Soil Microbial Ecology - Soil Molecular Ecology Laboratory

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E X E R C I S E 10
SOIL RHIZOSPHERE:
OBJECTIVE:
Isolate and directly observe your desire plant rhizosphere microorganisms using dilution
techniques and compound microscopes
INTRODUCTION
The rhizosphere is defined as the soil volume in close contact with plant roots. Roots
excrete a wide range of organic materials into the soil and these greatly influence the
development of a rhizosphere microbial community. Microbial populations differ both
quantitatively and qualitatively from those in the bulk soil. Bacteria populations in the
rhizosphere are often 10-100x higher than in root-free portions of the same soil. The
rhizosphere effect may be expressed in terms of a R/S ratio (rhizosphere population/rootfree
soil population). The rhizosphere also contains a higher proportion of gram
negative, nonsporulating, rod-shaped bacteria, and a lower proportion of gram positive,
nonsporulating rods, cocci, and pleomorphic forms.
Microorganisms in the rhizosphere can have a marked influence on plant growth
(positive or negative). Indeed, the rhizosphere is a complex zone of interactions among
microbial populations and between microorganisms and plant roots. In this exercise you
will use the dilution-plating technique to isolate rhizosphere microorganisms and also
observe microorganisms the root surface by direct microscopy.
METHODS
1. DILUTION-PLATE TECHNIQUE
Obtain a block of soil (approximately 10 cm 3 ) that contains herbaceous plant roots.
Crush the block, with as little tearing of the roots as possible. Remove roots and gently
shake to remove superfluous soil. Place roots, along with adhering soil in weighed flasks
containing 100 ml sterile H 2 0 and glass beads. Shake flasks vigorously for 5 min. on a