LANSING, Mich.—After more than a decade of research, a team of scientists has found that by releasing one pollutant into the environment, we might help capture another. Findings from one of the National Science Foundation's longest-running studies show that adding nitrogen to soil prompts northern hardwood forests to absorb more heat-trapping carbon dioxide.
As the atmosphere's most abundant element, nitrogen plays a significant role in ecosystems, and one to which scientists and policymakers are paying greater attention. Growing evidence suggests that as humanity pumps more nitrogen into the environment, forests could become bigger carbon sinks and help mitigate climate change. But experts warn that it's a dangerous experiment that could have serious consequences.
Nitrogen comes from a vast array of sources: farm fertilizer, car exhaust, factory and power plant emissions. Scientists expect nitrogen deposition this century to jump two to three times above current levels, dramatically influencing the planet's environment and thermostat.
"It is pretty important to recognize that human effects on the nitrogen cycle have significant effects on climate," said Alan Townsend, North America director of the International Nitrogen Initiative.
That's why Kurt Pregitzer of the University of Nevada, Reno and his fellow researchers essentially fertilized four experimental forests in northern Michigan. They applied nitrogen at two to three times current concentrations—about what society is expected to emit a century from now, thanks to increasing use of fertilizer and fuel.
Tree growth, as expected, took off, with the newly formed wood absorbing and storing more carbon.The surprise was what happened on the forest floor.
There scientists found that decomposition of twigs and other tree litter slowed. Lignin—the tough substance that gives vegetables their crunch and is quite good at storing carbon—proved more resistant to the forest floor's microbes.
Those microbes, said Pregitzer, are "the gatekeepers of carbon transformation."
"What we didn't anticipate was that storage of carbon in soil is directly altered by the addition of nitrogen," he said. "The really novel part, I think, is that the microbial community is actually altered."
This is significant, said team member Donald Zak of the University of Michigan. With microbes dining lightly on lignin, more dead plant matter stays in the soil.
Zak said this litter traps as much carbon as the robustly growing trees—a considerable boost to the forest's ability to sequester carbon emissions.
Scientists don't yet understand the mechanisms that slow the decomposition, Zak cautioned. It's a crucial question they'll need to answer if they hope to understand the role that forests play in carbon sequestration.
The team's findings could help build more accurate models of climate change, said member Andrew Burton of Michigan Technological University. "Now that [climate modelers] have the big fluxes down, there's a lot of these fine-tuning things left," he said.
There are major caveats, however. Nitrogen should not be seen as a climate savior, Pregitzer cautioned.
For one, pumping nitrogen into the atmosphere has many downsides, including biodiversity loss, smog formation, and acidification of forests, streams and lakes. "One thing that would be a mistake would be to give the implication that nitrogen deposition is a good thing," Pregitzer said.
Second, the experimental forests have all passed saturation, the point at which nitrogen can no longer be absorbed and instead leaches into groundwater. The group's models predict future forests will be equally saturated, Zak said. "If forests respond across the region as these have, then nitrogen leaching into aquatic ecosystems is going to increase," he said.
Some leaching might not be a bad thing, Burton said, since excess nitrogen could be taken up by plants in nearby areas. But people who drink water with too much nitrogen face health risks, most notably so-called blue baby syndrome, a potentially fatal blood disease that affects infants.
And once it enters waterways, nitrogen makes its way to lakes and oceans where it can contribute to "dead zones," coastal areas where excess nutrients cause explosive phytoplankton growth, robbing water of oxygen and suffocating the aquatic creatures that need it.
Finally, and perhaps most worrisome, saturating forests with nitrogen "will very likely" lead to more nitrous oxide emissions - a potent greenhouse gas - said Nathaniel Ostrom, co-director of the Biogeochemistry Environmental Research Initiative at Michigan State University.
Nitrous oxide emissions in other areas, scientists say, could even offset the climate benefit of northern forests as carbon sinks.
The jury is still out on whether human-caused nitrogen deposition accelerates climate change through those emissions or slows it by boosting plant growth and slowing decomposition, said Townsend."But I suspect that we're accelerating it," he said. "The speed and scale at which we're changing the nitrogen cycle are pretty scary."