Nitrogen Fixing Bacteria Rhizobium: How It Supports Legume Growth and Soil Fertility

Rhizobia are soil bacteria which are beneficial and enter into symbiotic relationships with legume plants and promote the fixation of atmospheric nitrogen into plant available forms. This is a natural process and one of the most significant biological processes involved in the nutrition of crops such as soybean, pea, chickpea, lentil, bean and other legumes related to nitrogen nutrition. The genus Rhizobium and several other genera and many species and strains of these microorganisms, such as Rhizobium, Bradyrhizobium and Mesorhizobium are broadly called Rhizobacteria. One such benefit of rhizobium relationships with nitrogen fixing bacteria rhizobium is that legumes are able to tap into a large atmospheric nitrogen supply as opposed to being dependent on external nitrogen supply.

Key importance of rhizobial bacteria is due to their capacity to form a specific relationship with compatible legume root. When the successful association is completed, the bacteria reside in the root nodules in which the biological nitrogen fixation formerly takes place. The plant offers some carbon compounds and energy from photosynthesis, the bacterial partner of the fixed nitrogen needed by the host plant. A relationship example is highly specialized symbiotic bacteria which live on plants. This biological relationship can be a key element in effective nutrient management for farmers, especially in demanding N applications such as cropping systems.

The close relationship of legumes with legumes and nitrogen fixing bacteria is because the legume-legume and legume-nitrogen fixing bacteria relationship may provide a natural contribution to the crop nitrogen nutrition. However, the crop species are legumes, so it is not always possible to have successful nitrogen fixation as this is.

What bacteria stay on the root nodules of legume?

The question is: "Which bacteria has been organically inherited by the legume plants to live in their root nodules where they facilitate nitrogen fixation?". The answer is: a group of bacteria called rhizobia that have successfully recognized and infected in compatible plants of a legume host. 

Early life in the association involves bacteria in the vicinity of the root reacting to the plant's chemical signals. Sometimes the bacteria can infect in a very coordinated manner and attach themselves to root hairs that are compatible. The root hair spirals around the bacteria and an infection court forms; bacteria enter plant tissue through the infection court. The plant then develops special legume root nodules which provide a controlled environment in which the microorganisms can participate and grow with nitrogen fixation.

Once in these nodules, rhizobial bacteria are able to get carbohydrates and other nutrients from the tissue of the plant. They in turn supply plants with fixed nitrogen that can be utilized for plant biosynthesis. Successful nodulation, and thus success of the bacteria in the nodule, must be more than just the presence of bacteria in the area near the root, it must involve successful recognition, infection, establishment of nodules, and continued metabolic activity within the nodule. Farmers can determine this relationship by inspecting the roots closely and by inspecting the quantity and quality of nodules.

What is Symbiotic Nitrogen Fixation?

People frequently may ask: How do legumes fix nitrogen? The answer is in the biological 'relationship' of the legume with the rhizobia within the nodules. Atmospheric nitrogen fixation is mostly bound in the form of molecular N₂ which is inutilizable for plant growth. However Rhizobia produces a nitrogenase enzyme that converts N₂ gas into reduced nitrogen compounds which enter the plant's N metabolism. This process uses a lot of energy, provided indirectly by the plants via the process of photosynthesis, which breaks down carbohydrates.

The role of rhizobium in nitrogen fixation is thus very specific. This exchange is very important in the legume cropping system for the preparation of N for atmosphere fixation. The plant provides C and energy to the bacterial partner and the bacteria provide biologically fixed N to the host. The symbiotic biological nitrogen fixation is an efficient natural association between plants and microorganisms, which is a mutual exchange. It is also the foundation for symbiotic (mutualistic) nitrogen fixation of great significance in sustainable crop nutrition and agricultural nutrient management.

The importance of Rhizobium in nitrogen fixation and nitrogen cycle

The role of rhizobium in nitrogen fixation is not only confined to the individual legume plant but also forms of biological atmospheric nitrogen fixation are a tie between the crop and other nitrogen cycling activities in agriculture. In a fixed state, atmospheric N₂ gets converted to plant-available N in the plant–microbe system.

In the case of industrial fertilizer production, the reactive N- compounds are produced using industrial processes, whereas in the case of the atmospheric N-fixation, the compounds are produced by biological activity. Specialised bacteria in legume systems can do this with the nitrogenase enzyme. It is important to note this difference as the nitrogen supplied by biological fixation is not produced outside the crop system and transported to the farm as a manufactured fertilizer.

Symbiotic biological nitrogen fixation therefore can have a part to play in an increasingly self-supported nutrient cycle within the agriculture system. Residues from legumes can replenish the soil with organic nitrogen and a rotation system using legumes can be beneficial to the soil following crops with proper management practices. Accordingly, the role of the bacteria role in nitrogen cycle is much broader than just colonization of the roots: microorganisms help in transformations that affect nitrogen cycling between the soil organic matter, mineral forms, plants and atmosphere.

What is the importance of legguminos inoculants to the farmers?

