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, scientists said on Thursday. "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.