Estimate made by Brazilian researchers in collaboration with foreign colleagues was published in the journal Nature Climate Change (photo: Wikimedia)

Payback time for soil carbon emitted from pasture conversion to sugarcane production is within three years

July 2, 2014

By Elton Alisson

Agência FAPESP – The reduction of soil carbon stock caused by the conversion of pasture areas into sugarcane plantations – a very common change in Brazil in recent years – may be offset within two or three years of cultivation.

The calculation appears in a study conducted by researchers at the Center for Nuclear Energy in Agriculture (CENA) of the University of São Paulo (USP) in collaboration with colleagues from the Luiz de Queiroz College of Agriculture (Esalq), also at USP. The study also included researchers from the Federal Institute of Alagoas (IFAL), the Brazilian Bioethanol Science and Technology Laboratory, the Institut de Recherche pour le Développement in France and Harvard University, Colorado State University and the Shell Technology Center Houston in the United States.

Findings from the project “Soil carbon stocks on land-use change process to sugarcane production in South-Central Brazil,” carried out with funding from FAPESP, were described in an article published in the online version of the journal Nature Climate Change.

“The study indicates that the soil carbon balance of pasture areas converted for the cultivation of sugarcane designed for ethanol production is not as negative as originally estimated,” said Carlos Clemente Cerri, project coordinator and researcher at CENA.

“The calculations we made could contribute toward ensuring that Brazil is producing and selling a low-carbon emission fuel in the domestic and international markets,” said Cerri, one of the authors of the study, during a lecture at the Workshop on Impacts of Global Climate Change on Agriculture and Livestock held at FAPESP on May 27, 2014, which was coordinated by Professor Carlos Martinez of the University of São Paulo (USP) in Ribeirão Preto.

According to Cerri, soil from pasture areas has a carbon stock whose volume varies only slightly over the years. However, the process of preparing this type of soil for conversion to sugarcane plantations causes part of the carbon stock to be emitted into the atmosphere as carbon dioxide (CO2).

In contrast, depending on the type of management, the introduction of sugarcane to pasture areas could compensate for, or even add to, the initial soil carbon stock when the organic matter and plant residue penetrate the ground.

Moreover, the ethanol produced from sugarcane grown in these areas over time ultimately offsets the CO2 emissions that occur during the conversion process because biofuel contributes toward reducing the burning of fossil fuel, explained the researcher.

No one knew the precise net carbon loss or gain, known as carbon debt or credit, respectively, or how much time sugarcane cultivation needed to take place in pastures to replace the carbon emitted from the land-use change.

“There have been few direct field measurements that quantify the effects of land-use change on soil carbon for the most common land-use transitions to sugarcane [cultivation] in Brazil,” said Cerri.

Field measurements

The researchers conducted measurements and collected 6,000 soil samples from 135 regions in south-central Brazil, which is responsible for more than 90% of Brazil’s sugarcane production.

At each of the sites, soil samples were collected from areas of sugarcane cultivation and from other areas to be used as reference. These reference areas included pastures, annual cropland (soybean, sorghum and corn) and Cerrado native vegetation.

“Seventy percent of the conversions we analyzed were from pastures converted to sugarcane plantations, 25% were from annual croplands and 1% was from conversion of the native Cerrado vegetation,” explained Cerri.

The samples were collected at different soil depths and in successive layers varying from 10 centimeters (cm) to one meter to facilitate comparisons with previous measurements, which were often restricted to surface layers at depths of up to 30 cm, and to provide a more complete soil carbon balance inventory as a result of land-use changes, said the researcher.

Laboratory sample analyses indicated that the carbon debt from converting soil from pastures to sugarcane production varied from 20 to 30 megagrams (Mg) or tons of CO2 per hectare in layers at depths of up to 30 cm and up to 100 cm.

However, the conversion of soil from annual croplands to sugarcane production presented a carbon credit of 36 to 70 Mg of CO2 per hectare for layers at depths of up to 30 cm and up to 100 cm, respectively.

Areas of conversion from Cerrado to sugarcane cultivation areas – which represent less than 1% of agricultural expansion in Brazil – presented a carbon debt of 77.8 Mg of CO2 per hectare.

“One hectare of sugarcane crop produces a certain number of liters of ethanol that mitigates 9.8 megagrams of CO2 emitted annually by burning fossil fuel,” Cerri stated.

“This means that sugarcane needs to be grown for two to three years in order to compensate for the emissions caused by the land-use change from pastures, which corresponds to approximately 80% of the conversions for this use,” he noted.

Production planning

According to the researchers, the study findings could contribute toward guiding expansion policies for sugarcane production aimed at producing ethanol to ensure the biofuel’s sustainability.

According to data from the Energy Research Company (EPE) presented during the FAPESP workshop, ethanol demand in Brazil is expected to jump from an annual total of 25 million liters to 61.6 billion liters by 2021.

The professor indicated that to reach this number, the area of sugarcane production in Brazil would need to expand from the current 9.7 million hectares to 17 million hectares.

Cerri notes that among the options for reaching the target area, the priority for expansion of production is expected to be the conversion of degraded lands, principally those used as pastures, into sugarcane plantations.

Between 2000 and 2010, three million Brazilian hectares were converted to sugarcane cultivation areas. More than 70% of this land consisted of pastures, and 25% had been used for growing grains, said the study’s researchers.

“Today, there are 198 million hectares of land dedicated to pastures in Brazil and 60 million hectares dedicated to agriculture,” said Cerri. “There is no question that the increase in land for agricultural production will need to come from pastures,” Cerri said.

The article, “Payback time for soil carbon and sugar-cane ethanol” (doi:10.1038 /NCLIMATE2239), by Cerri and colleagues can be read by subscribers of the journal Nature Climate Change at www.nature.com/nclimate/index.html.