Legume inoculants are especially beneficial when growers are not able to be certain that sufficient and compatible populations of nitrogen-fixing bacteria are present in the soil. This may be the case where a legume has not been grown on a field for several years, new bacteria have not established and spread into the soil, or when the natural soil bacteria are not effective for the legume to be planted. Under these conditions, selective introduction of the right partner of microorganisms may enhance the chance of successful nodulation.

A rhizobium inoculant will normally be used on seed, or in or near the planting area, depending on the system and type of inoculant used. The objective is to put a suitable bacterial population in close proximity to developing roots at the time when plant–microbe recognition and infection can occur. Crop-specific selection is crucial as each of the legumes needs to have the appropriate bacterial partner. Just because two products are alike in the sense that they belong to the same family does not mean that either will perform well on the other crop.

The efficiency of the rhizobial bacteria for the plant in question is also affected by the treatment of the inoculant prior to use. They are living microorganisms and may survive or not depending on handling and storage methods, the term and/or high temperatures during handling or possible chemical interactions with seed treatments. Finally, symbiotic bacteria act to their best advantage when used with a crop that is compatible and with soil moisture and fertility levels that are appropriate, along with temperature and pH levels.

Soybean Rhizobium Inoculant: What is the Have to Know?

Plant growers can check nodulation after plants are emerged and established by looking at the root system. When planting, avoid aggressive uprooting and remember to take care to avoid detaching the nodules from the root. Healthy, active nodules have a pink or red inner colour due to leghemoglobin and active nitrogen fixation processes. Producing a simple diagnosis by counting nodules is not sufficient – the position, growth and internal colour of the nodules give more information on effectiveness of biological fixation.

Thus, choice of rhizobium inoculant for soybeans must be made in light of the previous years' field experiences, rotations, soil factors, and handling of inoculant. Legume inoculants are most beneficial when they supply the proper and proficient microbial inoculant under circumstances that allow for the establishment. 

What could be the Factors Involved In the Nodulation of Rhizobium and N2-fixation?

An inoculant is not all that is needed to have an effective symbiotic nitrogen fixation. Factors beyond just light and moisture (such as soil pH and temperature, and the availability of nutrients and developing root systems) may also affect bacterial survival in the soil, infection and nodule formation. 

Rhizobial bacteria strains can also differ in their performance related to their strain compatibility and competitiveness. The bacteria strain needs to be able to recognize the host crop and develop an effective interaction with it. The introduced microorganisms can be useful for one field to the next, and may compete with the native microorganisms already in the soil. That's why it is important to take into account the crop history, especially when deciding whether or not to inoculate due to local soil conditions.

The role of rhizobium in nitrogen fixation of rhizobium may also be influenced by the pool of readily available N in soil. Under conditions where there is plenty of mineral nitrogen available, legumes may not be as motivated to invest in the BNF because acquisition from the soil can be achieved already. Excessive rates of nitrogen application is therefore preferred to balanced rates when the aim is to promote productive nodulation. The interaction between legumes and nitrogen fixing bacteria is best if both the root and bacteria, and also the soil environment around them, can help each other, rather than if there is too much for one or a weakness in the other.

How Novobac Rhizon helps with Legume management?

Novobac's Rhizon Legumes Rhizobium Inoculant is a microbial inoculant for legumes meant for promoting beneficial rhizobial associations with effective and compatible crops. The desired result of using such an inoculant for a grower is to provide an inoculant to soil that actually results in good root colonization, biological nitrogen fixation and nodule production. It should, therefore, be seen in the context of a comprehensive program of crop nutrition and soil management.

Farmers are required to take into consideration the compatibility of crops, the timing of application, storage conditions and plantings when applying nitrogen fixing bacteria rhizobium products. When soil fertility is seriously restricted and roots are damaged and/or conditions to crop establishment are poor, an inoculant will be ineffective if applied. Installation of appropriate moisture, management of balanced fertility and avoidance of practices that affect the survival of the microorganisms can contribute to creating an optimal environment for the association between plant and microbe.

The primary objective of a rhizobium inoculant is to ensure good microbiotic establishment, not to take the place of proper soil fertility management. Similarly, legume inoculants should be chosen based on the crop and the bacterial association which it will need. If applied correctly as part of an integrated nutrient-management approach, the appropriate inoculant can enhance the biological and agronomic processes that make legumes so important tools in sustainable crop production.

If farmers think of using legumes and nitrogen fixing bacteria, the major emphasis will be on seeing inoculation in conjunction with a whole production system. The result is dependent on microbial compatibility with the crop, planting practices, soil conditions and appropriate nutrition. Hence, all these factors are favorable for plant growth and contribute to the efficiency in the use of symbiotic biological nitrogen fixation as a source of nitrogen for crop growth and in the sustainability of the farming system.

Reference:

• Smith, R. S. "Legume inoculant formulation and application." Canadian journal of microbiology 38.6 (1992): 485-492. 
• Albareda, Marta, et al. "Alternatives to peat as a carrier for rhizobia inoculants: solid and liquid formulations." Soil Biology and Biochemistry 40.11 (2008): 2771-2779.