Forest fires major source of mercury emissions

It has long been recognised that burning forests release more than carbon, as the cocktail of chemicals absorbed by the trees as they grow is set free as they are consumed by fire.

But new research at the University of Michigan suggests that the role of forest fires has been underplayed when estimating the levels of toxic mercury levels, with wildfires responsible for about a quarter of the mercury released into the atmosphere in the US.

The study has implications for forest management and global mercury pollutionand was published in the January edition of science journal Global Biogeochemical Cycles.

The paper’s lead author, PhD student Abir Biswas, the paper’s lead author, was inspired to study the area after being surrounded by forest fires while studying at the university’s Rocky Mountain field station.

“There I was, watching forest fires around our field camp, and it seemed like the ideal place to study the problem,” he said.

At the time Mr Biswas had been reading a paper on the effects of fires on mercury emissions 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.

Forests act as mercury traps because mercury in the atmosphere, which comes from both natural sources and human activity such as burning coal and waste incineration, 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.

Based on their analysis and estimates of the area of forest and shrub land burned annually in the United States, the researchers calculated that wildfires and prescribed burns account for approximately 25 percent of human-generated mercury emissions in this country.

As well as helping to understand existing mercury pollution, Mr Biswas believes his findings have implications for forestry management.

“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,” he said.

“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.”

Sam Bond

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