"It's not as good a thing as some people would like it to be," said Peter Liss, of the University of East Anglia in England which led the study with Texas A&M University.

Fossil fuels burnt in cars, factories or power plants release nitrogen that can be absorbed by the seas. And nitrogen-based fertilisers are often washed off farmland and end up in the sea.

In theory, extra nitrogen acts as a fertiliser to spur growth of microscopic plants that absorb carbon dioxide, the main greenhouse gas blamed for global warming.

But the study, by researchers in Germany, Italy, China, the Netherlands, Switzerland, Canada and Chile as well as the United States and Britain, said there were unwanted side-effects.

And rising levels would only have a muted benefit in slowing climate change that the UN Climate Panel says will spur more heat waves, storms and raise sea levels.

"Extra nitrogen draws down carbon dioxide which is good news from the climate perspective," Liss told Reuters. "The bad news is that two-thirds (of the carbon absorbed) is offset by the production of another greenhouse gas, nitrous oxide."

Some scientists argue that artificially pumping nitrogen into the oceans should be eligible for credits as part of a UN plans to slow global warming beyond 2012.

"But you couldn't get carbon credits for all of it. Two-thirds of the carbon will be released again in the form of nitrous oxide," Liss said.

"There has been some net benefit" from nitrogen, he added. "Of the carbon dioxide that goes into the oceans about 10 percent of that might be due, in our calculation, to this increased nitrogen input by mankind's activities since 1860."

And adding more nitrogen to the oceans could disrupt marine life. "It might change the sort of species that grow, or perhaps the plants at the surface might start shading those lower down."

"I'd be very cautious about doing more," he said.

In some coastal areas, high levels of nitrogen from fertilisers create "dead zones" where oxygen levels are too low for fish.

-- For Reuters latest environment blogs go to: http://blogs.reuters.com/environment/