Effects of Agricultural Soil Contamination

1. Introduction

Agricultural soils contaminated with heavy metals such as lead (Pb) is a threat to human health. Plants grown in contaminated soil can accumulate these metals in the shoots and roots. Metals accumulate in Living organism, especially the liver and kidneys, when these plants are used as fodder for livestock and domestic animals. Thereby enter the human food chain (Akan et al., 2010). Lead is one of the heavy metal with high extension and toxicity. Although it is not an essential element for plants or animals, but can easily be absorbed by plants and animal (Sengar et al., 2008). Pb released into the environment by mining and smelting of Pb ore, burning of coal, effluents from storage batteries, automobile exhausts, metal plating, applications of fertilizers and pesticides, and additives in paint and petrol ((Eick et al.1999; Sharma and Dubey, 2005). Inhibition of the metals uptake and transport by plants is one of the adverse effects of lead (Fodor et al, 1998). Sharma and Dubey (2005) reported that, High concentrations of Pb the in soil causes imbalance of mineralin plants. So that it is prevented from entering cations (potassium, calcium, magnesium, manganese, zinc, copper and iron) and anions (nitrate) into the root. Pourrut, et al (2011) stated that decreased nutrient absorption in the presence of Pb may be results of the changes in physiological plants activity or presence of competition between elements and lead (e.g., the same atomic size). More research is often carried out to investigate the behavior of a single metal in the plants. Therefore, examining the relationship between heavy metals and nutrients in plants that are grown in contaminated metals is an important factor in preventing the toxic effect of these metals (Siedlecka and Krupa, 1999).

Interaction between iron and heavy metals such as Pb is important. Because, heavy metal influence on iron adsorption and accumulation in the root apoplasm, uptake by root and transfer from root to stem and leaves. Therefore, iron deficiency may affect the uptake and accumulation of heavy metals (Fodor, 2006). Iron is an essential element for plant growth and development (Curie & Briat 2003). Under aerobic conditions, iron (III) oxy-hydroxide is the dominant form of Fe in the soil. This form is a very low solubility. Thus, available form of iron for plants is limited under these conditions. Accordingly, one of the limiting factors in agriculture production is iron deficiency in agriculture soil of different parts of the worlds (Hansen et al 2006). Generally, two major strategies used by plants under the iron deficient condition. The Strategy I used by dicotyledonous and non-graminaceous monocotyledonous plants. These plants increased acidification of the rhizosphere and reduction of iron (III) to iron (II) in the rhizosphere through extrude H⁺ into the rhizosphere. Therefore, causing increased iron availability to plants (Kobayashi and Nishizawa, 2012). Strategy II is confined to graminaceous monocotyledonous plant species such as maize. Under iron deficiency condition, enhanced release phytosiderophores, which are non-proteinogenic amino acids with low-molecular-weight and high affinity for complex formation with Fe⁺³ (Meda et al 2007; Marschner, 1995).

Generally, the optimum soil pH range for uptake Iron by plants is 5 and 5.5. Therefore, Fe deficiency is one of the widespread nutritional disorders in the plants grown on calcareous and alkaline soils (with pH>7) (Bojovic et al., 2012). So, Iron deficiency in these soils can be corrected by spraying the plants with solution of iron chelates or ferrous sulfate more efficient than any other applications of Fe to the soil (Fageria et al., 2009). Grusak and Pezeshgi, (1996) and Vert et al, (2003) reported, when, Iron used as a foliar spray, the signals of shoot to root played an important role in iron utilization by roots. One of the topics that can be discussed, is related to the role of iron on Pb uptake and translocation in plants (Bojovic et al., 2012). The Fe nutritional status of the plants with Strategists I and II may influence heavy metal uptake (Fodor, 2006). Among crop plant species, maize (Zea mays. L) is the most important cereal crop in world and it is widely grown throughout the world (Mejia, 2005). Also, it is an important cereal crop of Iran and Is cultivated for fodder as well as for grain purpose in Iran (Nuraky et al., 2011). According to the material presented, further investigations are needed to examine the interaction between iron and lead. Thus, the present study aims to investigate; (i) Lead accumulation in roots and shoots of two cultivars of maize (260 and 704); (ii) Effect of foliar iron application on the uptake of lead by roots and its transport to the shoots and finally (iii) effects of lead accumulation and foliar iron application on Manganese, copper, zinc concentration and their transport.

