Table Of ContentTrace Elements
in the
Rhizosphere
1535/frame/FM Page 2 Tuesday, August 22, 2000 9:08 AM
Trace Elements
in the
Rhizosphere
Edited by
George R. Gobran
Walter W. Wenzel
Enzo Lombi
CRC Press
Boca Raton London New York Washington, D.C.
Library of Congress Cataloging-in-Publication Data
Trace elements in the rhizosphere / George R. Gobran, Enzo Lombi & Walter W.
Wenzel, editors.
p. cm.
Papers from a special symposium held during the Fifth International Conference on
Biogeochemistry of Trace Elements, July 11–15, 1999 in Vienna, Austria.
Includes bibliographical references and index.
ISBN 0-8493-1535-2
1. Soils—Trace element content—Congresses. 2. Trace element in plant
nutrition—Congresses. 3. Plant-soil relationships—Congresses. 4.
Rhizosphere—Congresses. I. Gobran, George R. II. Lombi, Enzo, 1968– III. Wenzel,
Walter W. IV. International Conference on the Biogeochemistry of Trace Elements (5th :
1999 : Vienna, Austria)
S592.6.T7 T73 2000
631.4′16—dc21 00-058577
CIP
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Preface
The term rhizosphere was introduced by Hiltner in 1904. Since then, the “hidden
half of the hidden half” has been investigated in its many aspects. Initially, the
interest of scientists and practitioners was directed toward plant nutrition, in partic-
ular, as to whether rhizosphere processes influence the uptake of mineral nutrients
by crop and tree species. With the general move of soil and plant scientists from
agricultural to ecological/environmental issues, the emphasis of rhizosphere research
has also increasingly shifted to studying the effect of plant–microbial associations
on the bioavailability, uptake, and transformation of inorganic and organic contam-
inants in soils. Most recently, contaminant–rhizosphere interactions have attracted
renewed attention as green plants have been proposed as being useful in the reme-
diation of contaminated soils (phytoremediation).
While potential practical applications are a driving force in rhizosphere research,
it is equally important to develop methodological tools employed to elucidate the
fundamental physical, chemical, and biological processes that create the highly
dynamic and reactive microenvironment around plant roots. Basically, two
approaches have been taken in the study of the rhizosphere: experimental investi-
gation and modeling. While a relatively large number of publications dealing with
various aspects of the rhizosphere is available, the information is still scattered and
we are far from a comprehensive understanding of the manifold interactions and
feedbacks that occur in the rhizosphere. In particular, only a few examples of studies
that link modeling to experimental work (model validation) are available, and little
is available on the fate of trace elements in the rhizosphere, even though such
information is urgently needed to improve food quality and to develop phytoreme-
diation technologies. Needless to say, the rhizosphere plays a key role in controlling
trace element contamination of the terrestrial food chain. Therefore, it is one of the
hot spots in sustaining ecosystem health.
Interest in this topic led to the organization of a special symposium entitled,
Fate of Trace Elements in the Rhizosphere, held during the Fifth International
Conference on the Biogeochemistry of Trace Elements, July 11–15, 1999, in Vienna,
Austria. This special symposium provided a forum to bring together scientists
involved in various aspects of rhizosphere research. Most of the chapters in this
book comprise work that was presented by the outstanding scientists invited to this
special symposium.
The book Trace Elements in the Rhizosphere is divided in three sections. Section
I (Chapters 1 to 5) emphasizes the bioavailability of trace elements, including
radionuclides in the rhizosphere and their significance in developing management
strategies for phytoremediation. Section II (Chapters 6 to 8) deals with the complex
interactions of root–microbial associations, in particular mycorrhizae, and the trans-
formation of trace elements. Tools and methods available for study of the dynamics
and fate of trace elements in the rhizosphere are discussed in Section III (Chapters
9 to 12), presenting examples of experimental techniques and modeling approaches.
Chapter 1 presents a review of the role that rhizosphere processes play in the
mobilization or immobilization of trace elements and summarizes the state-of-the-
art knowledge on metal hyperaccumulators as compared to nonaccumulating plant
species. Special reference is given to the application of rhizosphere processes to
phytoremediation, and the potential of manipulating the rhizosphere to control trace
element availability for phytoremediation of crops is explored. Chapter 2 provides
a review of complementary in-depth information on trace metal bioavailability in
the rhizosphere. Chapter 3 contributes to an understanding of the specific conditions
that modify trace metal interactions in the rhizosphere. Chapter 4 extends the scope
to radiocesium. Chapter 5 presents a comprehensive review of the fate of U and Pu
radionuclides in soil–plant systems. The role of rhizospheric interactions and their
application to the phytoremediation of U- and Pu-polluted soil and water environ-
ments are emphasized. Chapter 6 provides current information on exudates and
metal-organic complexes associated with root–microbial associations. Chapter 7
emphasizes the importance of metal speciation in symbiotic fungi (ectomycorrhiza),
presenting data generated by micro X-ray absorption spectroscopy, a promising
technique that might be more widely applied in rhizosphere research. Chapter 8
discusses bioavailability of trace elements in mycorrhizal rhizosphere systems,
hence, linking the information provided in Sections I and II. Chapter 9 explores the
application of a sequential chemical extraction technique as a means to study trace
metal fractionation in rhizosphere soils. Chapter 10 presents a technique to study
the effect of micronutrients or toxic trace elements on plant growth, using a steady-
state approach. This methodology may prove useful in rhizosphere studies, since
experimental conditions of microelement nutrition and their interactions with plant
physiological processes can modify root exudation and related processes in the
rhizosphere. Chapter 11 presents a model that describes cation exchange on plant
roots, a process that has been neglected in most models so far. Chapter 12 describes
a model of aluminum chemistry with reference to acidic forest soils.
