The study, which has implications for forest management and global mercury pollution, was published online today (Jan. 9) in the journal Global Biogeochemical Cycles.
Doctoral student Abir Biswas, the paper's lead author, came up with the idea for the project when he was a student at U-M's Camp Davis Rocky Mountain Field Station near Jackson Hole, Wyoming. Wildfires were burning all around the station that summer, and smoke blanketed the camp. Around that time, Biswas happened to read a new scientific paper suggesting the possible role of fires in global mercury emissions.
"There I was, watching forest fires around our field camp, and it seemed like the ideal place to study the problem," he said.
The study Biswas read investigated mercury emissions from the combustion of foliage at locations around the USA and extrapolated to estimate mercury release during forest fires. "I'm interested in earth surface geochemistry so I wanted to approach the question differently," Biswas said.
Over the next two summers, under the direction of U-M professor Joel Blum, Biswas collected core samples of forest soil from burned and unburned areas, using sections of PVC pipe sharpened at one end to obtain the cylindrical samples. He and Blum also collaborated with U-M professor Gerald Keeler and former research scientist Bjorn Klaue to take air samples at Camp Davis—measuring mercury and trace metals over two summers—which provided the scientists with a picture of the atmospheric background on which the fires were superimposed.
Forests act as mercury traps because mercury in the atmosphere—which comes from both natural and human-generated sources such as coal-fired power plants and municipal waste incinerators—collects on foliage. When the foliage dies, it falls to the forest floor and decomposes, and the mercury enters the soil. Because it binds strongly to organic molecules, mercury is most prevalent in the top several inches of soil, where organic matter is concentrated. By comparing the mercury content of burned soil with that of unburned soil, the researchers could estimate how much mercury was released when forests burned.
They found that both the type of trees in the forest and the severity of the fire affected the amount of mercury released. The type of tree makes a difference because evergreens take up more mercury from the atmosphere on their needles than do broad-leafed trees, leading to more mercury accumulation in the soil prior to the fire.
Based on their analysis and estimates of the area of forest and shrub land burned annually in the United States, Biswas, Blum and coworkers calculated that wildfires and prescribed burns account for approximately 25 percent of human-generated mercury emissions in this country.
Understanding the role fires play in mercury emissions is particularly important in light of predictions that forest fires will increase as global warming makes some parts of the world hotter and drier, said Blum, who is the John D. MacArthur Professor of Geological Sciences and director of Camp Davis.
The findings also have implications for forest fire management, Biswas said. "When you let fires run free in an area where they have been suppressed for a long time, as happened in the Yellowstone fire of 1988, the fires end up burning a huge area that has been accumulating mercury for a long time, so a lot of mercury is released. By contrast, when you allow fires to occur in natural 50- to 100-year cycles, you end up with more frequent, less severe fires, which release less of the mercury in the soil. So the current shift in management practices from suppressing fires to letting some of them burn suggests that in the immediate future there may be a lot of high mercury release fires, but that down the road the amount of mercury released from these fires should drop."
In a related project, the researchers are trying to identify the sources of the atmospheric mercury that ended up in the forests they studied. Preliminary results suggest that much of it came from mining operations in the western United States.
Studies of the sources and fate of mercury pollution are critical, Blum said, because it's a problem that won't go away. "Once mercury starts getting emitted and deposited into a forest, it then gets re-emitted and re-deposited and re-emitted again. So the legacy of mercury pollution will be with us for a very long time."
Funding was provided by grants from the National Institute of Environmental Health Sciences to Blum and from the department of Geological Sciences to Biswas.
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