Appreciable amounts of nitrogen (N) are present in the atmosphere, biosphere, hydrosphere, and geosphere. Given that the oxidation/reduction reactions of the N cycle occur in all four spheres, the N in all four spheres is, to varying degrees mobile, being affected by mediating chemical, physical and biological factors which may or may not be influenced by human activities.
The three mediating factors induce qualitative and quantitative changes - changes in the chemical form and quantity, respectively - of the N contained in the four spheres (Full Text Simplified N Cycle). The movement of a quantity of N from one sphere to another is often, but not always, triggered by some chemical (qualitative) change. Such chemical change can make the qualitatively-altered N prefer to be in one sphere over another. For example, the 3.86 x 1015 metric tons of N2 gas in the earth's 5.12 x 1015 metric ton atmosphere is principally the result of conversion of solid state and liquid state N by denitrification to N2 and other gases. Once converted to the gaseous state, N is predisposed to move from the hydrosphere, biosphere, and geosphere and into the atmosphere.
Inter-sphere fluxes of N also occur in the absence of chemical change. For example, wind erosion will move N into the atmosphere and water erosion will carry N into the hydrosphere in the absence of predisposing changes in the transported N. Likewise, rain will carry all forms of N found in the atmosphere into the geosphere and the hydrosphere in the absence of predisposing changes in the transported N. Conversely, rain will fill up soil pore space and vent soil gases, including N gases, to the atmosphere in the absence of predisposing changes in the vented N gases. Having said this, the moving of N into a new environment almost invariably results in qualitative (chemical) changes in the transported N (Properties of N, Atmosphere, Biosphere, Geosphere, Hydrosphere).
Quantitative and qualitative changes induced by the three modifying factors may not result in a
transfer of N from one sphere to another but, rather, a cycling of N within a given sphere. For example,
the oxidation and reduction reactions of the nitrification/denitrification sides of the N cycle within the
geosphere produces an estimated ~100 million (106) metric tons of the natural emissions of N
gases to the atmosphere. Whereas an estimated ~100 million (106) metric tons of N gases are so
generated and released to the atmosphere, the nearly 6 billion (109) metric tons N/yr of the biospheric
terrestrial N cycle in the upper 1 m of soil do not leave the geosphere but, rather, pass through
different pools that make up the soil's overall geospheric N reservoir.