Discovering how soil microorganisms process the fertilizer and organic residues used in sugarcane cultivation can help mitigate greenhouse gas emissions and make ethanol production more sustainable.

A series of studies with this objective were presented by Eiko Kuramae, a researcher at the Netherlands Institute of Ecology (NIOO-KNAW), during the third edition of the Brazilian BioEnergy Science & Technology Conference (BBEST). Organized under the aegis of the FAPESP Bioenergy Research Program (BIOEN), the event was held on October 17-19 at Campos do Jordão, Brazil.

“When the usual mixture of vinasse and nitrogen fertilizer is applied to sugarcane plantations, the activities of soil microbiota are stimulated and transformations occur, resulting in the production of such gases as carbon dioxide (CO2), methane (CH4) and, above all, nitrous oxide (N2O), whose atmospheric heat-trapping capacity is 289 times that of CO2,” Kuramae said. “Our research aims to understand the functions of soil microorganisms and the processes involved in N2O emissions, mainly in order to find ways to intervene.”

Vinasse is the main residue from ethanol production, she explained. Between 12 and 15 liters of the effluent are estimated to result from the production of one liter of ethanol. If vinasse disposal is not properly managed, it pollutes rivers and causes various kinds of environmental damage.

In recent years, Brazilian legislation has required the reutilization of the vinasse produced by sugar mills as sugarcane fertilizer. However, while vinasse is rich in nutrients, it cannot supply all the plant’s requirements and has to be mixed with inorganic nitrogen-based fertilizer.

In a study performed during Késia Lourenço’s PhD research, the group showed that simply applying vinasse and nitrogen fertilizer separately to soil mulched with leaves from the plant (straw) is sufficient to significantly reduce N2O emissions.

The experiment was conducted during the dry season in Piracicaba, São Paulo State (Southeast Brazil). Concentrated vinasse was applied 30 days before nitrogen fertilizer to the straw-covered soil. In this case, the N2O emissions were 39% lower than when the two substances were applied together. A similar experiment using normal (non-concentrated) vinasse led to a 27% reduction in N2O emissions.

In another test performed during the rainy season, the group applied normal vinasse 30 days before nitrogen fertilizer and measured a 49.6% reduction in N2O emissions.

“In both experiments, the biological processes involved in N2O emissions were found to be highly complex,” Kuramae said. “Nitrification occurs, mainly by bacteria but also by archaea [one of the domains of living beings, similar to bacteria morphologically but different genetically and biochemically]. Denitrification also occurs, by both bacteria and fungi. We showed for the first time that, in tropical sugarcane plantations, when cane straw combines with vinasse and nitrogen fertilizer, soil fungi also contribute to N2O emissions.”

Bilateral support

The research was supported by FAPESP and the Netherlands Organization for Scientific Research (NWO) under the aegis of acooperative agreement between the two institutions.

Part of the research was done in the Netherlands under Kuramae’s supervision and part was done in Brazil at the Agronomy Institute (IAC) under Heitor Cantarella’s supervision.

In the same field of research, Kuramae also partners with Janaina do Carmo at the Federal University of São Carlos (UFSCar) under anagreement between FAPESP and the BE-Basic Consortium.

“Generally speaking, the Brazilian students have led the field experiments in Brazil, in or near Piracicaba and Ribeirão Preto, while the soil microbiome studies and bioinformatics are performed at my lab in the Netherlands,” Kuramae said.

During Johnny Soares’s PhD research, also supervised by Cantarella and Kuramae, the group tested the use of nitrification inhibitors to reduce N2O emissions from sugarcane fields where the soil was treated with nitrogen fertilizer but without vinasse or straw.

“We found that, in the absence of straw and vinasse on the ground, the main process that occurs is nitrification caused by bacteria,” Kuramae said. “We therefore decided to test two compounds capable of inhibiting the enzyme ammonia monooxygenase in bacteria that converts fertilizer nitrogen into N2O. The compounds reduced N2O emissions by 95%, but didn’t affect bacterial diversity in the soil.”

The experiments lasted a total of three years. The results were published in 2016 in Scientific Reports.

Source : By Karina Toledo, in Campos do Jordão (Brazil)  |  Agência FAPESP