4.4 Nitrogen: a developing threat to health
A great deal of attention, by governments and the media, is focused on the environmental threat posed by carbon dioxide (CO2) emissions and on the urgent need to reduce them. Mainly due to the burning of fossil fuels, the level of CO2 in the atmosphere has increased by some 36% since pre-industrial times. According to recent estimates (IPCC, 2007), this increase has contributed more than 50% of the global warming attributed to human activities. The rest is due to enhanced atmospheric concentrations of a number of other ‘greenhouse’ gases, including nitrous oxide (N2O). Molecule for molecule, N2O is nearly 300 times more powerful as a greenhouse gas than CO2 and levels of the gas are rising rapidly, largely due to the widespread use of nitrogen fertilisers in agriculture; some of the nitrogen ends up in the air as N2O. However, the amount of N2O in the atmosphere is still very low, and the observed increase (up 18% above pre-industrial level) has contributed just 5% to global warming so far.
Nitrogen exists in the environment in huge quantities; as nitrogen gas (N2) it makes up 78% by volume of the Earth's atmosphere. As an element, nitrogen is an inert gas; that is, it is unreactive, is of little use to animals or plants and does no harm to them. It is when nitrogen becomes incorporated into compounds with other elements (notably oxygen or hydrogen) that it becomes reactive and can be harmful. In a natural environment, like that which existed before the industrial revolution, reactive compounds of nitrogen are produced naturally, in relatively small quantities. For example, lightning converts nitrogen in the atmosphere into compounds called nitrates that enter the soil in rain. Bacteria that occur in nodules on the roots of certain plants, known as nitrogen-fixing bacteria, also convert nitrogen gas into nitrogen compounds which enter the soil, providing a natural fertiliser.
The situation changed during the industrial revolution, with the burning of fossil fuels on a large scale, releasing various nitrogen compounds into the atmosphere in large quantities. More recently, the industrial production of nitrogen fertilisers has released into the environment a number of other, reactive nitrogen compounds. These are listed, with their effects, in Table 4 in Box 1.
Box 1: (Enrichment) Reactive compounds of nitrogen and their environmental effects
There are many compounds of nitrogen and they have a variety of effects on the environment. Table 4 lists a selection. The chemical formulae are included for completeness.
Table 4 Some compounds of nitrogen and their environmental effects
|Compound||Chemical formulae||Environmental effects|
|nitrate ion||NO3−||acid rain, eutrophication of water‡|
|nitric acid||HNO3||acid rain, eutrophication of water|
|nitrogen dioxide||NO2||smog, acid rain, eutrophication of water|
|nitrous acid||HNO2||smog, acid rain|
|nitric oxide||NO||smog, acid rain|
|nitrous oxide||N2O||greenhouse gas, destruction of ozone in the stratosphere|
|ammonia/ammonium ion||NH3/NH4+||smog, eutrophication of water, aerosols§|
The major source of nitrogen compounds in the environment is now the increasing use of fertilisers to enhance crop yields, driven largely by the need to feed the large proportion of the world's human population that is undernourished. This is a trend that is expected to continue (Figure 10).
Other sources of nitrogen being added to the environment are urine and manure from livestock, industrial emissions and human waste. As a result of all this release of nitrogen compounds, the natural cycle of nitrogen in the environment has become swamped by what is called the ‘nitrogen cascade’ (Galloway et al., 2003). In the natural nitrogen cycle (Figure 11), de-nitrifying bacteria convert nitrogen compounds back into atmospheric nitrogen, but these are now unable to cope with the massive quantities currently being released into the environment. As a result, levels of nitrogen compounds are building up in soil, in the atmosphere and in water.