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The nitrogen (N) cycle is a manifestation of oxidation and reduction reactions - the loss and gain of electrons. Oxidation and reduction reactions are also the hallmark of the biogeochemical cycling of carbon (C), hydrogen (H), and oxygen (O): the chemical elements with which the N cycle is most commonly and intimately interconnected.

The valence range which N undergoes in its biogeochemical cycling is full: going from loss of all five of its outer shell electrons (+5) to other elements to the gain of three electrons from other elements (-3) to complete all of the orbitals of its outer electron shell. As with its companion element C, biogeochemical cycling can potentially result in N losing all of its outer shell electrons to other elements and/or filling all of its outer shell electron orbitals with electrons from other elements.

On the right-hand side of the depicted N cycle, the N atom can eventually lose all five of its outer shell electrons to O - a more electronegative element (an element whose affinity for electrons is greater than that of N). With this, N can eventually become fully oxidized as nitrate (NO3-).

On the left-hand of the depicted N cycle, N can eventually add three electrons to fill all of its outer shell electron orbitals from elements such as hydrogen (H) and carbon (C) - elements less electronegative than N. With such gain of electrons, N can be fully reduced to ammonia (NH3) - which most commonly exists in its ionic form, ammonium (NH4+). Or N can be fully-to-partially reduced in a myriad of organic compounds. As with C, H, and O , the cycling of N also involves a zero valence form - N2 gas - in which the charge of the seven protons in N's nucleus are charge balanced by seven electrons orbiting the nucleus, the two electrons of the inner electron shell and the five of the outer shell.

The depicted N cycle is, by necessity, greatly simplified. As with C, H, and O cycles, there are many intermediate oxidation/reduction forms that N can assume.

Furthermore, the depicted N cycle is greatly idealized. As with the C, H, and O cycles, much of the N which is biogeochemically cycled does not pass through the entire potential range of oxidation and reduction reactions. 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. In spite of a popular perception that plants are limited to taking up N as NO3-, plants take up N in almost all of its forms, including NH3, NH4+, NO2-, and organic-N. 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.

And so, we depict the N cycle in simplified, idealized form using the six most commonly-depicted forms of N for the entire eight electron range of oxidation/reduction that N can undergo in its biogeochemical cycling.

While oxidation/reduction reactions of the N cycle are carried out in all four spheres, the transformations and fluxes of N within and between the spheres short-cut the full cycle of transformations to varying degrees. For example, of the estimated 5.8 billion (109) metric ton/yr biospheric N cycle residing in the top 1 m of soil of the geosphere, only a little more than 100 million (106) metric tons/yr is estimated to pass through the full eight-electron N cycle.

The oxidation/reduction reactions of the N cycle are mediated by biological, chemical, and physical factors. These mediating factors not only influence the form which N takes, but also the nature and extent of reaction(s), as well as the flux and accumulation of N, within and between the four spheres and the various storage components making up the spheres' reservoirs.

The importance of mediating factors can be seen from thermodynamic examination (see Transformation of Nitrogen Species in Nitrogen Properties) of the N cycle itself. That N exists in forms other than its most thermodynamically-stable form (which is NO3-), indicates that an external source (or sources) of energy - directed by mediating chemical, physical and biological factors - is(are) driving the N cycle.

An understanding of the factors driving and mediating the N cycle can be had from theories which scientists have developed concerning how the earth has developed into its present state.

Unlike the present-day atmosphere, the earth's original (primordial) atmosphere appears not to have been an oxidizing atmosphere. The early earth's atmosphere is believed to have been reducing. A reducing atmosphere would have been consistent with its being principally controlled by abiological, geochemical processes. This is because the overall mineralogy of the suite of primary minerals which make up the geosphere is reducing. The early atmosphere, unlike today's atmosphere, appears to have had much of its N gas in the form of NH3 gas.

That the original supply of primordial N was ammoniacal N is not contradicted by the geochemistry of the present-day earth. Indeed, theories of the past are drawn from observations of the present which are then extrapolated to the past. Geochemists estimate that the geosphere still contains about 50 times more N (mostly as NH4+) than the total mass of N2 gas in the earth's atmosphere.

Indeed, primordial (juvenile) N is still being emitted to the atmosphere today by volcanoes. And geothermal waters coming up from deep beneath the earth's surface often bring up with them extraordinarily-high concentrations of dissolved NH4-N which gets transformed to NO3-N under the influence of the earth's now-oxygen-rich atmosphere. And because of such N-rich geothermal activity, it remains scientifically uncertain to this day as to what the sources of the vast geologically modern Chilean nitrate deposits are (as well as what the source of other such similar deposits which have accumulated throughout the drier reaches of the North and South American Cordillera).

