Rhizosphere Microbiology

What is Rhizosphere? The region of the soil in contact with the roots of a plant. It contains many microorganisms and its composition is affected by root activities. In another word, it is an environment under the influence of plant. The Rhizosphere is the zone surrounding the roots of plants in which complex relations exist among the plant, the soil microorganisms and the soil itself.

The plant roots and the biofilm associated with them can profoundly, influence the chemistry of the soil including pH and the transformation of mineral compounds.

Rhizosphere can be divided into： Inner rhizosphere: the very root surface Outer rhizosphere: embracing the immediately adjacent soil ☆ ☆

Rhizosphere effect A stimulation that can be put on a quantitative basis by the use of the R：S ratio R：S ratio = number of microorganisms in the rhizosphere soil：number of microorganisms in the non-rhizosphere soil. R/S > 1, good stimulation R/S = 1, no stimulation R/S < 1, inhibition

Importance of Rhizosphere: 1. Plant health 2. Crop yield 3. Nutrient cycling

Methodology: Microscopy Plate count Molecular Biology technique

Plant affects the rhizosphere microorganisms through the release of root exudates What is root exudate? Small molecules that are released from plant roots, which include sugars, amino acids, organic acids and amides.  These molecules influence soil nutrient availability both directly and indirectly by stimulating the activities of certain microbial and fungal components of the soil biota.

The exudates contain carbohydrates, organic acids, vitamins and many other substances essential for life. From 5 percent to 40 percent of the total dry matter production of organic carbon from photosynthesis may be released as exudates!

When plants begin to decline, the amount of organic carbon released as exudates increases. Mineral deficiencies, low amounts of soil air and severe wounding are major causes for the increase. Another way to say this is that an increase in exudates would be caused by over-pruning, construction injury, planting too deeply, over-watering, compaction and planting plants in soils that have a pH too high or too low for their optimal growth.

Factors affecting the amount and the composition of root exudates

Plant species and developmental stage Variety Temperature Light Plant injury Atmosphere Foliar sprays Plant nutrient Moisture Microbial activity

Translocation of microorganisms in the rhizosphere When seeds are used as the carrier for introducing various bio-pesticide and bio-fertilizer. It is important to understand the movement of these agents in the rhizosphere.