Nitrogen-fixing bacteria in agriculture, an alternative to the use of inorganic nitrogen fertilization

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[:is]Nitrogen, being the most abundant element in the atmosphere, cannot be used by plants, however some bacteria can use it, and by associating with plants, they take advantage of nitrogen. Much progress has been made in the knowledge of this process, and it is a reality that they can be used not only to save on the use of fertilizers, but also to promote better crop development, increase yields, and promote the development of agriculture in poor soils. (For more information on use & application, contact Pedro Palazon – Technical Director of IDEAGRO, and a great expert and connoisseur of the use and management of natural solutions in agriculture).

Lavoisier called nitrogen "azoe" which means "non-life".«, and this because he saw it differently from oxygen, which was the other component of air, which was already clear to him that it was essential for breathing and therefore life. Today, with the advancement of knowledge, the name that Lavoisier gave it seems ironic, since it is known that nitrogen is absolutely essential for both animal and plant life.

Nitrogen is the most abundant element in the atmosphere (78% is made up of nitrogen). It seems paradoxical that being the most abundant, it is the most difficult to obtain. The reason is that the nitrogen in the air is inert and cannot be directly used by plants or animals.

Atmospheric nitrogen is immobilized with each other by a very stable and very strong triple bond (N2), and under these conditions it cannot be used by plants or animals. In order for it to be used, those bonds must be broken and nitrogen fixed or joined to other elements, such as hydrogen or oxygen. Only under these conditions, the nitrogen put into the soil is absorbed by the roots of the plants.

About Nitrogen Biofertilization

Nitrogen biofertilization is an interesting alternative to the use of traditional mineral fertilizers in modern agriculture. With the use in agricultural soils of bacteria capable of fixing a nutrient as essential as nitrogen, it will be possible, on the one hand, to reduce inorganic nitrogenous contributions, and on the other, to collaborate in obtaining non-polluting and appropriate methodologies from an environmental point of view.

The use of nitrogen-fixing bacteria represents a great opportunity for agriculture since the nitrogen fixed in the soil by the bacteria is directly available right in the place (rhizosphere) where it is required., while inorganic fertilizers applied to the soil suffer a loss of up to 50% due to natural leaching and denitrification processes. In addition, the excessive leaching of inorganic fertilizers can lead to the contamination of groundwater, rivers and lakes, causing ecological damage, and can constitute a risk to animal and human health.

Within symbiotic nitrogen-fixing bacteria we find two groups of organisms.

To the first group belong mobile soil bacteria, which are attracted to the root by compounds that it releases. They belong to the group of aerobic chemoorganotrophs. They are called rhizobia. Rhizobium (nodulates on roots of legumes from temperate and subtropical climates), Azorhizobium (nodules on stems and roots) and Bradyrhizobium (nodulates soybean roots) belong to this group. There are other nodule-formers of doubtful nitrogen fixation such as: Phyllobacterium (forms nodules on stems and leaves of myrsinaceae and rubiaceae) and Agrobacterium.

The second group is made up of Actinomycetes. (Gram positive bacteria) that nodulate roots of many trees and shrubs. They are those filamentous bacteria that live in symbiosis with actinorichic plants (angiosperms capable of forming nodules) and belong to the genus Frankia. It does not form aerial mycelium and its spores are immobile. It nodulates the genera Alnus, Myrca, Casuarina, etc. This nodulation is of great importance for perennial woody plants, because it provides nitrogen to the soil in poor or repopulated areas. Among the symbiont plants we will highlight the legumes (Fabaceae) for their important role in human evolution, providing food (lentils, beans and peas), fodder for animal nutrition (clover, peas, alfalfa...), obtaining wood (Acacia, Leucaena) or to colonize poor soils lacking in nutrients (broom, gorse, broom...).

Nitrogen-fixing bacteria of the genera Azotobacter, Rhizobium and Azospirillum have been the most widely used in agriculture as biofertilizers.The mechanisms by which these bacteria exert these effects are varied. Thus, they can fix atmospheric nitrogen and supply it to the plant; they can synthesize different phytohormones that act by improving different stages of plant growth; solubilize phosphorus minerals making it available to the plant, and synthesize various low molecular weight compounds or enzymes that are involved in plant growth and development. A certain bacterium can affect plant development through one or more of these mechanisms.

