Rhizosphere: what is it, what is it for, composition and importance

The rhizosphere , considered the largest terrestrial ecosystem, is the part of the soil near the roots of the plant, which extends specifically between 1 and 3 mm from the surface of the roots to the interior of the soil.

In this edaphic region, the roots of the plants interact with the soil and its microorganisms , which consequently provides benefits for the plants, improves soil fertility and favors the degradation of toxic chemical substances. It is an association known as rhizocenosis , which allows either the obtaining of food, as in mycorrhizae, or the fixation of nitrogen, in which bacteria, called rhizobacteria, generally participate, such as Azospirillum, Azotobacter and Bacillus (promoters of plant growth ) in grass roots and Beijerinckia in sugar cane roots.

The scientific community distinguishes 3 different parts that make up the rhizosphere:

• The endorizosphere or internal rhizosphere comprises the root cortex (tissue between the endodermis and the epidermis) invaded by microorganisms.
• The rhizoplane or rhizoplane is formed by the surface of the root and the microorganisms that are in it.
• The ectorizosphere, exorizosphere, rhizospheric soil, or outer rhizosphere is the part of the soil that is in close contact with the root surface of plants.

In general terms, we can say that the composition of the rhizosphere is: soil, water, radical depositions (exudates and mucilage) and microbiota (bacteria, fungi, algae).

On the one hand, the soil acts as a physical support and provides food for the plants. Their physicochemical properties determine the presence and distribution of microorganisms, while their conservation depends on them. We can highlight the pH as an abiotic factor that conditions and characterizes the functioning of the rhizosphere , since changes in its value can lead to the inactivation of the enzymes present in the microorganisms and disturb the fixation of nutritive minerals. In this region the pH values ​​are lower or more acidic compared to the rest of the soil, which is due to cation exchange and the production of organic acids.

On the other hand, water, which is an essential resource for life, plays a very important role in rhizospheric interactions . Its availability is directly related to the porosity of the soil and its potential to some extent is controlled by the microbiota . In fact, microorganisms improve soil aeration and infiltration capacity, thus favoring the maintenance of the water film in the rhizosphere.

In relation to radical depositions , plants release photosynthetic products (sugars, amino acids, vitamins, organic acids, hormones) through their radical exudates which, if they are of low molecular weight, can favor the diversity and microbe growth of the rhizosphere. These organic compounds attract microorganisms to the surface of the roots, constituting a very important source of food and energy for them. One of these exudates is mucigel, which is a gelatinous material that covers the surface of plant roots, made up of: plant mucilage, bacterial cells, polysaccharides, mineral colloids and soil organic matter.

Finally, the microorganisms that inhabit the rhizosphere ( bacteria , fungi, protozoa and nematodes ), mostly report benefits. The rhizosphere bacteria or rhizobacteria are responsible for decomposing radical exudates and plant residues. Fungi also stand out, establishing a symbiotic relationship with the roots, known as mycorrhiza , since, like bacteria, they are capable of degrading or mineralizing toxic and harmful substances in a process called detoxification. In this other article we will tell you more about what mycorrhizae are and their types .

The importance of the rhizosphere lies in the interactions that take place between plants and the soil microbiota. Why? Well, because in line with what was stated in previous sections:

• Through the rhizosphere, plants take in nutrients. The same happens with water and carbon that bacteria, fungi, insects, worms and protozoa need to survive.
• The microflora of the rhizosphere protects the root against pathogens, against radical diseases and produces substances that stimulate plant growth, such as indoleacetic acid, ghibellins and cytokinins.
• Biological Nitrogen Fixation (BNF) occurs, that is, microorganisms provide plants with assimilable nitrogen compounds (ammonium and nitrate), which they manufacture from the atmospheric nitrogen of the soil (N2).

It is well known that nitrogen is a limiting factor for plants because, despite its abundance in the atmosphere (it represents approximately 80% of its composition), they cannot take advantage of it in the molecular form in which it is found (N2). This situation has led to the massive use of chemical fertilizers to increase crop productivity. Consequently, important pollution processes have been unleashed in the natural environment that decrease soil fertility and the quality of soil and water resources. One way to avoid this is to make visible the fixing capacity of the microorganisms of the rhizosphere , which can satisfy the nitrogen demand of the plants , through the use of biofertilizers..