What is the Rhizosphere and how does it work?

The rhizosphere is defined as ‘the volume of soil surrounding roots in which bacterial growth is stimulated.’ This definition can be expanded to include the root tissue because it is involved in the release of exudates into the soil which promote microbial degradation of organic materials. (17) The rhizosphere is ~ 1 mm thick around the root tissues. The plant and the bacteria in the rhizosphere live in a delicate balance- if the plant is over-watered, injured, or subjected to drought, the bacteria are also affected. This mutualistic relationship is critical to the survival of all plants.

Microorganisms generally surround the roots, but are not present on the root apex because it is advancing through the soil. The apex does produce a polysaccharide called mucilage that can serve as a ‘glue’ to adhere the microbes to the rest of the epidermal root cells. (4)

The epidermal cells of the root release exudates which can induce microbial metabolism of organic materials in the soil (including pollutants). (17) These exudates include carbohydrates, lipids, organic acids, vitamins, and other essential nutrients for microorganisms. When a tree begins to die it will release the exudates in excess. (16)

The rhizosphere is a mutually beneficial relationship- bacteria also give advantages to plants. Bacteria in turn will release enzymes that inhibit the development of nematode eggs and their subsequent penetration into the root tissue. (15) They also are involved in nitrogen fixation that is later uptaken by the plant. (4)

The soil directly surrounding the plant generally has a different pH than the surrounding soil. This is due to the fact that as the plant uptakes/secretes different chemicals, it greatly alters the pH of the surrounding soil and affects in turn the microbes dependent on that plant. It is important for the bacteria present to be adaptable to changing environments.(7)

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The bacteria present in the rhizosphere are generally capable of great environmental stress, such as low water levels in the soil. (17) Gram-positive bacteria such as the Bacillus spp. and Arthrobacter spp. will form spores and become metabolically inactive until environmental conditions improve. Gram negative bacteria are especially resistant to hydrogen peroxide, which is released by the plant during growth. (17) It has been demonstrated that soil microbes are resistant to anthropogenic interference as well such as ploughing and liming, but generally are susceptible to death under biological additives to the soil or severe pollution accompanied by mine tailings. (9)

There are many factors that go into promoting a healthy rhizosphere capable of degrading a contaminant successfully. Soil factors include pH, water content, solubility, bioavailability of nutrients, and temperature.