Over the last 100 years the global population has increased four-fold to seven billion people and may reach nine billion by 2075. How to produce enough food to feed all these people is one of the biggest global challenges.

Throughout the twentieth and early twenty-first century, food production has been dramatically increased by improving agricultural yields, particularly by applying nitrogen fertilisers. In 1908, the German chemist Fritz Haber invented a method for producing ammonia fertilizer from the inert nitrogen gas which is in the air all around us.

Haber’s discovery earned him a Nobel prize and paved the way for cheap, industrially-produced fertilizer. A miracle of industrial chemistry, today around 150 mega-tonnes of nitrogen fertiliser are produced annually and half of the global population would not be alive without Haber’s process.

However increasingly it is being realised that this huge benefit to human development comes at a high price. Human production of reactive nitrogen is now more than double natural nitrogen fixation and we have unwittingly conducted a huge global-scale experiment in geoengineering.

The problem is that the nitrogen we spray on our fields does not stay there in perpetuity - much of it is washed into rivers or evaporated into the atmosphere. In water-bodies, nitrogen causes algal blooms that kill fish, impair drinking water quality and reduce aquatic biodiversity. In the atmosphere nitrogen from agriculture combines with oxides of nitrogen produced by burning fossil fuels.

In the UK, levels of this nitrogen pollution are often more than ten times higher than the natural background and scientists have observed acute nitrogen deposition events even in the remote Arctic. Atmospheric nitrogen is a severe threat to human health, reducing the life expectancy of at least half of Europe’s population by six months or more. Increasingly scientific research shows that nitrogen deposition is also a serious threat to ecosystems.

A paper just published in Proceedings of the National Academy of Sciences shows the impacts of nitrogen deposition in the environment may extend even further than previously thought. Dr Richard Payne and Professor Nancy Dise, of Manchester Metropolitan University, together with colleagues at Lancaster University and the Open University, studied more than 100 individual plant species’ reactions to nitrogen deposition at 153 grassland sites across Europe.

The scientists found that many species, particularly wildflowers such as creeping buttercup, harebell, yarrow, and autumn hawkbit, were much less abundant in areas with high nitrogen levels, such as central Britain, the Netherlands, northern Germany and Brittany. But particularly surprising was the discovery that many species declined at very low levels of pollution, often below the legally-recognised ‘safe’ level.

Professor Dise said: “One of the drawbacks of previous studies is that most field experiments to establish limits on pollution are near the populated and polluted areas where most scientists live. It may be that long-term exposure to even medium levels of pollution have already changed these ecosystems. In this latest research, we studied many grasslands along the natural gradient of pollution across Europe. And we found that at even relatively 'clean sites', low levels of pollution had an effect on the abundance of some plant species.”

This surprising result shows that even areas a long way from pollution sources and previously thought to be free from air pollution impacts may have been affected. But this is an environmental concern that most people have never heard about. Dr Payne said: “We have been very good at communicating the problem of climate change and carbon emissions but have failed at communicating the nitrogen problem. Until the public are aware of the issue then policy makers are unlikely to take action.”

The scale of the problem is huge. It has been estimated nitrogen pollution costs the countries of the European Union alone up to €320 billion a year- but progress in tackling it has been limited.

Over recent decades many developed countries have been quite successful at reducing nitrogen produced by fossil fuel burning; UK emissions of nitrogen oxides are down by almost 60% over the last 40 years. But tackling agricultural emissions has proved much more difficult. Nitrogen fertilizers are essential to feeding the world’s population but we can try to reduce the amount we use and the amount we lose into the environment.

The production of meat and dairy products contributes disproportionately to the nitrogen problem: nitrogen is released in the production of crops and then more nitrogen is released when these are fed to animals. In western countries we consume more animal products than is necessary for a healthy diet; if this can be reduced it would have big environmental benefits.

At the same time, better agricultural practices such as improved techniques for manure spreading can more than halve nitrogen emissions, as well as saving money. The Netherlands has some of the highest levels of nitrogen deposition anywhere in Europe but have achieved big reductions by legislating for low-emission farming techniques.

It is clear that the nitrogen problem isn't going to be solved quickly, which poses the question - if we must have nitrogen pollution where should it be?

Payne and Dise’s research might hold an answer. Dr Payne said: “We found that lots of species are lost with only a little pollution but once the ecosystem is very polluted adding more pollution doesn't make much difference”.

This suggests that to conserve as many species as possible new pollution sources should be put in areas that are already polluted and degraded to avoid any new pollution in currently unpolluted areas.

The full paper ‘Impact of nitrogen deposition at the species level’ and associated commentary can be found by searching ‘Impact of nitrogen deposition at the species on the PNAS website at www.pnas.org.

Dr Richard Payne is a former Research Associate at Manchester Metropolitan University and now Research Fellow at University of Stirling. His research interests are in the ecology and palaeoecology of environmental change, particularly in relation to climate change and pollution.

Professor Nancy Dise is Professor of Biogeochemistry at Manchester Metropolitan University. Her research interests encompass the biogeochemistry and biodiversity of terrestrial ecosystems, including greenhouse gas emission from wetlands and the cycling of nitrogen, sulphur, and carbon.

Image of wildflower meadow courtesy of www.shutterstock.com