Nitrogen is necessary for life on earth to continue and the nitrogen cycle is one of the most important nutrient cycles in terrestrial ecosystems . Everything that lives needs nitrogen; it is required for the manufacture of complex molecules such as chlorophyll, proteins and DNA, and also for the production of enzymes necessary for growth, reproduction and other vital functions. However, when present in excess, nitrogen poses risks both to the environment and human health.
The nitrogen cycle
The natural cycle of nitrogen is complex; although the concentration of nitrogen in the air is one million times its concentration in living organisms, it is not readily available for use. There are three main ways by which nitrogen can naturally be made available for use in ecosystems. Firstly, by plants themselves, when bacteria, most notably those associated with leguminous (bean or pea) plants, trap nitrogen from the air and combine it with hydrogen to form ammonia (NH3). The decomposition of plants and animals also releases organic nitrogen into the soil as ammonia. Bacteria and fungi in the soil then convert this ammonia into ammonium (NH4), which can be used by plants. Further chemical reactions by nitrosomonas bacteria transform the NH4 into nitrite NO2-. The nitrobacter bacteria then convert the nitrite NO2- to NO3- nitrate. This nitrate is very soluble, and can also be used by plants. The cycle is concluded when denitrifying bacteria in soil convert nitrates in anaerobic soil to either nitrogen gas (N2) or nitrous oxide (N2O) and these gasses then return to the atmosphere.
Nitrogen is also obtained from two other natural sources. The energy from lightening converts oxygen and nitrogen to nitric oxide (NO), which then oxidizes to nitrogen dioxide (NO2), and then to nitric acid (HNO3). These compounds are then carried to the ground in rain or snow. This process contributes between 5 to 8% of the total amount of nitrogen removed from the atmosphere . Finally, some free living bacteria can also trap nitrogen and convert it to nitrate.
Human intervention in the nitrogen cycle
There has been a recent intervention in this complex cycle; human modification of the nitrogen cycle has been profound and human activity now removes approximately 120 million tons of nitrogen from the atmosphere into reactive forms every year, more than all of the Earth’s terrestrial processes combined . Nitrogen is now as ubiquitous in water as carbon dioxide is in air , and significantly, the denitrifying bacteria which convert nitrogen compounds back into atmospheric nitrogen are unable to return the quantities of nitrogen now being released.
This excess nitrogen is largely from two sources. Firstly, approximately 80 million tons of atmospheric nitrogen a year is converted to ammonia for the manufacture of fertilizer , and over the last 21 years, an average of 375,000 tons of nitrate was spread per year in Ireland . As a result, much of the nitrogen in our bodies comes not from biological sources but from chemical factories . Secondly, nitrous oxides are an unintended by-product of the combustion of fossil fuels. Globally, 29 million tons of nitrous oxide are emitted each year  and Ireland emitted 89,000 tons of nitrous oxides to air in 2009 , almost a half of which came from the transport secto8 . In 2009, these emissions exceeded the ceiling of 65, 000 tons of nitrogen oxides from the National Emission Ceilings Directive, by over a third (37%). The highest emissions of nitrogen oxides to air in 2009 in Ireland were from power generation, the cement industry and the metal industry. Virtually all the ammonia emissions in Ireland are emitted from intensive farms, primarily intensive pig and poultry farms.
Impacts of excess nitrogen on the environment
Nitrates are the most significant pollutant of groundwater in Ireland and usually indicate human activity, such as intensive farming, effluent from waste water treatment plants, or septic tank leaching . An average of 41,000 kg of nitrogen was emitted to our water per year between 2007 and 2009 . The nutrient qualities of nitrogen encourage plant growth, and results in eutrophication, probably Ireland’s most serious environmental pollution problem . The bacteria that break down these plants when they die consume the dissolved oxygen in the water. The resulting anoxic conditions result in the collapse of other ecosystems, with a reduction in plant and animal life, including fish kills. Such rapid plant growth encourages fast growing species and can also lead to a reduction in biodiversity. In addition, the algae may contain the harmful toxin producing cyanobacteria, which poses a threat to human health. Excess nitrogen is similarly the cause of the growing number of oxygen depleted ‘dead zones’ in the oceans4. Irish maritime waters also have this problem; all of which show elevated nitrogen levels, with 49% of areas being classified as ‘problem areas’ . The top five emitters of total nitrogen to water in Ireland in 2009 were all waste water treatment plants.
