Are Lawns Worse than Corn Fields?

Which emits more carbon dioxide emissions—the nation’s corn fields or residential lawns? Turns out lawns trump corn fields according to findings from researchers at Elizabethtown College. David Bowne, assistant professor of biology, led the study in Lancaster County, Pennsylvania, and the results were published in the Soil Science Society of America Journal. His study, which measured carbon dioxide released from lawn and corn fields, found that urban heat islands may be operating on a smaller scale than was previously thought.

Bowne worked with undergrad Erin Johnson, co-author of the paper, to measure carbon dioxide emissions, soil temperature and soil moisture in both fields and lawns. Both carbon dioxide releases and soil temperature were higher in the residential lawns. Those higher temperatures were directly associated with carbon dioxide efflux.

“As you increase temperature,” Bowne says, “you increase biological activity — be it microbial, plant, fungal, or animal.” Increased activity leads to more respiration and increased carbon dioxide emissions.

The higher temperatures in home lawns appear to be the result of a small-scale urban heat island effect. Traditionally, the urban heat island effect has been thought to be a phenomenon that happens in cities where the glut of paved roads, dark roofs and buildings absorb and concentrate heat making cities much warmer during hot days than their country counterparts.

Bowne’s study found that even residential development can drive up surrounding temperatures, an effect that has not been widely studied. “Within a developed area, within a city or town, you could have local increases in soil temperature because of the amount of development within a really small area,” says Bowne. Even areas a short distance apart could have alternate soil temperatures because of differences in development.

In the future, Bowne will test carbon sequestration—or the way certain practices such as leaving grass clippings on lawns or no-till agriculture—can lessen the carbon dioxide emissions from lawns and corn fields.

Christina Milesi has been studying the impact of lawns for NASA’s Ames Research Center in California, particularly the way they impact the nation’s water supplies and carbon cycle. Using satellite imagery, she was able to find that there are at least three times more acres of lawns in the U.S. than irrigated corn. That makes lawns America’s largest irrigated crop. Not only do these lawns require a huge amount of drinking-quality water to stay green, they also could function as a carbon sink (storing more carbon than they release) if handled responsibly.

An article on NASA’s Earth Observatory site notes that: “If people recycle the grass clippings, leaving them to decompose on the lawn, the U.S. lawn area could store up to 16.7 teragrams of carbon each year. That’s equivalent to about 37 billion pounds—the weight of about 147,000 blue whales. Even if people bag their grass clippings and compost them off site…lawn surfaces still appear to be a carbon sink, although at a much smaller rate of about 5.9 teragrams of carbon per year.”

When grass clippings decompose, the recycling of nitrogen more than compensates for the carbon being released. Those clippings also allow people to reduce the amount of fertilizer they use on lawns, which contributes to toxic runoff. “In fact,” Milesi said,“the model suggests that if we recycle the clippings on the grass, we can almost halve the amount of synthetic nitrogen fertilizer, and the carbon storage is still greater than it would be if we used the higher amounts of fertilizer but removed the clippings from the lawn.”