Bacteria replace nitrogen fertilizer in cowpea crops in Amazonas

Research carried out by Embrapa Western Amazon (AM) points out that the inoculation of rhizobia bacteria in cowpea can replace nitrogen fertilization in the Direct Planting System (SPD) (learn more in the table below), as it maintains the same productivity of around of 1,5 kg per hectare. In this process, the seeds are coated with inoculant, a commercial product that contains beneficial bacteria called rhizobia. They are able to take advantage of the nitrogen present in the air and make it available to plants. In the study in question, the use of the bacteria Bradyrhizobium spp. allowed a 20% reduction in costs, with productivity similar to the application of 40 kg per hectare of nitrogen in Oxisol soils in the state of Amazonas.

The conclusion is presented in Technical Circular 87, "Inoculation of rhizobia in cowpea cultivated in the Direct Planting System in the state of Amazonas, Brazil", authored by researchers Aleksander Muniz (Embrapa Amazônia Occidental), Inocencio de Oliveira (Embrapa Arroz and Feijão) and Enilson Sá (Federal University of Rio Grande do Sul (UFRGS). The research was supported by the Agrisus Foundation.

Cowpea (Vigna unguiculata) is produced mainly in the North and Northeast regions of Brazil. Depending on the region, it is known as black-eyed peas or black-eyed peas. In Amazonas, where it is called beach bean, the average productivity achieved by farmers is around 900 kg per hectare.

Based on the price of inputs, in December 2022, under the conditions of Manaus, in Amazonas, the savings using the inoculant instead of nitrogen fertilizer are around 560 reais per hectare, according to a calculation made by researcher Inocencio de Oliveira.

Biological nitrogen fixation: science enhances natural process in agriculture

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

In research carried out in Amazonas with cowpea inoculation in direct planting, the inoculant used was the BR 3267 strain (SEMIA 6462), authorized by the Ministry of Agriculture and Livestock (Mapa) since 2006, as a recommended microorganism for the production of inoculants for beans. -cowpea in Brazil. 

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

The two cowpea cultivars achieved productivity gains with inoculation without the addition of nitrogen. Oliveira emphasizes that in cultivation it is important to use seeds of good health and good vigor. 

The results of the research showed that in the experiment where cowpea seeds were inoculated with Bradyrhizobium spp. and the inoculant SEMIA 6462, without nitrogen fertilization, a grain productivity of 1.498 kg per hectare was achieved. Using urea in sowing (20 kg of nitrogen per hectare) and covering (also 20 kg of nitrogen per hectare) yielded 1.580 kg per hectare. In cultivation without inoculant and without nitrogen via fertilizer, productivity was 1.187 kg of grains per hectare.

Direct Planting combines production and environmental benefits 

Other studies had already found that inoculation has the same effect as nitrogen fertilization on cowpea productivity, but this was a pioneer in terms of the use of inoculation with rhizobia in the Cowpea Direct Planting System in Amazonas. 

SPD is an agricultural cultivation technique aimed at soil conservation, which allows the accumulation of nutrients and organic matter in the surface layer of the soil and reduces the process of water erosion.

Studies previously carried out at Embrapa Amazônia Oeste with this system in corn cultivation showed that it also favors the conservation of soil carbon during cultivation, maintaining crop productivity, and that, therefore, it can be applied as a management and conservation technology for corn. ground. 

Researcher Muniz, who conducts direct planting research at Embrapa Amazônia Oeste, reports that the results obtained with these researches in the state of Amazonas demonstrate that the Direct Planting System increases the biological quality (microbial activity) and carbon of the soil. Therefore, its use directly contributes to the storage of carbon in the soil and, indirectly, reduces greenhouse gas (GHG) emissions, such as carbon dioxide.