Scientists have found that urban green spaces may actually contribute to global warming, than curb it down, in a new research.
Turfgrass lawns help 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, the study from UC Irvine researchers has shown.
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.
"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," said Amy Townsend-Small, Earth system science postdoctoral researcher and lead author of the study.
Turfgrass is increasingly widespread in urban areas and covers 1.9 percent of land in the continental US, making it the most common irrigated crop.
In the study, Townsend-Small and colleague Claudia Czimczik analyzed grass in four parks near Irvine, California.
Each park contained two types of turf: ornamental lawns (picnic areas) that are largely undisturbed, and athletic fields (soccer and baseball) that are trampled and replanted and aerated frequently.
The researchers 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 showed that nitrous oxide emissions from lawns were 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 calculated, released about four times more carbon dioxide than the plots could take up.
"It's impossible for these lawns to be net greenhouse gas sinks because too much fuel is used to maintain them," Townsend-Small concluded.
The research results are important to greenhouse gas legislation being negotiated.
"We need this kind of carbon accounting to help reduce global warming," Townsend-Small said.
"The current trend is to count the carbon sinks and forget about the greenhouse gas emissions, but it clearly isn't enough," she added.