There is a growing consensus among researchers that the scale of disruption to the nitrogen cycle may have global implications comparable to those caused by disruption to the carbon cycle. On the positive side, it appears possible that the nitrogen and carbon cycles are interacting. Since nitrogen is normally a limiting factor in plant growth, increased available nitrogen may be enhancing overall plant growth which, in turn, would enhance the Earth's carbon storage potential. This extra vegetation may explain the puzzle of the world's 'missing' carbon - the difference between the amount of carbon emitted and the amount known to be accumulating in the atmosphere each year (Vitousek and others 1997).
Nitrogen emissions to the atmosphere are contributing to global warming. Nitrous oxide is a potent greenhouse gas, accounting for about 6 per cent of the enhanced greenhouse effect. It is long-lived in the lower atmosphere, and concentrations are currently increasing at the rate of 0.2 to 0.3 per cent per year. In the upper atmosphere, the gas also contributes to ozone depletion. Most of the atmospheric nitrous oxide is of biological origin, being produced by bacteria in soils and surface waters. Recent increases in emissions are attributed to human activities, in particular related to agriculture and land use (Environmental Pollution 1998).
We are fertilizing the Earth on a massive scale. Specific impacts are being studied but we are still largely in the dark about the overall effects of this huge disruption of the nitrogen cycle. Additional synergistic effects between major biogeochemical cycles (nitrogen and carbon, for example) and human activities are still very uncertain.