Research proposes an alternative for a more rational and efficient use of nitrogen fertilizers in tropical pastures

The concern with agricultural sustainability, especially in the vast tropical pastures, occupied by marandu grass (Urochloa brizantha cv. Marandu), enhanced the article “Nitrogen fertilization regulates crosstalk between marandu palisadegrass and Herbaspirillum seropedicae: An investigation based on 15N isotopic analysis and root morphology".

The work has just been published in the journal Environmental Research, IF: 8,3, a scientific journal of great international relevance in the area of ​​environmental sciences. Researcher Cássio Carlette Thiengo explains that the objective was to evaluate the compatibility between inoculation with an endophytic diazotrophic bacterium (Herbaspirillum seropedicae HRC54) in marandu grass plants grown under nitrogen fertilization levels, with a focus on better exploring the synergistic effect of these inputs on sustainable agricultural production. 

“In the search for promising alternatives to reduce dependence on nitrogen fertilizers and increase nutrient use efficiency, inoculation with plant growth-promoting bacteria has been considered a potential solution. This approach, which is low-cost, easy to acquire and apply, and does not generate polluting waste, is aligned with sustainable and low-carbon agriculture, meeting contemporary demands”, explains the author.

In practice, the researcher examined the role of inoculation of Herbaspirillum seropedicae combined with different levels of N fertilization focusing on the contribution of fixed-N and N fertilizer recovery using two isotopic techniques (isotopic dilution 15N and natural abundance δ15N‰) and also considering root assessments, nutritional and productive measures. The study was conducted in a greenhouse, under controlled conditions. The treatments consisted of a combination of four levels of nitrogen fertilization, named as control, low, medium and high (0, 25, 50 and 100 mg N kg soil-1, respectively), in plants inoculated or not with the HRC54 strain of H. seropedicae. Urea was the nitrogen source used.

In summary, Thiengo explains that nitrogen fertilization regulated the 'crosstalk' between the marandu grass and Herbaspirillum seropedicae. Inoculation was most effective in promoting marandu grass growth when little (25 mg kg −1 N) or no nitrogen fertilizer was applied. “This compatibility was evidenced by the increase in biological nitrogen fixation and changes in root architecture, resulting in better exploitation of soil resources (more N, P, K, Mg and Fe accumulated) and the nitrogen fertilizer applied, which consequently increased forage production”, he adds. The author also highlighted the notable role of methodologies based on stable nitrogen isotopes, which made it possible to quantify biologically fixed nitrogen, from fertilizer and from the soil reservoir absorbed by plants. “This provided us with solid clues about the main growth-promoting mechanisms triggered by inoculation with H. seropedicae and to what magnitude they were affected by the intensification of nitrogen fertilization”.

The research is part of Cassio Thiengo's master's thesis, presented in the Postgraduate Program in Soils and Plant Nutrition at Esalq/USP and supervised by Prof. José Lavres, from the Division of Development of Analytical and Nuclear Methods and Techniques, Stable Isotopes Laboratory at Cena/USP. It also has support from the São Paulo State Research Support Foundation (Fapesp - process: 2021/05372-0) and was supported by Prof. Fernando Galindo (FCAT/Unesp) in monitoring and methodological validation of the project and Prof. Fábio Lopes Olivares (LBCT/UENF), who provided the HRC54 strain of H. seropedicae for the study.

“Our findings highlight the importance of controlled application of nitrogen fertilizers to optimize the positive effects of inoculation with endophytic diazotrophic bacteria and achieve greater efficiency in agricultural production, thus contributing to sustainability and responsible management of natural resources”, concludes the researcher.