Study reveals low nitrous oxide emission factor in sugarcane fields

Work in the field reveals that the percentage of the nitrogen in fertilizers that is transformed into greenhouse gases is lower than expected. The study reinforces the environmental benefits of ethanol.

By Fábio de Castro

Agência FAPESP – A percentage of the nitrogen contained in the main fertilizers utilized in agriculture is lost to the atmosphere in the form of nitrous oxide (N20), the most potent greenhouse gas. According to international studies, if this percentage is above a certain limit in sugarcane crops, Brazilian ethanol will have its environmental benefits nullified by the N2O emissions and cannot be considered a clean fuel.

Conducting an empirical study on the N2O emissions in sugarcane fields for the first time, a group of Brazilian researchers demonstrated that the emission factor of the nitrogen fertilizers used in sugarcane production is lower than the guidelines set forth by the Intergovernmental Panel on Climate Change (IPCC), strengthening the favorable arguments regarding the sustainability of ethanol.

The study published in Global Change Biology Bioenergy revealed that the emission factor in Brazilian sugarcane production is only elevated when the fertilizer is applied with vinasse, a residue from ethanol manufacture that is frequently reutilized in crops.

Funded by FAPESP under the Bioenergy Research Program (BIOEN), the study was coordinated by Janaína Braga do Carmo of the Department of Environmental Sciences at Universidade Federal de São Carlo (UFSCar) in Sorocaba (SP).

The study also included the participation of researchers and students from the University of Maryland, the Agronomy Institute (IAC), the Center for Nuclear Energy in Agriculture (CENA) at Universidade de São Paulo (USP), the Brazilian Agricultural Research Corporation (Embrapa), the São Paulo Technology Agency (APTA), the Sugarcane Technology Center (CTC) and the Universidade Federal Rural de Pernambuco.

According to Carmo, the article’s first author, the estimates conducted to date on the emission factor of sugarcane fields had been calculated based on mathematical models and indirect methodologies and not on work in the field.

“Our objective was to provide the emission factor of the fertilizers used to grow sugarcane based on empirical data. We managed to determine that the values are near the 1% estimate made by IPCC and lower than those suggested by the international literature. The emission factor is only high when there is a joint application of vinasse and fertilizers and when the quantity of the straw in the soil exceeds 14 tons per hectare,” said Carmo in an interview with Agência FAPESP.

According to Carmo, the study provides another strong argument for the large-scale use of ethanol as a clean biofuel. “In identifying the emission factors that are most worrisome, the study indicates that we have a very attractive biofuel from an environmental standpoint. The study, however, indicates that we have to improve the application of vinasse in sugarcane crops,” she affirms.

Carmo states that the study recommends the intensification of multidisciplinary studies regarding the quantity of straw that can be left in the soil. “Straw has an important role in soil protection and conservation, but part of it can be considered for the production of second-generation ethanol,” she said.

Vinasse is a rich source of potassium and, because of this fact, is utilized as a fertilizer in sugarcane fields. Nevertheless, in addition to the large quantities of potassium, the residue also has a high concentration of organic carbon that is readily available.

The presence of this organic carbon stimulates the microbial activity in the soil, which can potentiate the nitrification, principally, the denitrification processes, a process by which the nitrogen in the fertilizer is transformed into N2O,” explains Carmo.

According to Carmo, the field measurements were conducted in two experiments: the first was initiated in April 2010 in Jaú (SP) and the second in November 2010 in Piracicaba (SP). Each of the experiments involved different phases of the sugarcane crop.

“The Jaú experiment was conducted during the sugarcane planting, when planting begins and the culms are inserted into the furrows. The Piracicaba experiment was conducted in the ratoon phase, a stage when the sugarcane is cut. The samples were obtained for over a year, with intense collection in the first two months. They were analyzed in a gas chromatograph at UFSCar,” explains Carmo.

Emissions below 3%

The 2008 article published by Paul Crutzen – the 1995 Nobel Laureate in Chemistry – in Atmospheric Chemistry and Physics established that if the emission factor of N2O in sugarcane crops surpasses 5% of the quantity of nitrogen in the fertilizers, the environmental gains of using ethanol as a biofuel would be too slight for this product to be considered a clean energy source.

The environmental damages caused by N2O emissions would not offset by the carbon sequestration resulting from photosynthesis and the high energetic efficiency of sugarcane.

“Our study showed that the fertilizer emission factor was well below 3%. Only when there is joint application of vinasse and fertilizer and a large quantity of straw in the soil does the emission factor reach 3%. From this point forward, we will conduct a more-detailed experiment on the application of vinasse in soil and its effects on the gas emission throughout the year and in association with nitrogen fertilizers, in addition to evaluating the use of concentrated vinasse as a more sustainable alternative,” explains Carmo.

The article “Infield greenhouse gas emissions from sugarcane soils in Brazil: effects from synthetic and organic fertilizer application and crop trash accumulation”, by Janaína Braga do Carmo and others, can be read by the subscribers of Global Change Biology Bioenergy at http://onlinelibrary.wiley.com/doi/10.1111/j.1757-1707.2012.01199.x/full.