Soil nitrogen leaves methane up in the air.

Soil nitrogen leaves methane up in the air

Increased soil nitrogen, from acid rain or fertilizers, may lead to higher levels of methane in the atmosphere, a new study indicates. But researchers disagree on whether such an increase can significantly accelerate global warming.

Atmospheric concentrations of methane -- a colorless, odorless "greenhouse" gas produced mainly by rice paddies, ruminant animals, termites, wetlands and burning vegetation -- are increasing by about 1.1 percent per year, according to previous studies. While hydroxyl radicals (charged molecules produced by sunlight, water and ozone) remove much methane from the lower atmosphere, certain soil microbes are the predominant biological scavengers of the gas.

Working in an experimental forest in central Massachusetts, Paul A. Steudler of the Marine Biological Laboratory at Woods Hole, Mass., and his co-workers exposed soils to high and low levels of ammonium nitrate fertilizer. They recorded methane changes in the air trapped by plastic boxes over plot sections. After six months, methane emissions in the highly fertilized plots were 33 percent higher than in unfertilized plots, the researchers report in the Sept. 28 NATURE.

Microbes exposed to the fertilizer apparently feast more on it and consume less methane, Steudler says. He and his colleagues strongly suspect that decades of exposure to acid rain and nitrogen fertilizers significantly reduce a soil's role as a "sink" for removing methane.

Using data from this and other studies, the group also estimates that soils in temperate and boreal forests -- the largely coniferous woodlands running from the Arctic treeline into the northern United States -- consume 3.7 times more methane than do tropical forest soils.

M. Aslam K. Khalil of the Oregon Graduate Center in Beaverton, who studies methane sources and fluctuations, calls the report interesting "but nothing that's spectacularly new." Since roughly 550 trillion grams of methane now enter the atmosphere each year, and Steudler's group estimates that temperate and boreal forest soils worldwide remove about 12.5 trillion grams, Khalil says a decrease in methane consumption by forest soils won't dramatically increase atmospheric methane and global warming. In fact, "cows are doing more [to increase] the greenhouse effect than the soils are doing to prevent it," he told SCIENCE NEWS.

Steudler argues that "you may not need large changes in the rate of the [methane] sink" to trigger important changes in global methane. Since nitrogen depositions worldwide have increased substantially, scientists ought to learn whether that will affect global greenhouse gases, he says.

William S. Reeburgh, a geochemist at the Institute of Marine Science at the University of Alaska at Fairbanks, says he's observed significant methane uptake by microbes in the soils of tundra and high-latitude spruce-birch forests. But other tundra microbes and plants efficiently release methane, he says, at an estimated rate of 30 to 50 trillion grams per year, or about half the methane emission rate of the world's wetlands.

Steudler's new findings bolster other research suggesting that controlling nitrogen is important in improving air quality (SN: 9/17/88, p.180). President Bush's proposed Clean Air Act revision calls for halving sulfur dioxide emissions--a reduction of 10 million tons per year from 1980 levels -- but proposes a reduction of only 10 percent from the estimated 20.4 million tons of nitrogen oxides emitted into the atmosphere in 1980 (SN: 6/17/89, p.375). "We feel there should be as much attention paid to nitrogen oxide emissions as to sulfur dioxide emissions," Steudler says.

His group has begun research in the same forest to learn whether the nitrogen and sulfur in acid rain increase soil releases of carbon dioxide and nitrous oxide.