Ears of corn on display at the 2014 Iowa State Fair in Des Moines.
Ears of corn on display at the 2014 Iowa State Fair in Des Moines. - 
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A new study, which shows farmers have recently converted millions of acres of grassland to plant crops like corn, highlights a not-so-obvious downside: converting grassland to cropland has a large carbon footprint. 

The primary reason is not the diesel farmers burn with their tractors. Rather, the release of carbon comes from microbes that lie dormant in the soil, according to the study's lead author, Tyler Lark, a graduate student at the University of Wisconsin.

"When they are tilled up and exposed to oxygen, they 'wake up,' in a sense," Lark says. "When they go through that process, just like humans, they release CO2 when they 'exhale.'”

In addition to that one-time release, harvesting annual crops like corn does not store as much carbon in the soil as allowing perennials like grass to stay put.

That’s the biology. Markets play a role too, with high prices in recent years creating big incentives for farmers to convert more grassland to grow corn.

Government policies contribute as well. For instance, one driver for those high prices was a federal mandate to add more ethanol to the gasoline supply, which pushed up demand. Another policy may have played a role too: subsidized crop insurance, which mitigates financial risk to farmers.

"A lot of those lands that are being converted are high-risk lands," says Bruce Babcock, an Iowa State University economist who studies corn markets. "I think you can point the finger directly at the crop-insurance program for de-risking that conversion decision."

Those decisions may be more difficult to undo, thanks to a third policy decision: a provision in the 2014 farm bill, cutting back a program that pays farmers to set aside land for conservation. 

Two years of bumper crops have created huge surpluses, and dramatically lower corn prices — conditions that would make the conservation program more attractive to farmers — but the program's smaller size means fewer farmers can participate.

Babcock is less concerned about the carbon impacts of these conversions than about the fact some of these grasslands provided habitats for songbirds.

"We can do lots of CO2 mitigation by simply converting coal-fired plants into natural gas or solar," he says. "It's very hard to replace songbird habitats."

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Follow Dan Weissmann at @danweissmann