A new research by an international team of scientists has found that as much as a third of the nitrogen entering the world's oceans from the atmosphere is man-made.
The research, led by Texas A and M University and the University of East Anglia (UEA), has significant implications for global climate change because the nitrogen causes increased marine biological activity and CO2 uptake, which in turn produces the potent greenhouse gas nitrous oxide (N20).
The study found that increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for around one third of the ocean's external (non-recycled) nitrogen supply and up to three per cent of the annual new marine biological production.
While the increased biological activity has the beneficial effect of drawing down man-made CO2 from the atmosphere, the researchers found that around two-thirds of this is offset by the increase in harmful N20 emissions.
"This fertilization of the ocean by human activities has an important impact on the exchange of the greenhouse gases carbon dioxide and nitrous oxide and should be considered in future climate change scenarios," said Professor Robert Duce of Texas A and M University, lead author of the paper.
It has long been known that man is enhancing the global nitrogen cycle through the use of fertilisers in agriculture and the burning of fossil fuels in power stations and cars.
Though the effect of this on the land has been extensively studied, this is the first time its impact on the open ocean has been properly quantified.
"The natural nitrogen cycle has been very heavily influenced by human activity over the last century - perhaps even more so than the carbon cycle - and we expect the damaging effects to continue to grow," said Professor Peter Liss, an environmental scientist at the University of East Anglia.
"The solution lies in controlling the use of nitrogen fertiliser and tackling pollution from the rapidly increasing numbers of cars, particularly in the developing world," he added.