In nature, nitrogen remains constant. The Earth's atmosphere has 78 per cent nitrogen, but very little of this vital nutrient can be absorbed by plants. A chain of chemical reactions deposits atmospheric nitrogen in rain and then takes it to the soil. Nitrogen-fixing bacteria and lightning help plants to absorb this nitrogen, which is then consumed by humans and animals through food. The nitrogen returns to the soil through excrement, and then again goes to the atmosphere through bacterial decomposition of organic matter. For millions of years, this cycle kept nitrogen in closed loops (see box: Nature's check and balance). No longer. Nitrogen is entering water in millions of tonnes instead of going to the soil.

Changing the cycle
Human activities in the last 100 years have terribly damaged the nitrogen cycle (see graphic: Breaking the loop). Fossil fuel combustion has increased the amount of nitrogen oxides, which are blamed for the greenhouse effect. Large-scale cultivation of legumes, which support nitrogen-fixing bacteria, have also increased nitrogen in the soil. Fertilisers improved global food production in the last century by increasing the amount of nitrogen available to plants. But now agricultural fields in some countries have nitrogen in such massive quantities that plants can't absorb them. The nitrogen in fertilisers, present in a form called nitrate, is extremely soluble and so easily goes from soil to groundwater reservoirs feeding lakes and streams.

Flush-and-forget sewage disposal systems improved sanitation, but have aggravated the damage to the nitrogen cycle. Instead of returning plant nutrients in human waste to the soil, sewage systems take them to treatment plants. Thus, the nitrogen in human waste too is reaching surface water bodies and oceans.

Humans each year ingest nearly 20 million tonnes of nitrogen through food, all of which enters the environment. What's terrible is that while we consume 20 million tonnes of chemical nitrogen, 100 million tonnes of it is produced and supplied in the form of chemical fertilisers.

About 32 per cent of the nitrogen fertilisers applied in USA each year (about 3.6 million tonnes) reaches surface water and groundwater, say Robert Howarth, David R Atkinson professor of ecology and environmental biology, Cornell University, USA, and other researchers.

Their study shows that the excess nitrogen in USA's rivers had increased from 3 million tonnes in 1961 to 5 million tonnes in 1997, which is 45 per cent of the total 11.2 million inorganic nitrogen fertiliser consumed in the US in 1997. The Mississippi river in 1996 alone carried 1.5 million tonnes of nitrogen, says a paper by W R Raun and G V Johnson of the department of plant and soil sciences at Oklahoma State University, USA.

And paying for it
Scientists say the global rate of increase in nitrogen was slow from 1860 to 1960 but the amount has now increased. The amount of nitrogen added to the environment between 1860 and 2000 has increased from 15 Tg (One terragramme is equal to one million tonnes) of nitrogen per year to over 165 Tg of nitrogen per year, write James Galloway, professor of environmental sciences, University of Virginia, USA, and others in a paper published in the April 2003 issue of BioScience.

Half of all the nitrogen ever applied for agriculture was done in the last 15 years, says Howarth in a paper published in Ambio in 2002.

Changing the natural nitrogen cycle has given us benefits: Almost 40 per cent of humans are alive today thanks to the substantial increase in nitrogen through fertilisers and cultivation-induced biological nitrogen fixation. The flip side: the runaway increase in nitrogen in the ecosystem has several serious environmental consequences, many of which are being noticed first in advanced countries.

Deluge
Scientists predict the global production of nitrogen will reach between 250 million tonnes to 900 million tonnes per year by 2100. What are the exact effects of this nitrogen deluge?

High nitrate levels in drinking water can cause