Urban 'green' spaces may contribute to global warming

Turfgrass lawns remove carbon dioxide from the atmosphere through photosynthesis and store it as organic carbon in soil, making them important “carbon sinks.” However, greenhouse gas emissions from fertilizer production, mowing, leaf blowing and other lawn management practices are four times greater than the amount of carbon stored by ornamental grass in parks, a new study shows.

These emissions include nitrous oxide released from soil after fertilization. Nitrous oxide is a greenhouse gas that's 300 times more powerful than carbon dioxide, the Earth's most problematic climate warmer.

Previous studies have documented lawns storing carbon, but this research was the first to compare carbon sequestration to nitrous oxide and carbon dioxide emissions from lawn grooming practices.

“Lawns look great — they're nice and green and healthy, and they're photosynthesizing a lot of organic carbon. But the carbon-storing benefits of lawns are counteracted by fuel consumption,” says Amy Townsend-Small, Earth system science postdoctoral researcher at University of California, Irvine. Townsend-Small is the lead author of the study, which has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

The research results are important to greenhouse gas legislation being negotiated, Townsend-Small says. “We need this kind of carbon accounting to help reduce global warming,” the lead author says.

“The current trend is to count the carbon sinks and forget about the greenhouse gas emissions, but it clearly isn't enough.”

Turfgrass is increasingly widespread in urban areas and covers 1.9 percent of land in the continental U.S., making it the most common irrigated crop.

In the study, Townsend-Small and colleague Claudia Czimczik analyze grass in four parks near Irvine, Calif. Each park contains two types of turf: ornamental lawns (picnic areas) that are largely undisturbed, and athletic fields (soccer and baseball) that are trampled a lot and replanted and aerated frequently.

The researchers took and evaluated soil samples over time to ascertain carbon storage, or sequestration, and they determined nitrous oxide emissions by sampling air above the turf. Then they calculated carbon dioxide emissions resulting from fuel consumption, irrigation and fertilizer production using information about lawn upkeep from park officials and contractors.

The study shows that nitrous oxide emissions from lawns are comparable to those found in agricultural farms, which are among the largest emitters of nitrous oxide globally.

In ornamental lawns, nitrous oxide emissions from fertilization offset just 10 percent to 30 percent of carbon sequestration. But fossil fuel consumption for management, the researchers calculate, releases about four times more carbon dioxide than the plots can take up. Athletic fields fare even worse, because — due to soil disruption by tilling and resodding — they don't trap nearly as much carbon as ornamental grass but require the same emissions-producing care.

“It's impossible for these lawns to be net greenhouse gas sinks because too much fuel is used to maintain them,” Townsend-Small concludes.

The UCI study was supported by the Kearney Foundation of Soil Science and the U.S. Department of Agriculture.