About half the world's population depends on the nitrogen in fertilisers to live, according to Dr Mark Sutton, of the Centre for Ecology & Hydrology in a ground-breaking new report on the impact of nitrogen as a pollutant: European Nitrogen Assessment (ENA), unveiled at a conference in Edinburgh this week. We are talking here of ''reactive nitrogen'' rather than the inert nitrogen which makes up 78% of the air we breathe and keeps it stable.

''Reactive nitrogen'' is produced mostly by industrial processes and is in fertilisers and chemical pollutants like those given off by car exhausts and power stations. Without the food grown with fertilisers, Dr Sutton estimates that the Earth could only support about 50% of the current population of 6.8 billion.

Nitrogen in the form of ammonia was first produced industrially using the Haber Bosch process developed in 1909. During World War One this process allowed Germany to synthesize explosive chemicals which were not available due to the naval blockade in place at the time.

However its primary use has long been as the main ingredient in artificial fertilisers for agriculture. This has spurred a massive increase in food production, but like many scientific advances, there are unanticipated downsides. Dr Sutton says, ''Because of this, we have a larger environmental footprint than we would have done otherwise. This is an example of geo-engineering our planet which has unexpected side-effects.'' Reactive nitrogen leaks into the eco-system where it has causes problems. His report identifies five key threats, using the acronym WAGES:

- Water quality

- Air quality

- Greenhouse gas balance

- Ecosystems and biodiversity

- Soil quality

Reactive Nitrogen pollution affects every part of the natural environment. 70% of the nitrogen comes from agricultural use, 21% from motor vehicles and power stations, and most of the rest from biological nitrogen fixing from plants like legumes: peas, beans, lentils and the like. ''We are swamping the environment with more nitrogen than is natural. If we unpack this a bit more, we find nitrates in water produce dead zones in coastal areas; nitrous oxides and ozone are in the air we breathe, as well as particulate matter, which have significant adverse health effects; Nitrous Oxide - laughing gas - is a greenhouse gas responsible for some of the global warming that is occurring.''

Sutton proposes that we should use fertilisers more efficiently, which would reduce costs to both the farmers and the environment - a lot of these these agro-chemicals are applied incorrectly and very wastefully. Some EU states have voluntary guidelines but others, like the Netherlands and Denmark have made it a regulatory requirement for farmers to use them properly.

Sutton notes that some sewage systems are converting Reactive Nitrogen back to its stable form. This uses energy, so it is a poor employment of our energy resources which are in many cases in decline. He observes that biogas can be produced from this waste. Though this generally operates on a small-scale on farms, it would be possible to do this on a large scale using the waste from cities and other urban areas, recycling the nitrogen. 85% of the European nitrogen goes to feed animals and only 15% to feed people. Cutting down meat consumption would reduce nitrogen pollution by a considerable amount.

So the ENA conference in Edinburgh is taking the not just ''Talking the Talk,'', but is also, ''Walking the Walk,'' by cutting down its meat consumption. ''There will be about 300 people at the conference,'' Sutton says, ''and we are speaking with the caterers, so the buffets at the conference will be - demi-vegetarian - with half the meat content they would normally cook.'' Sutton laughs at his new term, demi-vegetarian. ''We've done this before. The response of the delegates was very good. It made a talking point for the event. People seemed satisfied with the amount of meat they were eating.'' Sutton points out that we in the West over-consume meat which has adverse health consequences. ''If we reduce our meat eating to a healthy amount, we benefit the environment as well: it's a win-win situation.''

This report opens a new chapter on how we approach the issue of feeding the world and its consequences.

Photo: Biogas installation near Neuerkerode, Lower Saxony, Germany, by Elmschrat

Links:

Nitrogen in Europe

Nitrogen and Global Change