It never rains but it pours

After acid rain and the greenhouse effect, the current environmental concern is fertilizer falling from the sky. Simon Hadlington reports on how excess nitrogen could change our ecosystem

Hold on to your hats for the next global environment crisis: we've had acid rain, the hole in the ozone layer and the greenhouse effect. The new menace is an excessive accumulation of compounds of nitrogen in the earth and in the air. In many parts of the world fertilizer is, literally, falling from the sky. As the Weather Girls might have sung: "It's raining N!"

Many of us are aware of the problem of nitrogen oxides from car exhausts causing smog in cities. However, the subtle effects of nitrogen compounds are less immediately visible. A gas called nitrous oxide is accumulating in the upper atmosphere. It is 200 times more potent than carbon dioxide as a greenhouse gas, and has been implicated in the thinning of the ozone layer. Ammonia from animal waste dissolves in rain water and falls back to the earth as fertilizer. This disrupts delicate plant communities that thrive only in nitrogen-depleted environments. Other compounds of nitrogen also cause rain to become acidic.

In an effort to understand the problem, the Natural Environment Research Council, with the environment ministry DEFRA and the Scottish Executive, has launched a £7m research programme called Gane – "global nitrogen enrichment". The programme's scientific co-ordinator is Professor Alan Davison, of the University of Newcastle. "Nitrogen enrichment has been ignored for too long," he says. "It needs to be raised much higher up the agenda."

The key to the problem lies in the way that nitrogen is cycled between its different chemical forms, and how man has radically interfered with the natural balance.

As a gas, nitrogen makes up four-fifths of the atmosphere. It is almost completely chemically unreactive in this form, but compounds of nitrogen – nitrogen atoms chemically combined with other atoms – are essential for life. There are two main ways by which nitrogen in its gas form becomes chemically altered to make it available to living organisms, a process called nitrogen fixing. The first is by populations of microbes that inhabit the soil and roots of leguminous plants. These convert atmospheric nitrogen into amino acids, the building blocks of protein, which are taken up by plants and incorporated into the food chain. Naturally occurring nitrogen-fixing organisms are thought to process 110 million tons of nitrogen a year. A further 10 million tons is fixed by the action of lightning – a high-temperature chemical process that forces atmospheric nitrogen to react with oxygen.

This 120 tons of fixed nitrogen was, for millennia, sufficient to keep the world's natural ecosystems supplied each year. Then man came along. Early farmers discovered that crops would grow better with manure spread on the land; that livestock thrived on certain plants, such as clover; and that other plants, such as soybean and other legumes, were rich in protein and nutritious. Deliberate cultivation of these nitrogen-fixers started to tilt the natural balance.

The big change came in Victorian times. In a speech to the British Association for the Advancement of Science in 1898, the scientist Sir William Crooke said: "England and all civilized nations stand in deadly peril of not having enough to eat." There was insufficient nitrogen fertilizer to grow the quantity of food needed to satisfy demand. Man had to find ways of fixing nitrogen. A few years later, the German chemist Fritz Haber showed how it was possible to take nitrogen from the air and react it with hydrogen to make ammonia, from which nitrates could be produced. When his countryman Carl Bosch showed how to do this on an industrial scale, the era of mass-produced artificial fertilizers had begun.

Nowadays, around 80 million tons of nitrogen is fixed each year as fertilizer. A further 30 million tons is fixed by cultivation of legumes, notably soybean. Together, these have doubled the quantity of nitrogen taken out of the air and made chemically reactive. The final "big fixer" is the combustion of fossil fuels in power stations and motor cars. The high temperatures cause the nitrogen in the air to form a variety of oxide gases. These account for a further 25 million tons of fixed nitrogen each year.

"In other words, through our efforts we are more than doubling the quantity of fixed nitrogen in the Earth and its atmosphere than would be achieved if nature was left to its own devices," says Professor Davison. "The polluting aspects of nitrogen oxides, the smogs in cities, are only a small part of the problem. Huge areas are being affected in ways that are not so obvious."

For a start, there is the production of nitrous oxide by microbes in the soil. An excess of fixed nitrogen results in more nitrous oxide being generated. This finds its way into the upper atmosphere, where it is a potent greenhouse gas. "Nitrous oxide is of great concern," says Davison. "The UK has obligations under various agreements to reduce greenhouse gas emissions, so Gane aims to provide DEFRA with the science to underpin policy decisions."

Another problem is the precipitation of nitrogen compounds from the air – it literally rains fertilizer. Ammonia is produced in significant quantities from intensive animal production – pigs, cattle and poultry. This enters the atmosphere, where it dissolves and is precipitated in rain. An estimated 230,000 tons of nitrogen in the form of ammonia is deposited on the UK in rain. Around 150,000 tons of nitrogen oxides also come back to earth in rainwater.

"It is difficult to give accurate figures, but most of England gets somewhere between 15 and 30 kilograms of nitrogen per hectare of land each year from the sky," says Davison. "Some places might get as much as 60 kilos. If you consider that a farmer growing wheat will add between 150 and 250 kilos of nitrogen fertilizer over the same area, the quantities are very significant."

The problem with this is the effect on wilderness areas that have, otherwise, remained largely undisturbed by man. "If you start to fertilize areas you will change the flora," says Davison. "Some plants will take advantage of this – stinging nettles and blackberries, for example – and begin to dominate at the expense of others. By adding fertilizer to natural vegetation you will almost certainly damage the biodiversity."

As well as its fertilizing effect, nitrates dissolved in rainwater are acid. "Since we have managed to control acid rain from sulphur, nitrogen in rain is the biggest acidifier," says Professor Davison. "This results in soils and streams becoming acid, which can have harmful ecological effects."

What can be done? Research programmes such as Gane are attempting to fill the huge gaps in our knowledge of the "new" nitrogen cycle. "When political decisions are being made, the decision-makers need to have reliable science to hand," says Davison. "We are trying to understand the underlying science of the way that the environment is becoming enriched with nitrogen. It is only recently that people have recognised this as an important global issue. It is clear that there are difficult scientific problems and political challenges ahead."