Biogeochemical Processes in the Rhizosphere: Role in Phytoremediation of Metal-Polluted Soils
W. W. Wenzel1, E.Lombi1 and D.C. Adriano2
1University of Agricultural Sciences, Institute of Soil Science,
Gregor-Mendel StraBe 33, A-1180 Vienna, Austria
2University of Georgia, Savannah River Ecology Laboratory P.O. Drawer
E, Aiken, SC 29802, USA
Introduction
According to most legislative schemes, a soil may require remediation
if certain concentrations of one or more heavy metals is exceeded in a
designated part (topsoil, subsoil) of the soil profile. A multitude of
remediation technologies has been developed for clean-up of heavy-metal-polluted
soils (Iskandar and Adriano 1997; Pierzynski 1997). Classic methods, such
as excavation, thermal treatment and chemical soil washing are typically
expensive and destructive.
Recently, the potential role of higher terrestrial plants in remediation
of metal- polluted soils has been studied by an increasing number of scientists
from various disciplines, including plant and soil sciences. Comprehensive
reviews of the emerging phytoremediation technologies and the underlying
fundamental processes are available (Baker et al. 1994; Cunningham and
Ow 1996; Entry et al. 1996; McGrath 1998; Wenzel et al. 1999), with one
of the most recent being part of this book (see Chap. 14 in this Vol.).
A focal point of soil plant interactions is the microecosystem surrounding
the plant roots, the rhizosphere (Hiltner 1904). Thismicroecosystem is
characterized by different physical, chemical, and biological conditions
that differ from the bulk soil. These are created by the plant roots and
microbial associations. These rhizosphere-related biogeochemical processes
are variable because they are influenced considerably by edaphic and climatic
conditions. The edaphic influence is in turn modified by the soil physical,
mineralogical, chemical, and biological features. Due to the limited spatial
extent of the rhizosphere, special tools and techniques
are required to study its characteristics and processes (Brown and Ul-
Haq 1984; Youssef and Chino 1988; Zoyas et al. 1997).
The
role ofrbizosphere processes in metal tolerance (Ryan et al. 1995; Pellet
et al. 1995, 1997) and phytoremediation has been considered by several
authors (Stomp et al. 1994; Entry et al. 1997; Wenzel et al. 1998), and
only very recently has it been addressed in experiments (Bernal et al.
1994; McGrath et al. 1997). However, no comprehensive review onrbizosphere-based
biogeochemical proces-
ses in phytoremediation is yet available, although rbizosphere research
has been considered a focal point in the development of phytoremediation
technologies (Wenzel et al. 1999).
The focus of this chapter is centered on the role ofrbizosphere biogeochemical
processes in phytoremediation. The fundamentals required for this topic
are discusseq briefly. In Sect. 13.2, important aspects of the fate of
metals in contaminated soils are highlighted. In Sect. 13.3 a summary
of phytoremediation processes and technologies is presented. For complementary
information, the reader is referred to chapter 14 in this volume. A more
detailed picture of the rbizosphere as a micro-environment and the biogeochemical
processes involved is presented in Sect. 13.4. Special attention is given
to interaction of rhizosphere processes with metals. Finally, rhizosphere
biogeochemical processes involved in the phytoremediation of metal-contaminated
soils are discussed by presenting the state-of-the-art technology and
research in this emerging field, and identifying gaps in knowledge and
research needs.
SREL Reprint #2871
Wenzel, W. W., E. Lombi and D. C. Adriano. 2004. Biogeochemical processes in the rhizosphere: role in phytoremediation of metal-polluted soils. p. 273-303. In Heavy Metal Stress in Plants: From Biomolecules to Ecosystems, edited by M. Prasad and J. Hagemeyer. Springer Verlag Berlin Heidelberg New York.
