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Data that has been lost in the weeds—or more accurately the turfgrass—could help improve estimates of carbon dioxide emissions from urban areas, according to a team led by scientists at Penn State.

As a part of a larger project to in Indianapolis, the researchers found that turfgrass lawns, like those surrounding our homes and in parks, golf courses and cemeteries, have impacts distinct from other urban vegetation, such as trees and wild grasslands.

Adding turfgrass to simulations of urban ecosystems increases not only the understanding of vegetation's role in carbon dioxide levels, but it could also improve estimates of human-caused emissions, the scientists in the Journal of Geophysical Research: Biogeosciences.

"This study has implications for guiding greenhouse gas policies and for reducing the amount of uncertainty when we're trying to estimate anthropogenic emissions to guide those policy decisions," said Jason Horne, a doctoral candidate in meteorology and atmospheric science at Penn State and lead author on the study. "There has been a push to better understand the processes that are going on in these areas, because it's really complex."

Urban greenhouse gas emissions result from humans burning fossil fuels while driving vehicles, powering factories and heating homes, but plants also play a role. Plants remove carbon dioxide from the atmosphere during photosynthesis, but they also produce carbon dioxide as they respire and decompose. Teasing out the impact of vegetation is important to produce accurate estimates of emissions from human activities, the scientists said.

"And for that, we need to have a good idea of what the biology is doing," Horne said.

The work was part of the Indianapolis Flux Experiment (INFLUX), which has built emissions estimates for the city and its surroundings using an array of measurement techniques, including towers around the city that take continuous measurements of carbon dioxide.

"INFLUX is this long-running project that is one of three greenhouse gas testing sites in the United States," Horne said. "It offers one of the best and most abundant and diverse records from urban areas compared to just about anywhere else in the world."

In this study, Horne analyzed data collected from eddy covariance flux towers—instruments that can measure the exchange of gases between Earth's surface and the atmosphere. One sensor was placed over a golf course and another over a cemetery lawn.

The researchers found that existing vegetation types that are already factored into their models could not capture seasonal patterns observed in the exchange of carbon dioxide from the ground to the atmosphere at these two turfgrass-covered locations.

For example, in winter months, when air temperatures dip below freezing, the models previously predicted no photosynthesis activity. But the team's observations showed there is still photosynthesis in grass lawns removing some carbon dioxide from the atmosphere—even at subfreezing temperatures.

"Our models were not able to capture the carbon dioxide being removed from the atmosphere by photosynthesis in the middle of winter," Horne said. "The model showed vegetation was a net source of carbon dioxide during the middle of the day."

Using their observations, the researchers created a unique turfgrass vegetation type in the model. Including the new turfgrass representation allowed the model to capture the photosynthetic activity in the middle of winter as observed.

"Turfgrass photosynthesis is not highly active during the winter, but it's active enough to make a difference in the models—and that could make a difference in how we understand every emission source," Horne said.

He explained that if the models show that vegetation is putting more carbon dioxide into the atmosphere than it is removing—or acting as a source rather than a sink—then when scientists look at total emissions during that time frame, they may underestimate the impact of human-caused emissions, the scientists said.

And while the impact of photosynthesis from turfgrass in winter may be small, extrapolating that over the entire urban area means it can have a real impact, Horne said.

"Something like 20% to 30% of the surface area in Indianapolis is estimated to be turfgrass," he said. "Even if we see a small drawdown of carbon dioxide in the middle of winter, it's not insignificant. If you are not considering that, you may be underestimating anthropogenic emissions."

The scientists noted differences between the two locations—the golf course's grass is fertilized, mowed and irrigated, while the cemetery's grass is less managed. Given the variability, they said additional studies are needed to further improve estimates of turfgrass impacts on carbon dioxide emissions.

"But it's clear from our work that turfgrass lawns are worthy of dedicated study," Horne said. "This could help reduce the amount of uncertainty when we're trying to estimate anthropogenic emissions to guide policy decisions."