The nitrogen cycle is arguably the second most important cycle, after the carbon cycle, to living organisms. Nitrogen is essential to plant growth, and therefore is a significant contributor to the human food chain, but its presence in the environment is strongly influenced by anthropogenic activities.

Here, we will describe the global nitrogen cycle; we then examine the long-term trends at national and global scales for both terrestrial and aquatic ecosystems; next we describe how nitrogen is transported at local and long-distance scales; finally we consider how public policy for environmental protection aims to mitigate against pollution effects.

Nitrogen was discovered in 1772 by Daniel Rutherford, who called the gas 'noxious air'. During the late eighteenth century other chemists, such as Scheele, Cavandish, Priestly, and Lavoisier were also studying 'dephlogisticated' air, the term then used for air without oxygen. By the late nineteenth century its vital role as a plant nutrient was understood and by the early twentieth century, the Haber-Bosch process was able to 'fix' nitrogen from the atmosphere on an industrial scale. Nitrogen fixation influences the amount of food present within an ecosystem. Prior to the industrial process ofN production, crop growth was sustained by recycling crop residues and manures on the same land where food was grown. Any 'new' N was created by growing rice and legumes, or by mining guano and nitrate deposits. However, as the human population increased so has the demand for food

Atmosphere 3.8 x 109

Denitrification 30 NOx deposition 11 NH3 deposition 23 Biological fixation 40

Biological Industrial

Microbial NOv

Deposition Deposition fixation fixation volatilization

89 36 150 110 122 8

Denitrification 147

Runoff / rivers

Denitrification 30 NOx deposition 11 NH3 deposition 23 Biological fixation 40

Runoff / rivers

Oceans Flora 3 x 102 Fauna 1.7 x 102

Weathering 14

Sedimentation 14

Rocks Sediments 4 x 108

Figure 1 Global nitrogen reservoirs (units kg Nyr 1) and fluxes (units x 109kgNyr 1).

and with that the dependence on inorganic fertilizers to sustain agriculture. This trend has affected the nitrogen cycle at global, national, and local scales.

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