SREL Reprint #2833
Root exudates and microorganisms
B-J Koo1, D. C. Adriano1, N. S. Bolan2,
and C. D. Barton3
1University
of Georgia, Aiken, SC, USA
2Massey University, Palmerston North, New Zealand
3University 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 sparingly 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. pp. 421-428 In D. Hillel (Ed.). Encyclopedia of Soils in the Environment. Elsevier Academic Press. Amsterdam, The Netherlands.