SCIENTISTS have discovered an emerging threat to the nation's forests - and it's coming from America's car tailpipes.
The culprit: Nitrogen oxide.
Nitrogen normally helps trees grow. But when it saturates the soil, trees begin to die and streams become more acidic. European scientists noticed the problem a decade ago. Now, it's showing up in the forests and streams of West Virginia.
Nitrogen saturation poses a long-term environmental challenge to the acid-rain-laden forests of the Northeast. Eventually, it can leach nutrients out of the soil and lead to acidification of streams, harming fish.
But nitrogen saturation presents an even bigger challenge for policy-makers who would try to control it. Although spread by the same acid rain that brought excess sulfur to ecosystems in the Northeast, nitrogen emissions will be much tougher to reduce because they stem largely from automobiles.
The Clean Air Act of 1990 did a fairly good job of cutting sulfur-dioxide emissions because the main culprit was a fixed number of power plants, says Frank Gilliam, a plant ecologist at Marshall University in Huntington, W.Va. Controlling nitrogen oxide - or NOx - emissions, on the other hand, may require cutting back the number of miles Americans drive, perhaps by encouraging mass transportation or moving to new types of cars that don't rely on the internal-combustion engine.
Engine improvements that cut NOx emissions are likely to be incremental, rather than dramatic. "Engineering will take us there to a certain extent," says John Aber, director of the complex-systems research center at University of New Hampshire in Durham. "But as the population continues to grow and we continue to drive more miles, that will just offset the gains engineering gives you."
Although the Northeast won't feel the full impact of nitrogen saturation for several decades, the problem is already evident in parts of West Virginia, where acid-rain deposition is high. Mr. Gilliam, who published a paper earlier this year in the Canadian Journal of Forest Research, documents how a nitrogen-saturated watershed in West Virginia began to leach nutrients out of the soil when researchers added more nitrogen. The result, Gilliam found, is something of an environmental double-whammy.
Increasing nitrogen is usually a signal for trees to grow faster. But in nitrogen-saturated soils, the growth signal is not accompanied by increases in other nutrients, such as calcium and magnesium. In fact, the nitrogen eventually works to leach these two elements out of the soil. So, just as the tree tries to grow faster because of the increased nitrogen, it is actually receiving less calcium and magnesium, according to the research by Gilliam and his co-authors, Mary Beth Adams of the US Forest Service and Bradley Yurish of Marshall University.
The environmental damage does not end with trees. Leaching calcium and magnesium make the soil more acidic, which in turn liberates the aluminum in forest soils. Aluminum is what harms fish and other aquatic life. Too much will kill fish.
BECAUSE many of the Northeast's forests were clear-cut for farming sometime in the last two centuries, they are nitrogen poor. That means they can accept increased nitrogen for several decades before the real environmental damage sets in, says Mr. Aber. In Europe, where deposition rates of nitrogen have been higher, the threat is more immediate.
"For years we have seen the forests as areas that protect the groundwater," says scientist Per Gundersen at the Danish Forest and Landscape Research Institute in Copenhagen. "Now we see forests where the groundwater is no longer protected."
Although ammonium in agricultural areas is causing the greatest nitrogen-saturation problem, European countries next year are due to make commitments on how much they will reduce NOx emissions. But "I doubt that it will be possible to reduce the nitrogen input significantly without reducing the number of cars and the distance they drive," Mr. Gundersen adds.