2. Materials and Methods

2.1. Soil sampling and preparation

This study was conducted in the research greenhouse of the college of agriculture and natural resources, university of Tehran. The uncontaminated soil (Normally contains 1.5 mgPbkg-1) used in this study was collected from a depth of 0–25 cm of the research farm, university of Tehran. It was air-dried at room temperature and pass through a 2 mm mesh sieve. Then, the soil was artificially contaminated by adding PbCl₂. Chemical and physical characteristics summarized in Table 1.

2.2. Analytical methods

2.2.1. Soil characterization

Soil texture was determined by hydro-metric method (Bouyoucos1962), total nitrogen (N) by Kjeldal method (Bremner1996), extractable phosphorous by Olsen method (Kuo1996), exchangeable potassium through normal acetate ammonium method (Hemke and Spark1996), electrical conductivity on saturated extract by Rhoades method (Rhoades1996), organic carbon content by Walkley–Black method (Nelson1982), the elements concentration were determined by atomic absorption spectrometry (Shimadzu-AA6400; Shimadzu Corp., Tokyo, Japan)) according to Waling et al (1989), cation exchange capacity (CEC) by Bower method (Sumner and Milker1996). Measurement of soil pH was done on saturated extract and equivalent calcium carbonate content was determined according to Carter and Gregorich (2008).

2.2.2. Plant analysis

After a growing period (75 days), the harvested plants separated into shoots and roots. Thoroughly washed with deionized water. The roots and shoots were oven-dried at 70 ± 50 C for 48h. Dry ash method (muffle furnace at 550 ÌŠ C for 6 h) was used for determining metal concentration in plant samples. After extraction (Cottenie1980) mineral concentrations were measured in plant samples by atomic absorption spectroscopy (Shimadzu-AA6400; Shimadzu Corp., Tokyo,Japan) (Waling et al.1989). Root volume was determined by water displacement in a graduated cylinder (Messenger et al., 2000).

Method of water displacement in a graduated cylinder used for determining the root volume (Messenger et al., 2000)..

2.3. Experimental treatments

A Factorial experiment based on a completely randomized design with three replications was conducted in a calcareous soil under greenhouse conditions. Experimental treatments Included four levels of Pb (0 as a control, 100,200, and 400 mgPb kg-1 soil), two varieties of maize (260 and 704), four levels of iron sulfate spraying (0: without spraying, 2, 4 and 6 gr (FeSO4.7H2O) in thousand ml distilled water, respectively) at the stage of eight leaves. Lead chloride (PbCl2) was used to contaminate soils in pots. it was dissolved in distilled water and sprayed on soils. Before cultivation of plants, treated soils were incubated at 25â-¦C and field capacity moisture for 150 days to allow Pb to achieve a balanced condition in contaminated soil. The control treatment (non-spray iron) sprayed with distilled water for the same conditions. (Pb0: Uncontaminated soil and Pb1, Pb2, Pb3 are 100, 200 and 400 mg Pb kg-1 soil, respectively / Fe0: Without Spraying and Fe1, Fe2, Fe3 are Foliar Spray with dissolve 2, 4 and 6 gr Feso4.7H2O in thousand ml distilled water, respectively).

2.4. Plant culture and greenhouse condition

Seeds of maize (Zea mays L) were obtained from the Seed and Sapling Research Institute of Karaj, Iran. The experimental plants (four per pot) were grown in plastic pots (18 cm height, 15.5 cm diameter, containing 3 kg of dry soil). The pots were watered based on 80% of the soil field capacity. Artificial light was used and daily light was adjusted to 12–14 h and 10000 luxe. Greenhouse temperature was 25±2° C.

2.5. Translocation Factor

The translocation factor (TF) is used to investigate metal transfer from root-to-shoot. It was calculated by the following equation: (Han et al, 2013)

TF = (1)

2.6. Statistical Analysis

Data were analyzed by ANOVA using SAS software version 9.1 (SAS Institute, Cary, N.C.,USA). Duncan’s test was used to determine the significant differences between means (P< 0.05). Excel software (2010) was used to plot graphs.