Biological nitrogen fixation reduces N2O in beans by 50% in the Cerrado region.

Research by Embrapa indicates that co-inoculation, in this case the inoculation of nitrogen-fixing bacteria and another that produces indoleacetic acid, a hormone linked to plant growth, reduces nitrous oxide (N2O) emissions from bean crops in the Cerrado region by up to 50% in integrated crop-livestock systems (ICLS), when compared to the use of urea, a synthetic fertilizer widely used as a source of this chemical element. Nitrous oxide is a gas formed by the loss of nitrogen from the soil to the atmosphere and contributes to global warming because its heat-trapping capacity is greater than that of carbon dioxide (CO2) and its duration is longer than that of methane (CH4).

The research was conducted in a 7,5-hectare area of ​​the Capivara Farm, belonging to Embrapa Rice and Beans, in the municipality of Santo Antônio de Goiás (GO), which has been under crop-livestock integration for 20 years. The management involves the cultivation of brachiaria grass for three consecutive years, with the forage used to feed beef cattle during the dry season in the Cerrado. After the three-year period, the forage is desiccated, and grain crops are established on the brachiaria straw using no-till farming during the summer growing season (October to March). The annual grain crops undergo crop rotation and also remain in the same area for up to three years until a new pasture cycle begins with the planting of forage crops. 

Under these conditions, the Embrapa experiment evaluated the performance of the BRS FC104 carioca bean variety with urea and co-inoculation in two summer crops, in the years 2019/2020 and 2021/2022. Urea (between 200 and 280 kilograms per hectare) was applied conventionally in split doses at sowing and as topdressing. 

For co-inoculation, a mixture of three bacteria was used: two of the species *Rhizobium tropici* and one of the species *Rhizobium freirei*, applied via seed treatment, to promote Biological Nitrogen Fixation (BNF). In addition, a bacterium of the species *Azospirillum brasilense* was sprayed onto the plants and soil to stimulate the production of indoleacetic acid, a substance linked to plant development. Co-inoculation is a practice in which bacteria with different functions are used, and normally, the combined effect is greater than the isolated impacts of each bacterium. 

The results of this research indicate that, in the context of integrated crop-livestock systems (ICLS), with diversified crops combining forage and grain crops – millet, corn, soybeans, rice, and beans – in a no-till system, urea can be replaced by co-inoculation for bean cultivation. One of the benefits is the reduction of nitrous oxide greenhouse gas emissions.

According to Embrapa researcher Márcia Thaís de Melo Carvalho, one of the coordinators of this work, the study demonstrated the viability of co-inoculation for common beans in intensive and integrated production systems in the Cerrado. “Conducting a field trial was crucial to evaluate the impact of co-inoculation compared to urea. The total emission of nitrous oxide from the soil was up to 50% lower in beans cultivated only with co-inoculation (0,208 kilograms per hectare) than in beans cultivated only with urea (0,404 kilograms per hectare) in the ILP system,” she states. 

The researcher emphasizes that there was no loss in bean crop productivity, since the average profitability with co-inoculation was 3,2 kilograms per hectare, which can be considered high, given that the national average is 1,1 kilograms per hectare. "It became evident that co-inoculation of rhizobia with Azospirillum can reduce dependence on expensive fertilizers such as urea, with less climatic and environmental impact."

Progress in studies

Márcia Thaís further reports that co-inoculation of common beans with rhizobia and Azospirillum as sources of nitrogen supply for cultivation is not unprecedented, but there are few studies when this subject is addressed in integrated crop-livestock systems (ICLS). "There is not much research on co-inoculation of common beans in integrated crop-livestock systems, and for this study, we investigated a series of variables related to the soil and the plant, including the bacterial community in the bean rhizosphere," adds Márcia Thaís.

In this sense, integrated crop-livestock systems (ICLS) consolidated over 20 years, such as the one at the Capivara Farm of Embrapa Rice and Beans, have a distinct characteristic, presenting soils with high physical, chemical, and biological quality, in particular. These soils are rich in organic matter, which function both to accumulate carbon and to help improve biological nitrogen fixation (BNF), maintain bean productivity, and reduce dependence on synthetic fertilizers. 

“The synergy between soil quality, cultivars, microorganisms, no-till farming, and diversified systems with integrated production components promotes the best use of natural resources and soil and rainwater in the summer crop season, mitigating vulnerability to extreme weather events and providing greater resilience to crops, even offering a solution for low-carbon agriculture in the Brazilian Cerrado,” points out the researcher.

An additional finding of this research is that, even with the use of urea, the nitrous oxide emission factor from the soil to the atmosphere varied between 0,1% and 0,4%. This rate is lower than that recommended by the IPCC (Intergovernmental Panel on Climate Change). The Panel indicates to governments and policymakers that nitrous oxide emissions should be below 1% for the use of nitrogen fertilizer in tropical soils. The IPCC is a United Nations (UN) body that, based on scientific evidence, makes recommendations related to climate change, its impacts and future risks, and adaptation and mitigation options. Brazil is one of the 195 member countries of the IPCC.

Bio-inputs on the rise

The demand for inoculants and, in general, bio-inputs for crops as an alternative to traditional synthetic inputs has been stimulating agricultural research. Embrapa Soja (PR), for example, has been working on grasses such as corn. There are results showing that the use of selected strains of Azospirillum brasilense increases the efficiency of nitrogen fertilizer use by about 25%.

Embrapa Soja researcher Marco Nogueira explains that this effect is mainly due to the action of phytohormones, which stimulate plant roots. This results in the plant having a more abundant and functional root system, and enabling it to explore the soil more efficiently in search of water and nutrients, including the applied nitrogen fertilizer. This greater utilization of the applied fertilizer contributes to more efficient production, where increased productivity occurs with the same amount of resources.

Embrapa Agropecuária Oeste (MS) researcher Rodrigo Garcia points out that, in the current scenario, efforts from both research and the productive sector have increased the supply of bio-inputs. When used correctly, in conjunction with other conservation practices, they make agricultural production more sustainable. In this sense, he observes that the use of inoculants and co-inoculants, crop diversification in integrated crop-livestock systems (ICLS) or integrated crop-livestock-forestry systems (ICLFS), and no-till farming favor production in the field.

“The benefits of adopting the no-till farming system, which consists of crop rotation, straw on the soil throughout the year, and no tilling, can be intensified with integrated systems. The scientific literature is quite robust regarding the positive effects on greater efficiency in the use of natural resources and agricultural inputs, in addition to increased biodiversity and soil health. Its adoption by the productive sector, combined with the greater use of bio-inputs, is a major step towards regenerative agriculture,” says Garcia.