ROOT EXUDATES AND MICROORGANISMS
B-J Koo and D C Adriano, University of Georgia, Aiken,
SC, USA
N S Bolan, Massey University, Palmerston North, New Zealand
C D Barton, University of Kentucky, Lexington, KY, USA
Introduction
Plant roots influence the physical, chemical, and biological conditions of the soil in the rhizosphere. The biogeochemical reactions induced by microorganisms at the soil-root interface (i.e., rhizosphere) play an important role in the bioavailability of nutrients and metals to plants. This microenvironment is characterized by distinct physical, chemical, and biological conditions compared with the bulk soil, largely created by the plant roots and its microbial associations. Such associations can include nonsymbiotic and symbiotic organisms such as bacteria and mycorrhizal fungi. The microbial populations are an essential part of the rhizosphere and affect the rhizosphere soil by their various activities such as water and nutrient uptake, exudation, and biological transformations.
Organic acids, sugars, amino acids, lipids, coumarins, flavonoids, proteins, enzymes, aliphatics, and aromatics are examples of the primary substances found within this microzone. Among them, the organic acids have received considerable attention owing to their role in providing substrates for microbial metabolism and for serving as intermediates for biogeochemical reactions in soil.
Nutrients
and metals are typically present in the soil solution at low concentrations
and tend to form spar-ingly soluble minerals (except nitrogen, sulfur,
and boron), or may be adsorbed to a solid phase through ion exchange,
hydrogen bonding, or complexation. The extent to which they are transferred
from the soil to the .biota (i.e., microbes or plants) is dependent on
the biogeochemical interactions and/or processes among the soil, plant
roots, and microorganisms in the rhizosphere. At this interface, the presence
of root exudates may influence chemical reaction kinetics within the soil
environment and subsequently affect biological activities. As such, understanding
the role of the rhizosphere on biogeochemical processes within the soil
is essential for developing bioremediation technologies of inorganic and
organic contaminants.
SREL Reprint #2833
Koo, B-J, D. C. Adriano, N. S. Bolan and C. D. Barton. 2005. Root exudates and microorganisms. p. 421-428. In Encyclopedia of Soils in the Environment, edited by D. Hillel. Elsevier Academic Press. Amsterdam, The Netherlands.
