The rhizosphere is the word used to describe the area of soil surrounding plant roots. It is the most biologically active layer of the soil; populated with micro organisms interacting and benefiting from chemicals released by plant roots (1,2,7). There are more micro organisms present in a teaspoon of soil than there are people on the earth; the rhizosphere can carry 1000-2000 times this amount making it highly populated with microbial life (2). The rhizosphere has three zones (2,9,10)-the endorhizosphere, the rhizoplane and the ectorhizosphere (figure 1).
1. The endorhizosphere: This is the inner section of the rhizosphere. It is the section of the plant root occupied by micro organisms (which benefit from organic compounds released by roots).
2. The rhizoplane: This is the middle section of the rhizosphere. It includes the plant root surface with soil particles adhering to it.
3. The ectorhizosphere: The outer area of soil surrounding the roots.
Rhizodeposition is what makes the rhizosphere an interesting place for biological study. Plant roots release organic compounds into their surrounding soils (1). These compounds are what scientists refer to as rhizodeposits and the process is called rhizodeposition. As plants release organic compounds from their roots they lose some of their carbon to the surrounding soil; this means that they contribute to the soil carbon content (3). So the soil benefits from the plants lost carbon. The release of organic compounds in the root zone also supports the soil microbial life (3). The amount and composition of rhizodeposits from plant roots vary depending on the type of plant, the climate, the nutrient deficiency, and the physical, chemical and biological properties of soil surrounding the root (2). Rhizodeposits are released through the following ways (3)
1. Production of mucilage: Mucilage is a thick, insoluble, polysaccharide rich substance produced by the cells of the root cap. Their function is to lubricate and protect the root tips as they penetrate through the soil (2). Mucilage is produced in many plants and also has the ability to improve the soil structure by binding together soil particles to form aggregates (5).
2. The release of root exudates: Root exudates are released by plant roots. Scientific research has shown that plants release exudates for a number of reasons (a) to restrict the growth of competing plant species (b) to attract symbiotic relationships (for example, the legume-rhizobia relationship which helps to fix atmospheric nitrogen) (C) to change the chemical or physical properties of the surrounding soil (d) as a way to obtain nutrients (2,4,6). The amount of organic carbon deposited by root exudates varies by different plant species (2,3).
3. Sloughing-off of outer root layers: The outer layer of roots sloughs-off as they push through the soil to reach down for nutrients and water (2,3). The sloughed-off layer of the root cap is released to the surrounding soil and becomes available for microbial decomposition.
Bibliography and further reading
1. Newman EI (1985) The rhizophere: carbon sources and microbial populations. Ecological Interactions in soil. A.H.Fitter. Oxford, Blackwell Scientific Publications. Pp 107.
2. McNear Jr., D. H. (2013) The rhizosphere – roots, soil and everything in between. Nature Education Knowledge 4(3):1.
3. Nguyen C (2003) Rhizodeposition of organic C by plants: mechanisms and controls. Agronomie 23: 375-396.
4. Badri DV and Vivanco JM (2009) Regulation and function of root exudates. Plant cell environment 32:666-81.
5. Czarnes et al (2003) Root- and microbial-derived mucilages affect soil structure and water transport, Eur. J. Soil Sci. 51: 435–443.
6. Nardi et al (2000) Soil organic matter mobilization by root exudates. Chemosphere 41: 653-658.
7. Hiltner, L. (1904) “On recent experiences and problems in the field of soil bacteriology and with special regard to the Grundungung and Brache,” Work of the German Agricultural Society, 98, 59-78.
8. Jones et al (2009) Carbon flow in the rhizosphere: carbon trading at the soil-root interface. Plant Soil 321:5-33.
9. Lynch JM (1987) The rhizosphere. Wiley Interscience, Chichester, UK.
10. Clark FE (1949) Soil micro-organisms and plant roots. Advances in Agronomy 1: 241-288.