Furthermore, the depicted N cycle is greatly idealized as much of the N which is biogeochemically cycled through the four spheres does not pass through the entire potential (eight electron loss-and-gain) cycle of oxidation and reduction. For example, N passing up the food chain is principally passed along as fully-reduced, organic forms and partially-oxidized inorganic and organic forms. Even much of the N taken up by plants circumvents the complete oxidation/reduction cycle of the depicted N cycle. Thus a realistic depiction of the N cycle would have an almost infinite number of intermediary steps along the circumference of a circle within which there would be a multi-dimensional spider web of internal, crisscrossing connections of dizzying complexity.
That N exists in forms other than its most thermodynamically-stable (least energy-containing) form indicates that the oxidation/reduction reactions of N - as well as those of the allied C, H, and O cycles - are powered by external energies directed by mediating chemical, physical, and biological factors.
These integrated elemental cycles and their mediating factors can be simply viewed as the gears of a solar-powered mechanical "watch."
And this solar-powered "watch" dominates global biogeochemistry. Photosynthesis, powered by the sun, is performed by a nitrogenous compound (chlorophyll a) that processes and creates the bulk of the earth's atmosphere and hydrosphere. Namely, all of the N2 and O2 gases that make up the atmosphere and all the water (H2O) of the hydrosphere are destroyed and remade by photosynthesis and its integration with the N cycle. For example, H obtained from H2O reduces N. The reduced N combines with C to form amino acids and other nitrogenous substances that are essential components of the biomass that make up all forms of life. The O which was ultimately obtained from water converts nitrogenous wastes to more oxidized forms of N - such as N2 gas and various oxides of N. These more oxidized forms of N are generally more mobile thereby making N widely available to living organisms over a wide range of geographic and environmental locations.
Through such cycling of the biospheric, atmospheric, and hydrospheric components of the "watch," the geospheric component is also cycled.
The large amounts of energy required to run the oxidation/reduction cycling of N (and the other
elements) is paid for by the sun. Since most of the N being cycled does not go through the full eight-electron
cycle of oxidation/reduction, the full amount of energy needed to run the eight-electron cycle is
paid for only by a small portion of the N cycle.
Mason, B. 1966. Principles of Geochemistry. Third Edition. John Wiley & Sons, Inc., New York.
Odum, E.P. 1963. Ecology. Holt, Rinehart and Winston, New York.
Rosswall, T. 1976. The internal nitrogen cycle between microorganisms, vegetation and soil. Nitrogen, Phosphorus and Sulphur - Global Cycles. SCOPE 7 Report, B.H. Svensson and R. Soderlund (eds.) Ecol. Bull (Stockholm) 22:157-167.
Soderlund R., and B.H. Svensson. 1976. The global nitrogen cycle. Nitrogen, Phosphorus and Sulphur - Global Cycles. SCOPE 7 Report, B.H. Svensson and R. Soderlund (eds.) Ecol. Bull (Stockholm) 22:23-73.
Stevenson, F.J. 1982. Origin and distribution of nitrogen in soil. Nitrogen in Agricultural Soils. F.J.
Stevenson (ed.). Agronomy Monograph No. 22. American Society of Agronomy, Madison, WI., pp.