How to stop nitrogen pollution

20 March 2013


Nitrogen pollution is a growing danger to the world’s climate, overall human health and the global economy. Now scientists have found a link between carbon and nitrogen cycles that could yield important insights into reducing nitrous oxide emissions and developing strategies for combatting global warming.

Nitrogen is an essential nutrient for life but is now being produced in far greater quantities than in the past. While greater nitrogen availability has allowed global food production to keep pace with growing human demands, it can also impair water and air quality, reduce plant species diversity and exacerbate global warming. Concerns have been raised about whether the biosphere is able to soak up this extra nitrogen and what that means for the future. However, recent research involving academics in the Oxford Martin School’s Biodiversity Institute has shown that despite an increase in nitrogen production through industrialization, nitrogen availability in many ecosystems has remained steady over the last 500 years.

The international team of researchers looked at how nitrogen availability changes when atmospheric carbon dioxide increases. To do so, they compiled palaeoecological records of nitrogen availability from the end of the last glacial period – when carbon dioxide increased rapidly – to recent times, when humans have contributed greater amounts of carbon dioxide to the atmosphere through the burning of fossil fuels. The team collected and analysed data from the sediment records of 86 lakes across six continents representing most biomes (ecosystems). With the data, they were able to compare past and present nitrogen cycling in various regions.

"There is growing concern that future plant productivity and carbon storage could be limited by nitrogen. When carbon dioxide rose in the past, plants responded by storing increasing amounts of carbon. Our results show that this was associated with a long-term, global decline in nitrogen availability," explained Elizabeth Jeffers, a James Martin Fellow with the Biodiversity Institute at the Oxford Martin School, who was one of the academics involved in the international study.

She continued, “And so although humans have nearly doubled the amount of nitrogen released to the environment, globally nitrogen levels have remained stable at most sites for the past 500 years as seen in this dataset.”

One reason may be that plants are using more nitrogen than they previously have, keeping nitrogen levels consistent with those thousands of years ago, suggested co-author Kendra McLauchlan, Kansas State University.

"Our best idea is that the nitrogen and carbon cycles were linked tightly back then and they are linked tightly today," McLauchlan said. "Humans are now manipulating both nitrogen and carbon at the same time, which means that there is no net effect on the biosphere.”

“Today people are relying on trees to offset our growing emissions of carbon, yet when trees sequester carbon, nitrogen is also locked away for long periods and this could have knock-on effects on plant productivity. This underscores the need to integrate policies relating to carbon and nitrogen as changes in one will have profound effects on the other,” said Jeffers.

McLauchlan and Jeffers worked with Joseph Craine, research assistant professor in biology and Joseph Williams, postdoctoral research associate at Kansas State University. The team published their findings, Changes in global nitrogen cycling during the Holocene epoch in the current issue of Nature, published 21 March 2013.