In the concrete jungle at the core of a city, carbon dioxide (CO2) emissions are dominated by the fossil fuels burned by the dense concentrations of cars and buildings. Boston University researchers now have shown, however, that in metropolitan areas surrounding the city core, plant roots and decomposing organic material in soil give off enough CO2 , in a process termed "soil respiration", to make an unexpectedly great contribution to total emissions. In fact, analyzing CO2 released from soil respiration at 15 sites across greater Boston, the BU scientists found that during the growing season, releases of the greenhouse gas from soil may approach those of fossil fuels in dense residential areas. The first study of urban soil CO2 to reach this wide scale and to integrate a high-resolution model of both soil respiration and local fossil fuel emissions, the research will help to improve assessments of climate action programs.

"Very close to this concrete jungle downtown, where you have a lot of fossil fuel emissions and no soil, you have residential areas that have lower fossil fuel emissions and a whole lot of soil," says Stephen Decina, a doctoral student and lead author on a paper published today in the journal Environmental Pollution. "Over the growing season, CO2 emissions from soil respiration are almost 75 percent of the fossil fuel emissions in those areas. In some places, they're actually higher than fossil fuel emissions."

"These biological fluxes are much larger than you'd expect," says Lucy Hutyra, Associate Professor of Earth & Environment and coauthor on the paper. "In our efforts to monitor, verify, and reduce greenhouse gas emissions in cities, we really need to pay attention to this."

Decina and his colleagues measured the emission of CO2 from the same patches of soil around metropolitan Boston every two weeks during the growing season when releases of the gas are highest and measurements are not impeded by snow.

"We found that the average rate of CO2 coming out of the soil is highest in landscaped areas such as gardens, followed by areas with a lawn, and it is lowest in the urban forests," he says.