The killing fields of Great Britain

Nitrogen is vital to plant growth, but too much of it is being pumped on to our farmland. The effect on biodiversity will be catastrophic. Steve Connor reports

Sometimes it is possible to have too much of a good thing. This seems to be true when it comes to nitrogen fertiliser, which has now been linked to a depletion in the diversity of wild plants and flowers growing in the British countryside. Scientists have discovered that the man-made increase in nitrogen compounds over the past century is leaving an indelible mark in the lowering of the biological richness of the natural landscape. Some species are being strangled in an overfertilised environment.

Sometimes it is possible to have too much of a good thing. This seems to be true when it comes to nitrogen fertiliser, which has now been linked to a depletion in the diversity of wild plants and flowers growing in the British countryside. Scientists have discovered that the man-made increase in nitrogen compounds over the past century is leaving an indelible mark in the lowering of the biological richness of the natural landscape. Some species are being strangled in an overfertilised environment.

Until the beginning of the 20th century, farmers struggled to get enough nitrogen fertiliser on to their fields. Sources of nitrogen-rich fertiliser included bird droppings - guano - from the South Pacific, saltpetre from the deserts of Chile as well as manure from farmyard animals. This changed with an industrial process invented in 1909 by the German chemists Fritz Haber and Carl Bosch who devised a way of converting the relatively inert nitrogen gas in the atmosphere into the more reactive ammonia.

Ordinary nitrogen gas - N ₂ - is of no use to plants, but being able to convert it into ammonia - NH ₃ - meant that it was then possible to make the ammonium salts that crops can use as a source of essential nitrogen. Ammonium nitrate (NH ₄NO ₃) for instance is now one of the principle sources of nitrogen fertiliser used by farmers. (It has also become infamous as the illicit ingredient of choice by terrorists who want to make a large bomb with a hefty explosive force.)

In nature, the conversion of atmospheric nitrogen into reactive nitrogen compounds such as ammonia or nitrates is largely carried out by bacteria. This recycling of nitrogen is essential for life. Scientists estimate that worldwide some 100 million tons of reactive nitrogen is produced, or recycled, each year by microbes living in the soil or the gut of herbivores. By comparison, human activity it estimated to produce about 160 million tons of reactive nitrogen, significantly more than the amount produced naturally.

If the current increase in the rate of production continues, by the end of the century scientists estimate that we will be producing anywhere between 250 million and 900 million tons of nitrogen through pollution from cars and factories and by the production of nitrogen compounds such as fertilisers.

The world is set to become even more awash with active nitrogen than it has been over the past century and the consequences could be dramatic.

It is now clear that adding nitrogen fertiliser to farmland leads to significant pollution of water courses, rivers, estuaries and coastlines. Scientists from the University of Virginia in Charlottesville, for instance, estimate that about half of the nitrogen spread on to American fields is not taken up by crops but washed away into rivers and water courses.

Excess nitrogen in the water environment causes the explosive growth of algal blooms which soak up dissolved oxygen and suffocate fish and other aquatic wildlife. The nitrogen runoff from the Mississippi river for example has caused a giant patch of algae covering 20,000 square kilometres to grow in the Gulf of Mexico.

But excess nitrogen is not just a problem for rivers, streams and the sea. Carly Stevens, an ecologist with the Open University, has just completed a major study of the impact of nitrogen on the countryside, funded by the Natural Environment Research Council. She and her colleagues looked at 68 grassland sites across Britain to investigate the impact of nitrogen pollution that is able to get into the air and "rain down" on even quite remote upland or island habitats.

This type of atmospheric nitrogen pollution comes in two forms, Stevens says. "These are nitrogen oxides that mainly come from burning fossil fuels and industrial processes and car-exhaust fumes. And the second form is ammonia, which mainly comes from intensive agriculture, especially intensively reared livestock.

"Over two years I looked at grasslands ranging from areas of low nitrogen pollution such as the Scottish Highlands and Lundy Island in the Bristol Channel, to areas of high pollution such as the Peak District and Staffordshire," she says.

"The results show a clear relationship between the number of species in a given area of grassland - its species richness - and the amount of nitrogen pollution. We have found strong evidence for a decline of species richness of acid grasslands in the UK that may be linked to nitrogen pollution," she says.

The idea that nitrogen pollution from farms, cars and other sources is finding its way into the atmosphere and then being recycled back on to the land is not new. Scientists at Rothamsted Research near Harpenden in Hertfordshire have been measuring the nitrogen content of rainwater since 1853. They found that the nitrogen in rain has increased threefold in about 150 years.

In fact, one Rothamsted scientist estimated 10 years ago that the nitrogen falling in rain on a typical one-hectare plot of land in central England over a period of one year amounts to the equivalent of about 8,000 cowpats being deposited on the same area of land over the same period of time.

Superficially this might seem a good thing to both farmers and the wild plants of the countryside which need nitrogen to grow. But not so, according to Carly Stevens, who says that a nitrogen-limited environment is sometimes a good thing for species diversity. "The low fertility stops any one species from growing too rapidly and forcing out slow-growing neighbours. This means a lot of species can co-exist together," she explains.

In the 68 sites she investigated as part of her study, it was important to compare areas of high nitrogen pollution with areas of low pollution. "In areas of high pollution, species richness is significantly lower than in areas of low pollution. And no other environmental variable can explain these observations so well," she says.

What was important about this study, published last month in the journal Science, was that it allowed the scientists to form judgements about what level of nitrogen pollution is acceptable in terms of affecting biodiversity. "The strong relationship we found allowed us to make predictions about the amount of species we'd expect at different levels of nitrogen pollution and to estimate how many species we are losing in comparison to what we'd expect in unpolluted areas," Stevens says.

The average level of nitrogen pollution in Britain and central Europe at present is about 17 kilograms of nitrogen per hectare per year. "At this level we'd expect to find a reduction of over 20 per cent in the average number of species," she says.

This does not bode well given the current legally acceptable limit for nitrogen. "At the current legislative limit for nitrogen pollution in the UK, which is known as the critical load, we would see a 40 per cent reduction [in species diversity]. If this is the case, then biodiversity is not being protected," says Stevens.

Although the study investigated grasslands, there is every reason to believe that it applies to other types of ecosystem. Studies of nitrogen-soaked forests, for instance, have shown that the trees initially thrive and grow faster but after a while there is evidence that the soil chemistry begins to change. As nitrates - which are negatively charged - begin to build up in the soil they become attracted to positively charged ions such as calcium and magnesium, causing these essential nutrients to become more mobile and so leach out of the soil.

Stevens and her colleagues believe that any environment where nitrogen is naturally limited will suffer from being overfertilised. "All of the species we have found to be sensitive to nitrogen pollution have been found to decline in other studies where nitrogen or fertiliser is added," she says.

"This loss of biodiversity is important for many different reasons, not least because other species of insects or birds may depend on the plants in decline. Nitrogen pollution has been a long-term problem in the UK, and the effects we are seeing now are accumulative. It could take a very long time for sensitive ecosystems to recover."

There is now abundant evidence to suggest that species are becoming extinct faster than at any time for thousands of years. Nitrogen overload could be a part of the problem, says Stevens. "We've only looked at grasslands but everything is interconnected and if you take enough bricks out of the wall then the wall is going to fall down - and that wall is biodiversity."