Since the late 2000s, there has been a biofuel boom, as demand for this clean fuel has soared. Buses, trams, trucks and trains that run on biofuel are now commonplace, as are biomass boilers to heat and power homes and businesses. In the US, this biofuel boom has been fed by farmers growing corn and soybean. But where are these new biofuel crops being cultivated? Has more land been converted to farmland, or has biofuel feedstock displaced existing crops?

Traditional agricultural statistics alone cannot answer this question: they track only aggregated, net changes that often mask land conversion. Instead, Tyler Lark from the University of Wisconsin-Madison and colleagues performed a field-level analysis of cropland changes across the US between 2008 and 2012, combining new satellite data with historic land-cover information to measure how land use has changed through time.

Their results show that the biofuel boom has coincided with significant transformation of the landscape across the entire US, with large swathes of grassland, and land not recently used for agriculture, being converted to cropland.

"Nationwide, 7.3 million acres of land – an area the size of Maryland – were converted to crop production," said Lark. In some local hotspots in the Dakotas, Southern Iowa and the western parts of Kansas, Oklahoma, and the Texas panhandle, more than 10% of the landscape was converted to cropland during the study period.

The scientists also found that much of the conversion occurred on grasslands and marginal land. "This means the environmental impacts and economic costs of new croplands are likely to be greater than already-existing croplands," said Lark. The findings are published in Environmental Research Letters (ERL).

When it comes to climate change, the conversion of all this extra land has had a significant impact. "The estimated greenhouse-gas emissions from land converted to corn and soy alone were equivalent to a year's release from 34 coal-fired power plants or an additional 28 million cars on the road," said Lark. "That's comparable to over a 10% increase in the amount of US vehicles and their emissions."

By understanding this cropland expansion, Lark and his colleagues hope that their findings can be used to implement better controls and prevent further expansion. In particular, the scientists are supportive of a nationwide "Sodsaver" provision, which discourages conversion by reducing crop-insurance subsidies on cropland converted from native sod. "Currently, this provision exists, but only in six states that accounted for just 36% of conversion of previously uncultivated land," said Lark.

Second, the researchers believe that better monitoring and enforcement of the US Renewable Fuels Standard is needed, to ensure that biofuels are not planted on land cleared since 2007.

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