Grass lawns may belch more greenhouse gas than grain fields. Urban lawns released more carbon dioxide than corn fields in a recent study, but blades of grass might not be the carbon culprits.

The study’s authors suggested that the urban heat island effect may warm soils and cause city-dwelling plants and soil microbes to respire more rapidly than in the cooler countryside.

PHOTOS: Grow a Truly Green Lawn for Earth Day

“Since soil is the main source of carbon dioxide from terrestrial ecosystems… even a small increase in soil respiration would have implications for climate change,” wrote the authors in study published in the Soil Science Society of America Journal.

Cities tend to be warmer than surrounding rural and wilderness areas. This phenomenon, known as the urban heat island effect, results from the release of heat absorbed by asphalt, cement, bricks and other human constructions, as well as the heat generated by other human activities.

The world’s population continues to move to cities. As of 2010, more than half of the planet’s population lived in cities, according to the World Health Organization. The heat island effect grows along with the expanding cities.

The recent study suggested that the urban development influences soil temperatures and carbon dioxide emissions even on relatively small scales of a few hundred meters.

“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,” said lead author David Bowne of Elizabethtown College.

Future studies on the effects of urbanization should account for neighborhood-scale effects on soil temperature and soil carbon dioxide release, suggested the authors in the discussion section of the study.

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Bowne’s study did not include measurements of the amount of carbon stored in the soil in urban versus rural areas. Nor did the study include carbon dioxide emissions from lawn fertilizer production or lawnmowers.

Perennial lawn grasses may be storing significant amounts of carbon in their roots, which could offset some of their emissions. However, fuel inefficient lawn mowers and the overuse of synthetic fertilizers could be increasing the carbon footprint of lawns.

Converting one’s yard to native wildflowers and gardens provides a solution to some of the problems of lawns. In my own home, much of the land has been replanted with hardy fruit bushes, including raspberries and grapevines, or low-maintenance, bee-feeding native flowers. I spend less time cutting the grass, and get a harvest of tasty treats and flowers to give to my wife.

IMAGE: Young boy mowing lawn with toy mower in King City, Ontario. (Corbis)