Park turfgrass may hurt climate more than it helps

Science News

December 19th, 2008

By Sid Perkins

The lush green turf in well-maintained parks can absorb carbon dioxide from the air as it grows and store carbon in the underlying soil, thereby helping to reduce the amount of the gas building up in the atmosphere. This buildup is one cause of detrimental climate change.

Yet new analyses of greenhouse gas emissions from the soil in such parks hint that the parks may still contribute to global warming.

Amy Townsend-Small, a biogeochemist at the University of California, Irvine, and her colleagues studied the greenhouse gas emissions from various patches of turf at four parks in Irvine. All the parks had been established between 1975 and 2006 on sites formerly occupied by open grassland. Also, all of the parks’ leisure areas, such as those used for open lawns and picnic sites, are watered and fertilized according to the same procedures, Townsend-Small notes.

At the Las Lomas Community Park, which was established in 2005, the top 5 centimeters of soil beneath each square meter of turf contains about 600 grams of carbon. The same of amount of soil beneath the turf in the Harvard Community Athletic Park — which opened in 1975 and is the oldest of the group the team studied — contains about 2.6 kilograms of carbon. The researchers found that, in general, the older the park was, the more carbon it stored beneath its turf.

The field tests also indicated, however, that soils beneath turf in the older parks emitted higher amounts of nitrous oxide (N2O) than the grassy areas of the younger parks. That gas, while emitted in small quantities, is a greenhouse gas whose long-term planet-warming effect is about 300 times that of carbon dioxide. In the oldest park, in fact, the harmful effect of emitted N2O eclipsed any gain from stored carbon. Tests conducted so far don’t pin down the cause of the N2O emissions. However, Townsend-Small and her colleagues suggest that fertilizer may be building up in the soils in parks as they age, and soil bacteria may be converting some of those accumulated nutrients into the greenhouse gas.