"It's not, 'if a little is good, a lot is better,'" Law said in a telephone interview. "It can reach a point where there is saturation of the effect of increased growth."

Law and her colleagues studied how much carbon was being absorbed and sequestered -- taken out of the atmosphere and locked up in the growing plants -- in forests in North America and Europe.

They also monitored how much nitrogen was being deposited in those forests by automobile engines, factories and intensive agriculture. One of nitrogen's uses is as a plant fertilizer, and it appears to have this effect in forests in this study.

Law stressed that the level of nitrogen that can actually nourish a forest and help it suck up the greenhouse gas carbon dioxide is about 10 percent of what is annually applied to farmlands.

The amount of nitrogen deposited in the forests studied ranged from about 11 pounds to 33 pounds (5 kg to 15 kg) per 2.47 acres (1 hectare) per year.

This provided only a small boost to how much carbon a forest can absorb by its natural processes of photosynthesis and respiration, she said.

"One of the things that we've heard in the past from research was that nitrogen deposition can be bad for forests," Law said. "Those (bad) levels we're talking about are very high levels."

When these levels are reached, soil becomes more acid and other nutrients like calcium decrease.

So would it be legitimate for a car company or a factory to claim that the nitrogen they emit was actually helping the forests grow and cutting down on global warming?

"I'd exercise caution, in that we're talking about the effects of continuous low-level nitrogen deposition," Law said. "If the levels were to increase to some level that it becomes detrimental then that's not good, and there are areas around the globe where (nitrogen deposition) might be higher than the levels we examined."

These findings give a complex picture of how carbon and forests interact. For example, it was already known that logging or other events that can wipe out a whole stand of trees can create periods of from five to 20 years when there is a net release of carbon into the atmosphere, instead of the carbon sequestration that occurs later on.