Corn and soybean prices went sky high between 2008 and 2012, and so did the number of acres that went under the plow in Minnesota and the rest of the country, according to a new study.

About 250,000 acres, or nearly 400 square miles, of Minnesota land was converted into row crops during that period. Most of it was grassland, but 25,000 acres were wetlands — more than any other state. And 13,000 acres of forests were converted — the second largest forest conversion in the country.

The researchers from the University of Wisconsin-Madison used satellite data from three different sources to analyze land conversion. The study was published Thursday in the journal Environmental Research Letters.

"In the Midwest we saw a lot of croplands expanding outside the traditional corn belt area, and Minnesota in particular was a key hot spot of land conversion," said Tyler Lark, a university graduate student and the study's lead author. "Much of the conversion came at the cost of natural ecosystems."

Converting forests, wetlands and grasslands into row crops has raised concerns in Minnesota and elsewhere for two reasons. First, farming practices can degrade water quality. Second, undisturbed land can capture carbon dioxide, a greenhouse gas contributing to climate change, so plowing those lands to grow crops increases CO2.

This map shows an estimate of the net conversion to or from cropland from 2008 to 2012. Most of the land conversion that happened in Minnesota during that time went from non-cropland to cropland. The darker the red color, the larger percentage of the landscape that was converted into cropland. Courtesy of Tyler Lark

The study estimated that the 7.3 million acres nationwide — roughly the size of the state of Maryland — were converted to everything from corn to wheat to alfalfa. The increased carbon emissions from the additional corn and soybean crops alone would be equal to a year's worth of CO2 emissions from 34 coal-fired power plants, the study said.

The high corn and soybean prices came after the federal Renewable Fuel Standard took effect. The standard calls for a certain amount of ethanol and other renewable fuels to be added to gasoline. Part of the policy's goal was to reduce greenhouse gas emissions, but some environmental groups have argued the standard is coming up short in that regard because of the consequences of land conversion.

The researchers aren't the first to point out the land conversion, but they used data that allowed them to determine how much of the converted land had not been used for crops in at least the past four decades. In Minnesota's case, 22,000 acres that were converted to cropland between 2008 and 2012 hadn't been used for that purpose in recent history.

In addition, the study found that converted land was often considered "marginal," meaning previously farmers hadn't wanted to plant on it.

"This marginal land is often characterized by increased risks for erosion, flooding or drought, and as such can have much greater consequences of cultivating new croplands as opposed to preexisting cropland area," Lark said.

Corn and soybean prices have plummeted since the study period, so Lark said it will be interesting to see what effect that has on land conversion. Because the new federal Farm Bill limits the number of acres where farmers can earn money for keeping land as grasslands, wetlands or forests, some farmers might choose to continue planting on lands that they recently converted into row crops.