TF. Pb accumulation reveals an opposite regularity (Fig. 3). Thus, two indices allow judging about the plant tolerance to heavy metals: (1) intensive accumulation of heavy metals in plants when the metal accumulation in plant is behind increasing the content of its mobile forms in soil, (2) the metal redistribution in plant organs when its accumulation in grain is behind that in straw. One should notice that the values of TF serve as evidence of protective possibilities of the soil–plant system, because they demonstrate changes in metal mobility in soil, on the one hand, and, on the other hand, the reaction of plants to these changes.
4. Conclusion In the present study a concept has been formulated to show ecological resistance of the soil–plant system to pollutants. Ecological resistance of soils to pollutants including heavy metals is a soil ability to eliminate direct adverse effects of excessive amounts of chemical substances on soil living organisms as well as indirect effects exerted by these chemical substances on water, air and the other
environmental conditions. This is explained by the ability of soil components to pollutant fixation. Ecological resistance of plants to pollutants is the plant's ability to eliminate their input to generative organs. The plants are capable to accumulate pollutants in the composition of their organs which are not so valuable for plant homeostasis. The results of this study have clearly demonstrated peculiar ecological resistance of the soil–plant system to heavy metals in the steppe zone of Rostov region. In unpolluted ordinary chernozem and chestnut soil Zn is intensively absorbed by barley, Cu—to a lesser extent. Pb is characteristic of a low absorption by plants. With increasing the soil contamination by heavy metals the total content of these metals in soil is also increased. The amount of their mobile compounds becomes higher as well what speaks about decreasing the ecological resistance of soils to heavy metals; the protective functions of soils to pollutants start to weaken. The plant resistance to pollutants is increased. The barley as a test crop reveals different resistance to a higher content of Cu, Zn and Pb in soil. Their resistance is testified by limiting the input of metals to plant generative organs due to declining accumulation intensity of metals in plants and activating barrier mechanisms. The plant tolerance to heavy metals may be expressed in the following way: Zn > Cu > Pb. The plants are tolerant to Zn and Cu when the latter are in mobile forms and to Pb in case within a low range of its concentration. Hence, apart from the soil buffering capacity to heavy metals the mechanisms responsible for self-regulation of plant organs are activated in order to absorb metals from soil and to decrease their accumulation in generative organs. This capacity of plants is especially manifested in terms of biophyle elements. The buffering capacity of soils and protective mechanisms of plants allow decreasing the toxic effects of heavy metals in case of increasing their content in the environment. From the obtained results it is concluded that TF may be used as a quantitative measure of protective function of the soil–plant system based upon the ratio between the amount of any chemical element in the dry mass of plant or its organ and the content of mobile compounds in soil. TF is promising to determine the regional threshold of maximum heavy metal concentration that is allowed in plants. Acknowledgments This work was supported by the Russian Foundation of Fundamental Research (project no. 11-05-90351-RBU_а) and the Ministry of Science of the Russian Federation (project nos. РН 5.5349.2011, 16.740.11.0232, 16.740.11.0528 and 16.740.11.0054). References Anisimova, L.N., 2008. Co, Cu and Zn accumulation by barley depending on the content and forms of these metals in a soddy-podzolic soil. Journal Agrochemistry 10, 62–68.
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