Brazil - Bacteria replace nitrogen fertilization in cowpea plantations in Amazonas state

A study by Embrapa Western Amazon shows that the inoculating cowpeas with Rhizobium bacteria can replace nitrogen fertilization in a No-Till Farming System (NTFS), as it maintains the same yield of about 1,500 kg per hectare. In this process, the seeds are coated with the inoculant, a commercial product that contains beneficial bacteria called rhizobia. They can absorb the nitrogen present in the air and make it available for the plants. In the study in question, the use of the bacteria Bradyrhizobium spp. reduced costs by 20%, with a yield that was similar to the application of 40 kg per hectare of nitrogen fertizer in latosol-type soils in the Brazilian state of Amazonas.

The conclusion is presented in the publication Technical Circular 87, "Inoculação de rizóbio em feijão-caupi cultivado no Sistema de Plantio Direto no estado do Amazonas, Brasil"[Rhizobium inoculation in cowpeas grown in No-Till Farming System in the state of Amazonas, Brazil], authored by researchers Aleksander Muniz (Embrapa Western Amazon), Inocencio de Oliveira (Embrapa Rice and Beans) and Enilson Sá (Federal University of Rio Grande do Sul (UFRGS). The research was funded by the Agrisus Foundation. 

Reducing nitrogen fertilization generates savings and increases environmental sustainability.  Photo: Inocêncio Oliveira

Cowpeas (Vigna unguiculata) are mostly produced in the North and Northeast regions of Brazil. Depending on the region, it is known as black-eyed peas. In the Brazilian state of Amazonas, the average yield farmers achieve is around 900 kg per hectare.

Based on the price of inputs in December 2022 under the conditions of Manaus, in Amazonas, savings from using the inoculant  of nitrogen fertilization total around 560 reais per hectare, according to calculations made by the researcher Inocencio de Oliveira.

Biological nitrogen fixation: science enhances natural process in agriculture

The technology to inoculate cowpea seeds with Bradyrhizobium spp. is based on a natural process called Biological Nitrogen Fixation (BNF), which occurs between bacteria from the rhizobium group found in the soil and roots of leguminous plants, such as cowpeas. Such rhizobic bacteria are beneficial for legumes as they have the ability to fix nitrogen from the atmosphere and supply it to plants. Embrapa conducts research with BNF and develops inoculant products that enhance such natural process. Thus, several strains were selected and recommended for use as microbial inoculants of agricultural interest.  

In the study held in Amazonas with cowpea inoculation in no-till farming, the inoculant used was strain BR3267 (SEMIA 6462), which has been authorized by the Ministry of Agriculture and Livestock (Mapa) since 2006 as a recommended microorganism for the production of cowpea inoculants in Brazil.

The cowpea cultivars used in the research were BRS Guariba and BRS Novaera, both of semi-erect size and white grains. These cultivars were launched by Embrapa's Cowpea Breeding and Genetic Improvement Program and have already been evaluated and recommended for sowing in Amazonas.

The two cowpea cultivars achieved productivity gains with inoculation without the addition of nitrogen. Oliveira points out that in crops it is important to use seeds with sanitary quality and good vigor.

The research findings showed that in the experiment where cowpea seeds were inoculated with Bradyrhizobium spp. and the inoculant SEMIA 6462, without nitrogen fertilization, achieved the grain yield 1,498 kg per hectare. The use of urea at the time of sowing (20 kg of nitrogen per hectare) and as a maintenance fertilizer (also 20 kg of nitrogen per hectare) yielded 1,580 kg per hectare. In cultivation without inoculants and without nitrogen fertilizer, the productivity was 1,187 kg of grains per hectare.

No-Till Farming combines production with environmental benefits

Other studies had already found that inoculation has the same effect as nitrogen fertilization on cowpea yields, but this was a pioneering one with regard to the use of rhizobia inoculation in no-till cowpea farming in Amazonas.

NTF is an agricultural technique that aims to conserve the soil, which allows nutrients and organic matter to accumulate on the surface layer of the soil and reduces the process of water erosion.

Previous studies by Embrapa Western Amazon with the NTF system in corn crops have shown that also favors soil carbon conservation during the cultivation while maintaining crop yields, and thus it can be applied as a soil management and conservation technology.

Muniz, who conducts no-till farming research at Embrapa Western Amazon, informs that the results obtained with such studies in the state of Amazonas demonstrate that the No-Till Farming System increases biological quality (microbial activity) and carbon in the soil. Hence its use directly contributes to storing carbon in the soil and indirectly reduces greenhouse gas (GHG) emissions, such as carbon dioxide's.