Sugarcane expansion over pastures increases Brazilian ethanol carbon footprint
AGÊNCIA FAPESP/DICYT - With a growing global demand for renewable energy, large areas of pasture with low productivity and intensity have been converted into sugarcane plantations in Brazil. According to researchers linked to the Federal University of São Carlos (USFCar) – Sorocaba campus (São Paulo State, Brazil), the total area under sugarcane has increased by approximately 60% in the last decade alone. Nearly half of all new sugarcane crops have replaced areas which were originally degraded or abandoned – part of an expansion process which occurred mostly at the expense of natural or unmanaged and unproductive pastures.
Since it avoids the clearing of tropical forest, the expansion of sugarcane over these areas is considered positive. However, UFSCar researchers found that the conversion of pasture into sugarcane plantations sets forth a process which increases the emissions of greenhouse gases (GHGs) such as carbon dioxide, methane and nitrous oxide. The conclusion was derived from a study supported by the São Paulo Research Foundation -- FAPESP.
The authors of the study, which was published in the Journal of Environmental Management, set out to evaluate a hitherto unknown phenomenon: how changes in land use derived from sugarcane expansion over degraded pasture affect the GHG balance of sugarcane ethanol in Brazil.
The adoption of higher-yield grass varieties, combined with the use of nitrogen fertilizers as well as the replanting at the end of each sugarcane production cycle (crucial practices for improving the productivity of the remaining pastures) alters the balance of carbon and nitrogen in the soil and the atmosphere.
“We found that the land use changes associated with sugarcane expansion into pasturelands and with intensive management of the remaining pastures had increased GHG emissions and that the magnitude of the increase was determined by the type of soil management practice, use of nitrogen fertilizer, and environmental conditions,” said Janaina Braga do Carmo, a professor at UFSCar and the lead author of the study.
Moreover, the researchers stressed that the more intensive management of the remaining pasturelands, including conventional tillage and the use of nitrogen fertilizer, which was hitherto rarely applied to pasture in Brazil, can increase GHG emissions still further, since fertilizer is applied to the entire area and not just to the planting furrows, as in the case of sugarcane and no-till systems.
According to Carmo, an increase of GHG emissions as a result of changes in land use was the initial hypothesis of the study – which included participation from researchers in the US at the University of Maryland and in Brazil at the Campinas Agronomy Institute (IAC), the São Paulo State Agribusiness Technology Agency (APTA) and the University of São Paulo’s Center for Nuclear Energy in Agriculture (CENA-USP).
“To test it, we would have to find out what would happen after sugarcane was grown in an area previously devoted to pasture for 40 years, for example, with very low GHG emissions and a considerable soil carbon stock, that then became managed. We also wanted to see what would happen in pastureland to which fertilizer was applied to ensure adequate production, avoid degradation, and support non-extensive cattle raising with animals in feedlots of a small size and with rotational grazing,” said the FAPESP-funded researcher.
To evaluate and quantify the GHG emissions in these different scenarios, the researchers studied an experimental area of half a hectare, or approximately 500 square meters, on a dairy farm with year-round grazing in Sorocaba.
They divided the area into three plots. The first was unmanaged pasture without the use of fertilizer and was left as it had been for 40 years. The second plot was pasture that was plowed, harrowed, leveled, and planted with grass seeds either directly or with the application of fertilizer to simulate the creation of new pasture. The third plot was tilled and planted with sugarcane with or without nitrogen fertilizer.
For 339 days, they collected gas samples using cylindrical chambers located in all three plots to measure the fluxes of carbon dioxide, methane and nitrous oxide during a period equivalent to one year after the conversion of unmanaged pasture into intensively managed pasture or sugarcane fields.
Laboratory analysis of the gas samples showed that the carbon dioxide and methane fluxes in the pasture and sugarcane areas increased significantly with the use of fertilizer and conventional soil management.
The emissions of carbon dioxide, methane and nitrous oxide varied greatly in the different scenarios analyzed. Cumulatively, however, the annual emissions of carbon dioxide equivalents, obtained by multiplying the tons emitted by their global warming potential, were higher for areas of intensively managed pasture and sugarcane than for unmanaged pasture.
Fertilized pasture produced higher emissions of carbon dioxide equivalents than pasture converted to sugarcane. “Theoretically, planting sugarcane in degraded pasture would be better for the GHG balance than trying to rehabilitate it,” Carmo said. “But this conclusion is premature, as the entire sugarcane cycle has to be evaluated, including the several ratoon crops yielded by a field over the years. Moreover, this study did not consider soil carbon stocks in pasture or sugarcane fields.”
Role of management
According to the researchers, better management practices can contribute to reducing GHG emissions in the early stages of the conversion of degraded pasture to sugarcane or of the intensive management of natural pasture.
Plowing before planting sugarcane or pasture increases carbon dioxide emissions, for example, because all the carbon sequestered in the ground is rapidly lost in the form of gas owing to rapid mineralization and the exposure of soil organic matter.
If sugarcane trash is preserved and left on the ground instead of being burned before the harvest, the carbon losses via soil conversion can be minimized, depending on the amount of straw left. Nitrogen fertilizer should be applied when pasture vegetation is more developed to synchronize its use with uptake by plants, thereby minimizing emissions.
“Management is essential. Using no-till or minimum tillage systems when introducing sugarcane or new pasture can mitigate emissions,” Carmo said.
Emissions associated with sugarcane production in Brazil are still relatively low, as nitrogen fertilizer application rates are moderate compared with those of other countries as well as those for other crops in Brazil itself.
The expansion of sugarcane acreage in Brazil, however, may increase GHG emissions throughout the sugarcane lifecycle, according to the researchers.
“The control of rising emissions due to the expansion of sugarcane growing and other activities in Brazil is crucial to mitigate climate change,” Carmo said.
|The article “Impacts of sugarcane agriculture expansion over low-intensity cattle ranch pasture in Brazil on greenhouse gases” (doi: 10.1016/j.jenvman.2017.11.085) by Camila Bolfarini Bento, Solange Filoso, Leonardo Machado Pitombo, Heitor Cantarella, Raffaella Rossetto, Luiz Antonio Martinelli and Janaina Braga do Carmo can be read by subscribers to the Journal of Environmental Management at www.sciencedirect.com/science/article/pii/S030147971731160X.|