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).
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.