It is hoped that this book will contribute to the body of knowledge on the secrets
of the rhizosphere. The book is suitable for advanced undergraduate and graduate
courses in soil environmental chemistry, biogeochemistry, and ecology. It should
interest professionals in those areas, as well as consulting engineers and decision
makers involved in bioremediation technology and sustaining ecosystem health.
We greatly acknowledge the authors of this book and their cooperation and
support. We also wish to extend our special thanks to many anonymous reviewers
of each chapter, especially G. Ågren and R. Finlay (Swedish University of Agricul-
tural Sciences, SLU, Sweden), and K. Turnau (Jagiellonian University, Krakow,
Poland). The book’s manuscript was carefully read by M.B. Kirkham (Kansas State
University, U.S.A.) and M.P. Huang (University of Saskatchewan, Canada), who
deserve our special thanks and appreciation. We are also grateful for the financial
support received from the Faculty of Forestry, SLU, Sweden, and the Swedish
Council for Forestry (SJFR); the Agricultural Research, IACR-Rothamsted grant-
aided support received from the Biotechnology and Biological Sciences Research
Council of the United Kingdom; and the support of the University of Agricultural
Sciences, Vienna, Fonds zur Förderung der Wissenschaftlichen Forschung, Austrian
Federal Ministry of Science and Transport.
April 2000 George R. Gobran, Walter W. Wenzel, and Enzo Lombi
Editors
George R. Gobran is Professor of Ecology-Soil Sciences with specialization in
nutrient dynamics in the rhizosphere. Since 1985, Dr. Gobran has been working at
the Department of Ecology and Environmental Research, Swedish University of
Agriculture Sciences, Uppsala, Sweden. Dr. Gobran received his Ph.D. in soil chem-
istry from the Catholic University of Louvain-la-Neuve, Belgium in 1980. He
received his M.S. in soil chemistry in 1975 and his B.S. in soil and water sciences
in 1969 from Alexandria University, Egypt. Dr. Gobran was also awarded a post-
doctoral fellowship from Texas A & M University, U.S.A. (1982–1984).
Professor Gobran has significant experience in research dealing with soil chem-
ical processes, with special interest in soil–plant interactions and rhizospheric pro-
cesses. Currently, he focuses his research and teaching efforts on the reciprocal
effects of soil–plant interactions, especially in ecosystems under environmental
stress. Dr. Gobran has written many papers and book chapters, and has participated
in several international conferences, workshops, and symposia. Additionally, he is
a reviewer for several international journals and programs and is included in the
1999 edition of Who’s Who in the World. With his coeditors, Drs. Wenzel and Lombi,
Dr. Gobran organized a symposium entitled, Fate of Trace Elements in the Rhizo-
sphere, which was presented during the Fifth International Conference on the Bio-
geochemistry of Trace Elements, Vienna, July 11–15, 1999.
Walter W. Wenzel is Professor of Soil Science at the University of Agricultural
Sciences in Vienna, Austria. He has been working on various fundamental aspects
of trace element biogeochemistry, with emphasis on several cationic metals and the
metalloids As and F. Currently, Professor Wenzel’s work is centered around exper-
imental assessment and modeling of rhizosphere processes and metal hyperaccumu-
lation. His involvement in applied research has been largely related to risk assessment
and the development of phytoremediation technologies for the cleanup of soils
contaminated with metals and organic pollutants. Professor Wenzel is coeditor of
the Journal of Environmental Quality and the Journal of Plant Nutrition and Soil
Science. He organized the Fifth International Conference on the Biogeochemistry
of Trace Elements, Vienna, July 11–15, 1999. Additionally Professor Wenzel is one
of the initiators and founders of the International Society for Trace Element Bio-
geochemistry, for which he currently serves on the board of directors.
Enzo Lombi was born in Piacenza, Italy, 1968. After graduation in 1993, he earned
a Ph.D. in agricultural chemistry and conducted his research in Italy, Austria, and
the United States. During 1997–1998 he was Scientific Researcher at the Institute
of Soil Science, University of Agricultural Sciences, Vienna, and at the Austrian
Research Centre, Seibersdorf. His activities focused on the development and appli-
