Almost 80 percent of the atmosphere is nitrogen gas. It is essential for plant growth, but before plants can absorb nitrogen, it must become "fixed" – bonded with carbon, hydrogen, or oxygen. Certain kinds of algae, soil bacteria and lightning accomplishes this naturally. Before human beings began to alter the nitrogen cycle, these mechanisms provided 90-150 million metric tons of fixed nitrogen (nitrates and nitrogen oxides) a year. Now human activity adds 130-150 million more tons. Half the industrial nitrogen fertilizer used in human history has been applied since 1984. As a result, coastal waters and estuaries bloom with toxic algae while oxygen concentrations dwindle, killing fish; as a result, nitrous oxide traps solar heat. And once the gas is in the air, it stays there for a century or more.
The major human intrusion into the nitrogen cycle involves inputs of nitrogen oxide into the atmosphere and nitrates into aquatic ecosystems (through improper use of nitrogen fertilizer, animal wastes and sewage).
Humanity is fertilizing the Earth on a global scale through intensive agriculture, fossil fuel combustion and widespread cultivation of leguminous crops. Evidence is growing that the huge additional quantities of nitrogen being used are exacerbating acidification, causing changes in the species composition of ecosystems, raising nitrate levels in freshwater supplies above acceptable limits for human consumption and causing eutrophication in many freshwater habitats. In addition, river discharges laden with nitrogen-rich sewage and fertilizer run-off tend to stimulate algal blooms in coastal waters, which can lead to oxygen starvation and subsequent fish kills at lower depths, and reduce marine biodiversity through competition. Nitrogen emissions to the atmosphere contribute to global warming. Consensus among researchers is growing that the scale of disruption to the nitrogen cycle may have global implications comparable to those caused by disruption of the carbon cycle.
Evidence is mounting that human activities are seriously unbalancing the global nitrogen cycle. Nitrogen is abundant in the atmosphere but must be fixed by micro-organisms in the soil, water and in the roots of nitrogen-fixing plants before it is available for use by plants and the animal life dependent on them. The advent of intensive agriculture, fossil fuel combustion and widespread cultivation of leguminous crops has led to huge additional quantities of nitrogen being deposited into terrestrial and aquatic ecosystems. Human activities have at least doubled the amount of nitrogen available for uptake by plants (Vitousek and others 1997) and now contribute more to the global supply of fixed nitrogen than do natural processes: we are fertilizing the Earth on a global scale and in a largely uncontrolled experiment.
Large areas of northern Europe, where intensive agriculture and high fossil fuel combustion coincide, are now in a state of nitrogen saturation: no more nitrogen can be taken up by plants, and additional deposits are simply dispersed into surface water, groundwater and the atmosphere without playing any role in the biological systems for which they were intended.