Rising amounts of nitrogen entering the oceans from human activities are less beneficial than previously thought as a fertiliser for tiny marine plants that help slow global warming, according to scientists.

"As much as a third of the nitrogen entering the world's oceans from the atmosphere is man-made," according to a team of 30 scientists writing in the journal Science.

"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 US 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," Dr Liss siad. "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," Dr Liss said.

"There has been some net benefit" from nitrogen, he added. "Of the carbon dioxide that goes into the oceans about 10 per between 1970 and 2007 land-based species fell by 25 per cent, marine by 28 per cent and freshwater by 29 per cent.