Conversion of pasture into cropland increases greenhouse gas emissions
April 04, 2018
By Elton Alisson | Agência FAPESP – Growing global demand for renewable energy has driven the expansion of sugarcane plantations in Brazil into degraded or abandoned pasturelands. Given the importance of cattle ranching to Brazil, the occupation of these pasturelands by sugarcane has had to be counterbalanced by productivity improvements in the remaining pastures, such as using tillage, higher-yield grass varieties and nitrogen fertilizer. The effects of these changes in land use on greenhouse gas (GHG) emissions are unclear, according to experts in the field.
A study conducted by researchers affiliated with the Federal University of São Carlos (UFSCar) at Sorocaba, São Paulo State (Brazil), found that the conversion of pasture to sugarcane and the more intensive management of remaining pasturelands to compensate have increased the emissions of GHGs such as carbon dioxide, methane and nitrous oxide and that these land use changes could therefore alter the GHG balance of sugarcane ethanol in Brazil.
The study resulted from a project supported by FAPESP and from research conducted at UFSCar by Camila Bolfarini Bento for her master’s degree. Other participants included 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). The study was published in the Journal of Environmental Management.
“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, in an interview given to Agência FAPESP.
According to the researchers, large areas of pasture with low productivity and intensity have been converted into sugarcane plantations in Brazil. The total area under sugarcane increased by approximately 60% in the last decade alone, mostly at the expense of natural or unmanaged and unproductive pastures. Almost half the expansion occurred in degraded or abandoned pastures.
Although the expansion of sugarcane into these areas is considered positive because it avoids the clearing of tropical forest, the process both affects GHG emissions and alters the balance of carbon and nitrogen in the soil and the atmosphere owing to the annual application of nitrogen fertilizer and replanting at the end of each sugarcane production cycle.
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.
The growing use of nitrogen fertilizer in pasture areas and in sugarcane plantations could increase emissions of nitrous oxide and methane, changing the GHG balance of sugarcane ethanol production. However, according to the researchers, the magnitude of the increase in GHG emissions due to the conversion of pasture to sugarcane and the intensive management of the remaining pasture was unclear.
“Our hypothesis was that GHG emissions would increase as a result of the changes in land use,” Carmo said. “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.”
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 sciencedirect.com/science/article/pii/S030147971731160X.
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