Irish soils are also becoming overloaded; a recent survey found that under current legislative limits, 10% of Irish soils are at risk from exceedances of critical loads of nitrogen, and over a third of Irish forests and semi-natural ecosystems receive excess nitrogen in sufficient amounts to possibly lead to changes in the composition of plant species .
As nature’s ability to process this excess nitrogen and return it to the atmosphere is overwhelmed, nitrogen compounds build up in the air, water and the soil. The only way that nitrogen can return to its original inert state is through the action of the anaerobic bacteria.
Impacts of excess nitrogen on health
It is clear that excess nitrogen is not good for our environment; it is also not good for our health. Reactive nitrogen is an important driver of air pollution worldwide and as sulphur emissions have lessened, nitrogen is now the principal acidifying component in acid rain.
Nitrogen may join with oxygen to form nitrogen oxide (NO), a precursor of smog, and also a respiratory irritant. Nitrogen oxides, along with volatile organic compounds, contribute to the formation of ground level ozone. Long term exposure to moderate levels of ground levels ozone results in a reduction in lung capacity and can exacerbate respiratory and cardiac disease. Significantly, the levels of ground level ozone in Ireland are already classed as ‘moderate’ .
Aside from being a respiratory irritant, nitrous oxide (N20) is also one of the three most important greenhouse gases, being almost 300 times more potent as a greenhouse gas than carbon dioxide. Nitrogen oxides emitted from aircraft also act as a catalyst in the destruction of the stratospheric ozone layer, thereby increasing exposure to ultra-violet B radiation, with resulting impacts on the incidence of skin cancer, the immune system and cataracts. High levels of nitrates in water can result in the condition of methaemoglobinaemia, condition usually found in very young babies, where the haemoglobin is unable to carry oxygen properly, and results in cyanosis. Nitrates are also a precursor of N-nitroso compounds, and while there is strong biological plausibility for nitrates in drinking water as a possible cause of cancer, the epidemiological evidence is weak .
The threat that excess nitrogen poses to the environment and our health has been recognized. The Nitrates Directive came into force in December 1991 and is concerned with the protection of waters against pollution caused by nitrates from agricultural sources and needs to be fully implemented. The Stockholm Resilience Centre3 has proposed a limit of approximately 35 million tons of nitrogen being removed from the atmosphere by human activity on a global scale. Such a limit would greatly reduce the amount of reactive nitrogen released into the land, sea and air. This figure of 35 million tons per year is in stark contrast to the current figure of 121million tons, and the pre-industrial figure of 0. To achieve such substantial reductions in the emissions of nitrogen from the Irish perspective, would require less polluting methods of food production, including a reduction in intensive poultry and pig farming, alternatives methods of power generation and transport and improvements in waste water treatment plants. Aside from recognizing the vital importance of conserving our wetlands, the habitat of the denitrifying bacteria which enable nitrogen to return to the atmosphere, there are several ways by which we can work to restore the balance to the nitrogen cycle. Firstly, we need to consume less; one third of the food we buy is not eaten and we can also reduce our consumption of food that has been intensively farmed. Secondly, we must continue to improve the quality of our waste water treatment plants. Finally, we need to reduce our consumption of electricity derived from fossil fuels and increase our use of public transport.
The nitrogen cycle is one of our human life-support systems, supporting human life and life on our planet. Our disruption of the nitrogen cycle is a public health issue of profound importance.
The help of the Department of Agriculture in sourcing data on fertilizer use is much appreciated.
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Featured image: Irish woodland bog. Author: John Donovan Source: http://www.sxc.hu/photo/17031