Science & Technology: 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
Connor, Steve, The Independent (London, England)
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 - N2 - is of no use to plants, but being able to convert it into ammonia - NH3 - meant that it was then possible to make the ammonium salts that crops can use as a source of essential nitrogen. Ammonium nitrate (NH4NO3) 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. …