Study finds low carbon payback periods for sugar cane ethanol

Source: Amanda Peterka, E&E reporter • Posted: Tuesday, July 1, 2014

The soil carbon lost when pasture and crop lands are converted to sugar cane for ethanol production is small and can be made up within a few years, according to new research.

It takes three years to build back the soil carbon lost when pasture land in Brazil is converted to sugar cane for ethanol production, the study published this month in Nature Climate Change found. Planting sugar cane on lands that had been growing other crops yields, on average, an immediate 16 percent increase in soil carbon stocks.

The study shows “what we suspected all along but now with much more data than we’ve ever had before,” said Eric Davidson, an adjunct senior scientist at Woods Hole Research Center and co-author of a review of the study that will appear in next month’s Nature Climate Change.

Brazilian sugar cane growers planted more than 12 million new acres between 2000 and 2012, bringing total acreage to nearly 24 million, according to the study. In the south-central region of the country, where most production is centered, 70 percent of the recent expansion has been the result of converting pasture land to sugar cane. Twenty-five percent has been the result of converting land used for grain crops.

In the study, a team of international researchers led by the University of Sao Paulo sampled 135 sites to measure soil carbon.

Their results showed greenhouse gas benefits from converting pasture land and other crop land into sugar cane when the full life cycle emissions of ethanol production are taken into account.

The study confirms that the “amount of soil carbon you lose when you convert a cattle pasture into sugar cane is pretty small, and you make that up within a few years of burning ethanol from the sugar rather than burning fossil fuels,” said Davidson, who was not involved in the study. “From a greenhouse gas perspective, that’s generally a good thing to do.”

Converting native forestlands or savannah to sugar cane for biofuels, on the other hand, results in much longer time frames for soil to build back carbon, according to the study. The study found, though, that conversion of native forest and grasslands to sugar cane is low and that Brazil has enough degraded pasture land to support a growing biofuels sector.

The research supports policy decisions in Brazil that encourage the conversion of degraded pasture land into ethanol production and discourage the expansion of pasture land to new regions. It has implications for the United States when it comes to imports of Brazilian sugar cane ethanol. The United States allows imported sugar cane ethanol from Brazil to qualify under the renewable fuel standard based on its greenhouse gas emissions.