Azospirillum has been shown to stimulate root hair density and length, as well as growth of secondary roots and root surface. The intensity of these effects on the root depends on the plant species and the cultivar used, and above all on the concentration of Azospirillum in the medium. In most cases the optimal concentration is 10^7 CFU per seed or seedling. This microorganism influences the concentration of indole acetic acid and indole-3-butyric acid as well as the speed of specific respiration and the activity of enzymes related to the tricarboxylic acid cycle, and also the glycolysis pathway in the roots of maize and other plants.

Biological nitrogen fixation takes place in root nodules. The nodules are the result of a perfect symbiotic relationship between the plant and the bacteria. The bacteria that form part of these root nodules are called rhizobia. Rhizobia independently cannot fix nitrogen, but instead require a host plant. Neither the plant nor the bacterium can independently fix nitrogen.

Different inoculation tests of cereals and forage crops with nitrogen-fixing microorganisms carried out in the field have shown very interesting results from the performance point of viewHowever, as in any test, the results can be affected by a multitude of factors that can intervene affecting yield: contribution to the plant of fixed nitrogen, specificity and adaptation of nitrogen-fixing bacteria to the root habitat, influence of microbial metabolites, improvement of mycorrhization, displacement of other microorganisms from the rhizosphere that are harmful to the plant, differences in the genotypes of the host plant, ability of the inoculated microorganisms to survive in the medium, etc.

Inoculation with these nitrogen-fixing bacteria (Azotobacter & Azozpirillum) allows obtaining the same yields as by using inorganic nitrogenous fertilizers but reducing the applied nitrogenous fertilization by 50%, with the great environmental advantage that this entails.

This fact is particularly interesting within an Organic Agriculture, and for the development of crops in areas vulnerable to contamination by nitrates, in which the amount of nitrogenous fertilizer to be applied annually is limited, thus opening a wide field for the use of nitrogen-fixing bacteria in agricultureThe use of products of this type is particularly recommended in poor soils, with low microbial activity, since in them the competition between the autochthonous microorganisms of the soil and the inoculated ones will be less.

The use of biofertilizers in the soil constitutes a solution for the future in favor of a more sustainable agriculture. The positive effects that can be seen with its use are multiple, and it is that in combination with various factors such as the phytohormonal effect, nitrogen fixation, improved nutrient assimilation, etc.

During the last years, at IDEAGRO, we have worked extensively with bacteria, isolating different strains of microorganisms and developing new formulations based on microorganisms isolated from the soil that have the ability to solubilize phosphorus and potassium present in the soil, as well as fix environmental nitrogen. 

Likewise, we have developed numerous works with bacteria of the genera Pseudomonas, Bacillus, Azotobacter and Azospirillum, evaluating the efficacy in different soils, climates and crops, achieving compelling results. At present, we have a strain that, properly inoculated, allows us to obtain a reduction of 30% in the amount of inorganic nitrogenous fertilizers supplied; increasing not only the production, but also the final quality. 

The set of aspects and improvements achieved translates into a healthier plant/crop, with a greater nutritional contribution, also achieving greater resistance to diseases and higher productive yields. We are currently working with new strains of rapid implantation to colonize the soil with bacteria after disinfection to avoid possible re-infection with pathogens, as well as how developing new formulations and tests that allow us to continue at the forefront in the use of beneficial bacteria in agriculture. In addition, we are working intensively with a crop such as corn with the aim of reducing the contribution of mineral fertilizers.

If you want more information, please contact our Technical Director, Pedro Palazón by email, palazon[at]ideagro.es, or by phone at 968 118 086. Do not hesitate to ask us about it! At IDEAGRO we have a multidisciplinary team at your entire disposal.References:
The use of nitrogen-fixing bacteria in Organic Agriculture – MAGRAM
Symbiotic nitrogen-fixing bacteria – CT 3 (2011) 173- 186 – SCG – U. de Salamanca[:]

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