"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."