What is biological nitrogen fixation (BNF)?

It is the process through which nitrogen (N2) that is present in the atmosphere is converted into formats that can be used by plants. The reaction is catalyzed by an enzyme named nitrogenase, which is found in all nitrogen-fixing bacteria. In the context of agriculture, the symbiosis between nitrogen-fixing bacteria (called rhizobia) and legumes (family of plants that includes soybeans, beans, peas, and others) is the most important one.  

Do all plants biologically fix nitrogen in symbiosis with rhizobia?

Unfortunately, not all do. Such symbiosis is restricted to leguminous plants and is characterized by the formation of specialized root structures known as nodules, in which the BNF process occurs. After the root nodules are formed, bacteria start to fix atmospheric nitrogen into organic compounds that are used by the plants, eliminating or reducing the need to use nitrogen fertilizers.

Does that mean that legumes are the only plants to benefit from BNF?

No, other bacterial species that can fix atmospheric N₂ have already been found in association with grasses such as maize, wheat, and sugarcane. In these plants, root nodules are not formed, and the amounts of fixed N are very low. Therefore, it is not possible to successfully work with such crops without the use of nitrogen fertilizers. Nitrogen-fixing microorganisms have also already been found in plants such as coffee, African oil palm, cassava, papaya, and banana, and their contribution for these plants has been the object of many studies.

What does the inoculation process consist of?

Inoculation is the process through which nitrogen-fixing bacteria selected by research are added to plant seeds before sowing. Inoculation is performed with a product called inoculant or biofertilizer.

In which crops is the use of inoculants common?

The product that is sold in the largest scale is for soybean crops, for which 20 million doses are sold in the country. Other plants that also make use of this technology include beans, cowpeas, peanuts, different kinds of green manure, and tree legumes used in the recovery of degraded areas, all of which are plants that form nodules. This is an innovative product widely used in other countries such as Mexico and Argentina, for instance.

Should seeds be treated with fungicides before or after inoculation?

Before. It is very important that farmers are attentive to the fact of that the inoculant cannot be mixed with fungicides or micronutrients because, to some extent, they are toxic to bacteria. Generally speaking, fungicides based on heavy metals like zinc and copper and some organophosphorus insecticides hinder nodulation in legumes. Herbicides and nematicides are less toxic. As for the case of seeds treated with fungicides and then inoculated, they must be sown within 12 hours. If that is not possible, the seeds must be inoculated again.

What is the importance and the main advantages of the process of inoculation for soybean cropping?

In Brazil, thanks to the process of BNF, inoculation fully replaces the need for nitrogen fertilization in soybean crops. The inoculant contains selected Bradyhrizobium bacteria that, when associated with soybean roots, can convert N₂ from the atmosphere into nitrogen compounds, in quantities of up to 300 kg of N/ha, which will be used by the plants. In addition to the savings obtained from replacing the use of industrialized nitrogen fertilizers with the inoculation of soybeans with Bradyhrizobium bacteria, there is the fact that this is an extremely simple technology that does not pollute the environment.

What is the cost of inoculants for farmers?

Despite the investment in research and technology, the cost of enough inoculant to plant one hectare generally does not exceed the amount of R$ 8.00.

The most well-known N₂-fixing bacterium is the rhizobium applied in soybean crops. In which other crops can it be applied?

Beans, peas, cowpeas, forage legumes, tree legumes, and green cover crops. With regard to common beans, due to a series of factors related to the bacteria, the plant, and the environment, inoculation is not always enough to supply the N demanded by the crop, unlike the case of soybeans. Yet, some research results indicate that bean plants can considerably benefit from the biological process, especially because inoculants include more efficient strands.

In the case of peas and lentils, Embrapa Cerrados has selected and launched rhizobium strands that are adapted to Cerrado conditions, are capable of fully replacing the use of nitrogen fertilizers in these crops, similarly to the case of soybean crops. For cowpeas, in studies coordinated by Embrapa Agrobiology, the selection of efficient strands adapted to regional conditions has culminated in the launch of strand BR 3267, which has provided for productivity increases of up to 40% in experimental conditions and of up to 52% in experimental farm areas.

Is maize also a crop that can benefit from BNF?

Yes, it is. Research has discovered bacteria that are capable of reducing air nitrogen to a form that is assimilable by plants, a species which is called Herbaspirillum seropedicae. The bacteria were initially isolated from rice, maize, and sorghum, and later from other plants such as sugarcane and forage grasses. According to research, by using this  bacteria as a maize inoculant, farmers can reduce the N-fertilizer dose by up to 50% .

Besides nitrogen nutrition, can inoculation with N₂ fixing bacteria bring other benefits for the plants?

In addition to BNF, most of such bacteria are also known for their capacity to produce plant growth hormones. The production of such plant growth substances can stimulate an increase in root hair density, in the rate of the appearance of secondary roots, and in root surface. Such increment results in improved absorption of water and nutrients, thus increasing, the plant's capacity to produce and to withstand environmental stresses.