faeze lotfi; amir fotovat; reza khorasani; Mahdi Bahraini
Abstract
Introduction: The pollution of soils by heavy metals due to human activities poses a serious concern for human and environmental health. In order to evaluate the risks of heavy metal contamination such as cadmium in soil, it is necessary to understand its bioavailability which depends on its chemical ...
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Introduction: The pollution of soils by heavy metals due to human activities poses a serious concern for human and environmental health. In order to evaluate the risks of heavy metal contamination such as cadmium in soil, it is necessary to understand its bioavailability which depends on its chemical forms in the soil. According to Tessier (1979), heavy metals can be found in various chemical forms in soil including exchangeable, bound to carbonates, bound to iron and manganese oxides and bound to organic matter and residual. These fractions significantly influence the cadmium mobility and bioavailability. Distribution of metals in chemical forms in soil depends on soil pH, amount of organic matter, oxidation-reduction potential and ionic strength. Root exudation, soil texture, cation exchangeable capacity and amount of calcium carbonate may also impact chemical forms of cadmium. Many studies have showed that plant root may affect the chemistry of heavy metals in soil root zone. The objective of this study was to evaluate the effect of organic matter on the distribution of cadmium in corn root media.
Materials and Methods: To investigate the effect of organic matter (cow manure) and root activity on chemical forms of cadmium, a greenhouse experiment was conducted using rhizobox. The contaminated soil sample used in the study was collected from Zanjan. This greenhouse experiment was conducted in a factorial design, with 2 replications, two levels of organic matter (0 and 1.5%) and three zones classified based on their distance from root. The soil samples were air dried and crushed to pass through a 2-mm sieve. The cultivation was conducted using a rhizobox. The rhizobox consisted of three parts: 1.central compartment (rhizosphere), 2.close to rhizosphere, and 3. soil bulk. Soil samples were mixed with fertilizer and packed in rhizobox. Eight pre-germinated maize seedlings were transferred to the central compartment and five days after germination, thinned to four plants. Ten weeks after planting, corn plants were harvested for analysis. The compartments of rhizobox were separated. The collected plant samples (root and shoot) were rinsed with deionized water and oven-dried at 70 °C. Soil samples were also measured for pH, CEC and total organic carbon. The chemical forms of cadmium in the soil and plant samples were identified by the sequential extraction procedure proposed by Tessier (1979). Bioavailable cadmium in soil was also extracted by DTPA-TEA.
Results and Discussion: Results showed that the highest amount of soil cadmium was found in carbonate fraction. Adding organic matter increased the soil pH, CEC and organic carbon amount, whereas none of chemical forms of cadmium were significantly affected by adding organic matter. Bioavailability of cadmium, however, decreased by adding organic matter to soil, It can be therefore concluded that increment in cadmium uptake due to increased organic matter led to decreased cadmium bioavailability. The exchangeable cadmium was negatively correlated to soil organic carbon, while bioavailable cadmium was negatively correlated to soil pH, CEC and amount of soil organic carbon. Moreover, our results indicated that the fractions of cadmium were not significantly affected by distance from the root. Moreover, adding organic matter insignificantly increased concentration of cadmium in shoots, roots and total plants.
Conclusion: In this study, among different chemical forms of cadmium, only bioavailable cadmium was significantly affected by adding organic matter to soil. Additionally, soil pH, CEC and organic carbon were significantly increased by adding organic matter. These results indicate that addition of organic matter to soil may indirectly influence chemical forms of cadmium through impacting soil properties (soil pH, CEC and organic carbon). The addition of organic matter had the most influence on carbonate fraction of cadmium which may be potentially available to plant. It seems that addition of organic matter (cow manure) may result in increase of cadmium concentration in plant. Therefore, it can be concluded that addition of cow manure to calcareous soils with neutral to slightly alkaline pH may lead to increased cadmium uptake by the plant (corn) and reduced soil cadmium concentration.