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 pounds 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. …