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The rhizosphere is the region of soil that is immediately adjacent to and affected by plant roots. The rhizosphere is a very dynamic environment where plants, soil, microorganisms, nutrients and water meet and interact. The rhizosphere differs from the bulk soil because of the activities of plant roots and their effect on soil organisms.
A major characteristic of the rhizosphere is the release of organic compounds into the soil by plant roots. These compounds, called exudates, make the environment of the rhizosphere very different from the environment in the bulk soil. The exudes can be used to increase the availability of nutrients in the rhizosphere and they also provide a food source for microorganisms. This causes the number of microorganisms to be far larger in the rhizosphere than in the bulk soil. Their presence attracts larger soil organisms that feed on microorganisms and the concentration of organisms in the rhizosphere can be up to 500 times higher than in the bulk soil.
Another characteristic of the rhizosphere is the uptake of water and nutrients by plants. Plants take up water and nutrients into their roots. This draws water from the surrounding soil towards the roots and rhizosphere. The balance between the movement of water and nutrients towards the roots and their removal from the soil by roots means that their concentration in the rhizosphere is usually very different from what it is in the bulk soil.
The rhizoplane is the surface of plant roots in the soil. Like the rhizosphere, the rhizoplane is a constantly changing environment. As roots grow they cast off dead cells and must navigate around soil particles, making the rhizoplane highly irregular. The rhizoplane is the site of water and nutrient uptake and the release of exudates into the soil.