It appears that the earth's N cycle changed dramatically with what would have been the greatest environmental catastrophe of all time: the development of plants whose photosynthesis (chlorophyll a) came to use solar energy to hydrolyze water to release a most corrosive and deadly gas - O2 gas, e.g.: 6CO2 + 6H2O = C6H12O6 (glucose) + 6O2.

Release of this highly corrosive gas would have dissolved whole mountain ranges. Minerals, previously stable under a reducing atmosphere, would have dissolved under the corrosive attack of O2. The amounts of minerals dissolved by O2 appears to have been truly prodigious. It seems that 100s of millions of years passed before this deadly O2 gas could even begin to appreciably accumulate in the atmosphere.

With the accumulation of O2 in the atmosphere, any life which previously existed under the reducing atmosphere was literally "burned up" by oxidation. Only those cells able to form cell walls which could have survived the "burning" by O2 gas to protect the vital reducing conditions of cell interiors would have been able to live. Possibly a hold over from the early earth, plants today take up much of their N as NO3-N but spend large amounts of energy to reduce it to ammoniacal N.

Thus the apparent change in the earth's oxidation/reduction status appears to have profoundly altered the N cycle, as well as the allied C, H, and O cycles.

Such theorized changes in these interconnected elemental cycles would have been both qualitative and quantitative in nature (Spheres of the Nitrogen Cycle). And, because such theoretical changes are based on observations of how the world works today, they are most useful for providing us with a mental framework for understanding the N cycle as it exists today.

Qualitatively, oxidized forms of N and C now support the photosynthesis which enables their formation via generation of O2 by the splitting of H2O - the N and C being reduced within the plant cell in conjunction with the H cycle in spite of the almost prohibitive energy expenditure involved in driving such oxidation and reduction. Accordingly, the ultimate energy source driving the N cycle can be said to be external: principally solar.

Quantitatively, these intertwined 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. For example, the processing of 100 billion (109) metric tons CO2-C/yr by chlorophyll a produces an amount of O2 contained in the earth's atmosphere every 9,000 years. This rate of photosynthesis also processes an amount of water equal to that contained in all the world's oceans in less than 10 million years. The leakage of 200 million metric tons/yr of N2 gas from the 10 billion (109) metric ton biospheric N cycle will produce an amount of N2 gas equal to that of the earth's atmosphere in less than 20 million years. Thus, this solar-powered watch not only runs the biosphere, it also runs the atmosphere and the hydrosphere at its most fundamental level - making-destroying-making again. Indeed, as we have seen above and will see again below, the solar-powered watch has a powerful effect on the geosphere.

Using the theory of the earth's development, we gain further insight into how the world works today. We see from the theorized changes in C, H, O, and N cycles that the physical and chemical factors which we do not think of as being biologically-mediated are in fact biologically-mediated, albeit indirectly. For example, the biosphere influences the nature and quantity of energy passing through the atmosphere by maintaining the chemistry of the atmosphere to be that of O2 and N2. Atmospheric photochemical reactions that transform N in the atmosphere are mediated indirectly by the biosphere which influences the type and amount of chemicals present as well as the type and amount of sunlight driving the photochemical reactions. Even fixation of atmospheric N2 gas by lightning can be said to be indirectly mediated by the biosphere because the N2 gas being fixed is produced biologically - as is the O2 gas with which N2 gas is being combined. And even the physical and chemical properties of soils and sediments in which N cycling is taking place are also, in significant degree, products of the biosphere.

In turn, the feedback from physical and chemical mediating factors influence the biosphere which, in turn, influences physical and chemical mediating factors, and so on.

The more we understand the far-reaching effects that mediating biological, physical, and chemical factors have on the earth's N biogeochemistry and each other, the more we may be able to realize that clear-cut divisions between biological versus physical versus chemical and internal versus external reactions do not exist in the real world. But rather, such reductionist categorization is necessary for the human mind to understand complex phenomena via identification of underlying fundamental controlling factors. From this reductionist base more complex (and more realistic) mental structures may be built from which we may be able to better model reality.

As you read further in this web site, you may want to practice (along with us) simplifying the real world to a reductionist base and then building upon the reductionist base. One approach, for example, would be to think about how biology may have indirectly mediated the N cycle via its influence on physical and chemical factors which mediate the fluxes and forms which N undergoes.

Please let us know what ideas you come up with.

Suggested Reading:

Delwiche, C.C. 1970. The nitrogen-cycle. Sci. Am. 223(3):136-147, 264.

Furley, P.A., and W.W. Newey. 1983. Geography of the Biosphere. Butterworths, London.

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.

Sprent, J.I. 1987. The Ecology of the Nitrogen Cycle. Cambridge University Press, New York.

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., p. 1-42.

Viets, F.G., Jr., and R.H. Hageman. 1971. Factors affecting the accumulation of nitrate in soil, water, and plants. Agriculture Handbook 413. U.S. Government Printing Office, Washington, D.C.


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