Preface

Plant and soil form an integrated system. Technogenic contamination of soils with potentially toxic trace elements (PTE) are reflected in the functioning of plants and soil biota. Soil contamination by PTE has several implications for human health, as well as for the biosphere. Trace element "biogeogenic cycling" in the environment is an integral function of the ecosystem (aquatic, terrestrial, and atmospheric). Therefore, the aim of this collective work is to deal with the trace elements in the holistic environment, considering advancements in the state-of the-art analytical techniques, molecular biology, and contemporary biotechnology that enhance our knowledge of the behavior of trace elements in the biogeosphere and organismal levels, i.e., at the cellular and molecular levels. Various chapters of this book provide the background with appropriate examples to understanding the trace elements in the biogeosphere on bioavailability, biogeochemistry, biotechnology, bioremediation, and risk assessment.

Trace element behavior and fate depend on their chemistry in soil inorganic and organic phases; their bioavailability depends on a variety of factors concerning the ambient environment, soil, and/or sludge. Trace element enrichment in soil, water, and air may result from natural sources and/or anthropogenic activities such as smelting, mining, agricultural, and waste disposal technologies. For example, coal fly ash application to soils and its effect on boron and other trace element availability to plants; bioavailability of trace elements in relation to root modification in the rhizosphere; and availability through sewage sludge are some important issues discussed in this book. To better explore adaptive physiology of plants exposed to elevated doses of trace elements, knowledge of the behavior of the essential and nonessential elements, aspects related to biogeogenic cycling, accumulation, and exclusion mechanisms by target organisms is a must.

It is generally accepted that the rhizosphere plays an important role in the bioavailability of trace elements. The mechanisms involved in chemical modifications in the rhizosphere, as well as on uptake of trace elements, differ among plant species and soil conditions. The ability to manipulate siderophore production in the rhizosphere to improve plant trace element nutrition will remain a significant challenge for the future to investigate. The importance of mycorrhizal symbiosis for the establishment of a sustainable plant cover on soils with PTE is therefore obvious. Microbial genomics is an integrated tool for developing biosensors for toxic trace elements in the environment, arbuscular mycorrhizal fungi, and the role of arbuscular mycorrhiza and associated microorganisms are increasingly considered in phytoremediation of heavy metal polluted sites. Plant metallothionein genes; genetic engineering for the cleanup of toxic trace elements; and "metallomics," a multidis-ciplinary metal-assisted functional biogeochemistry — its scope and limitations as the crux of biotechnology and its role in dealing with the PTE in the environment are some of the themes reviewed in different chapters.

Self-cleaning of soils does not take place or, rather, takes place extremely slowly. The toxic metals in top soil, thus get accumulated in plants. Plants can remediate metal pollutants mainly in two ways: (1) phytostabilization, in which plants convert pollutants to a less bioavailable form and/or prevent pollutants' dispersal by wind erosion or leaching; and (2) phytoextraction, in which plants accumulate pollutants in their harvestable tissues, thus decreasing the concentration of the pollutants in the soil. Plants that accumulate and/or exclude toxic trace elements; tolerant plants and biodiversity prospecting to promote phytotechnologies for environmental cleanup; phytoman-agement of abandoned mines and biogeochemical prospecting; phytoremediation of contaminated soil with cereal crops; and the role of fertilizers and bacteria in biavailability of metals are reviewed.

Phytotechnologies using trees; stabilization, remediation, and integrated management of metal-contaminated ecosystems by grasses; applications of weeds more adapted to unfavorable soil conditions such as low moisture; presence of toxic metals easily acclimatized to local situation that would act as sentinels for monitoring trace element pollution; detoxification and defense mechanisms in metal-exposed plants; biogeochemical cycling of trace elements by aquatic and wetland plants and its relevance to phytoremediation; plants that hyperaccumulate PTE and biodiversity prospecting for phytoremediation; phytomanagement of radioactively contaminated sites; phytoex-traction of Cd and Zn by willows — advantages and limitations; adaptive physiology; and rhizo-sphere biotechnology are covered in the sections on biotechnology and bioremediation.

Bacterial biosorption of trace elements; processes and applications of electroremediation of heavy metal-contaminated soils; and application of novel nanoporous sorbents for the removal of heavy metals, metalloids, and radionuclides are some of the emerging areas of research that have been included in this book.

The increasing level of trace elements in the tissues of plants and animals due to bioaccumulation and trophic transfer has adverse effects on ecological and human health. Therefore, the risk assessment, pathways, and trace element toxicity of sewage sludge-amended soils and usage in agroforestry; trophic transfer of trace metals and associated human health issues; and PTE accumulation, movement, and remediation in soils receiving animal manure are also covered.

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