Newly plowed land in Minnesota released 1.6 million metric tons of carbon into the atmosphere each year between 2008 and 2012, a consequence of high corn prices and the ethanol mandate.

Only the Dakotas, Texas and Missouri released more carbon buried in soil by converting natural landscapes to row crops, according to an analysis by researchers at the University of Wisconsin, presented this week at a scientific and conservation conference on grasslands.

It adds another element to the hotly debated question of whether the renewable fuels mandate increases or decreases the greenhouse gas emissions that are driving climate change, and whether the resulting demand for corn has resulted in the loss of prairies, wetlands and forests while adding to agricultural water pollution. Researchers at the University of Wisconsin combined high-resolution satellite imagery and annual federal land use data with estimates of carbon lost when soil and native plants were plowed up across the United States. They found that since the 2007 increase in national biofuel goals, more than 7 million acres of habitat were plowed under to plant corn and soybeans. That released enough carbon to equal the amount produced by 20 million cars, they said. The calculations include the carbon saved by burning ethanol instead of gasoline.

Tyler Lark, one of the authors who studies land use and mapping, said that Minnesota was the fifth-highest state in the country in terms of land use changes and carbon emissions. It lost an average of 263 square kilometers of land each year, which produced an annual net increase of 1.6 million metric tons of carbon — about the same as 342,000 cars.

“If it involves land clearing, you see this large additional impact that shows it doesn’t pay,” Lark said.

The area of research — marrying federal data on how land is used with satellite imagery — is relatively new, and arguments over its accuracy are the focus of competing scientific papers.

Geoff Cooper, executive vice president of the national Renewable Fuels Association, said other analyses and studies have shown the increase in corn production has come from more productivity per acre, not more land converted to row crops.

“The authors continue to abuse and misrepresent unreliable satellite data, and they continue to present highly uncertain modeling results as if they were the gospel truth,” he said in an e-mailed statement. He cited recent estimates by the Environmental Protection Agency, which found that while crop production is up 18 percent since 2000, farm acres are unchanged.

Lark said that the accuracy of their research techniques has improved from earlier studies and has been vetted.

Others said that land use change varies widely across the country.

“Since 2008 there has been virtually no change in total land devoted to field crops in the United States,” said Bruce Babcock, a retired professor of agriculture at Iowa State University. “But clearly there has been … more acreage in the Great Plains.”

The ethanol mandate currently requires gasoline and diesel sold in the United States to include about 17 billion gallons of fuel derived from plants. Today about a third of the nation’s crop goes into the gas tank.

The mandate has become a target for conservation groups like the National Wildlife Federation, which provided some funding for the University of Wisconsin study, but which had no hand in the work or its conclusions, the authors said. It was presented at the Grasslands Conference in Fort Worth, Texas, which was sponsored by the wildlife group.

Political controversy over the mandate is reaching a new high now that the Environmental Protection Agency is expected to issue a new goal for 2018 — which it has already proposed to lower from this year.

That would make the energy and fuels industries happy, but it’s already generating an outcry from farmers and agricultural groups.