If you’re interested in sustainability, you’ve probably thought about how to reduce your carbon footprint, from how you fuel your car to how you heat your home. But what about carbon emissions from growing the food you eat?

Most of the crops in the United States are grown using chemical fertilizer – a lot of it: American farmers used over 24 billion pounds of nitrogen fertilizer in 2011. And making nitrogen fertilizer requires fossil fuels like natural gas or coal.

In a single year, production of fertilizer in the United States emitted as much carbon dioxide as two million cars. But plants need nitrogen to survive, so farmers can’t just stop using fertilizer. Without it, U.S. crop yields would fall by as much as 50 percent, according to some estimates.

What if we could help plants make their own nitrogen so they wouldn’t need manmade chemical fertilizers? Professor Sharon Doty, a plant microbiologist at the University of Washington, says nature has already figured out this problem — we just have to know where to look.