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Introduction

The most common heavy metal contaminations found in nature were Cd, Cr, Cu, Hg, Pb and Ni. Environmental pollution by these heavy metals has become extensive due to industrial activities and when they go in elevated level in the environment, they are excessively absorbed by roots and translocated to shoot, leading to impaired metabolism and reduced growth. Heavy metal contamination in soil results in decreased soil microbial activity, soil fertility and yield losses. Cadmium is released into the biosphere naturally by volcanoes, weathering of rocks as well as anthropoegenically through various urban/industrial wastes such as mining and metal refining,¹compost application²and cadmium rich phosphate fertilizers.³

Increased concentrations of Cd in agricultural soils are known to come from human activities such as the application of phosphate fertilizer, sewage sludge, wastewater, and pesticides,^4,5,6mining and smelting of metalliferous ores with high Cd content.⁷

Determination of Cd in soil is very important for evaluating its environmental impact or its risk to agricultural production and soil health. According to the FOREGS geochemical database for European soils, the level of total cadmium concentration ranges between 0.06 and 0.6 ppm. However, the estimated cadmium values are below the most limiting threshold value of 1 ppm for agricultural soils⁸. An accumulation of cadmium in soil will lead to an increase in pore water concentration and ultimately to increase in ground water and surface water concentrations.According to the Statutory Order 49/2000, the soil quality criteria forcadmium is 0.5 mg kg^-1soil.⁹Thismeans that areas applied with waste products must not exceed 0.5 mgCd/kg dw in the plough layer. The criterion was mainly focused onsludge application. In the guidance document on waste products used foragricultural purposes, it is mentioned that the criterion value is assumednot to prevent the obtaining of sufficient areas for the use of waste products on agricultural soil.¹⁰The maximum target permissible level of cadmium according to the waste water quality guidelines for discharge into water bodies or water courses is less than 0.1 mg L^-1.¹¹

Cadmium induced effects include oxidative stress, genotoxicity, inhibition of photosynthetic activity and inhibition of root metabolism. The possible interactions between these modes of toxicity waswell explained.¹²Chlorosis, leaf rolls and stunting are the main and easily visible symptoms of cadmium toxicity in plants. There is a growing interest in problems concerning heavy metal contamination of cultivated lands and little is known regarding metal homeostasis and tolerance at the organismic level. For that reason, it is essential to clarify those problems related to metal transport, accumulation, detoxification and tolerance, and in this sense, it would be of enormous interest to use model systems other than plants, such asSaccharomyces pombe and Saccharomyces cereviciae, as well as the molecular analysis of hyperaccumulators like Arabidopsis halleri and some of the Thlaspi species.¹³

The expected anthropogenic sources of cadmium in Coimbatore soils are more likely derived from a growing number of adverse anthropogenic sources viz., agricultural and urban runoff, garbage dumps, sewage disposals and gold making wastes.

The objectives of this research was to find out the hot spots of cadmium level in the contaminated areas of Coimbatore district, Tamil Nadu, India and explore the native microbial species viz., arbuscularmycorrhizal fungi and the plant growth promoting bacteria like Pseudomonas sp., etc. for cleaning up of contaminated sites.

Materials and Methods

Sampling sites

Coimbatore (Tamil Nadu state, India) is located at 11.0161°N and 76.971°E. Most areas in Coimbatore have a large number of open and covered sewage channels that are connected to each other which spread around the city premises. Almost all channels carry sewage disposed in an untreated form. The sampling locations were nearer to the places where sewage water and solid wastes are dumped for years together and collected from both barren and agricultural fields (Fig. 1). The samples were also collected from and near gold making waste disposed sewage areas (samples 20, 21 and 26) and sewage treated sludge sample (sample 7). Most of the agricultural lands where the soil samples were taken near the sewage disposal areas were cultivated with Palak greens and Amaranthus spp. as given in Table 1.

Table 1: Collection of soil samples near different sewage disposed areas in Coimbatore

Isolation and screening of AM and PGPR from collected soil samples

Plants in symbiosis with ArbuscularMycorrhizal Fungi (AM) have the potential to take up cadmium from an enlarged soil volume and provide an attractive system to advance plant based environmental cleanup. Pseudomonas,a plant growth promoter present enormously in the rhizosphere supports the AM symbiosis and thus the combination of AM fungi and Pseudomonasoffers suitable system towards the remediation of cadmium polluted soil.

ArbuscularMycorrhizal fungi as well as Pseudomonas were isolated from soils where cadmium concentration was found higher. Totally ten isolates of AM and Pseudomonas were obtained. AM isolates which were observed predominant have been selected for the study.Simultaneously Pseudomonas isolates were tested for IAA production and cadmium tolerance and the best one was selected for the further study.

Also cadmium content was high in samples due to direct sewage sludge dumping as soil heap in nearby agricultural fields (ranged from 4.5 to 8.2 mg kg^-1) and sewage water treated sludge application (3.55 mg kg^-1). The nearby Amaranthus field (irrigated by well water as well as direct sewage water) and Marigold field (fertilized with treated sludge) were found with increasing level of cadmium

Cadmium in soils can be immobilized by increasing the soil pH through addition of liming materials.²⁰Decrease in cadium uptake arise from increased Cd²⁺吸附引起的pH值,增加负charge, thereby decreasing its bioavailability. It is also recommended that soil pH be maintained at pH 6.5 or greater in land receiving cadmium containing soil amendments such as phosphatic fertilizers and biosolids.¹⁶The bioavailability of Cd could be decreased by 2-3 fold with ageing which may be attributed to increased immobilization.¹⁸

Conclusion

From the above study regarding cadmium contamination in sewage disposal areas of Coimbatore, it was important to report that despite high level of cadmium content in these areas, a significant proportion of added cadmium existed in a non-bioavailable pool in the soil. Moreover the soil physio-chemical properties were effectively correlated with the cadmium content in agricultural and barren fields. different land use locations. Exact mechanism in availability was not investigated.In due course of years there may be chances for the movement of cadmium into subsurface layer through leaching or soil diffusion mechanisms if this practice of sewage dumping takes place near the agricultural fields.The transfer of Cd to food chain can be managed through decreasing the bioavailability of cadmium by controlling sorption reactions.

Acknowledgement

We would like to acknowledge the Department of Science and Technology, Ministry of Science and Technology, Government of India, New Delhi for the financial support of this project. The suggestions ofDr.K.Arulmozhiselvan, Professor of Soil Science and Agricultural Chemistry, Tamil Nadu Agricultural University, Coimbatore regarding the presentation of is deeply honored and appreciated.

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