TIPS FOR THE TEACHER
Background Notes
Introduction
Before discussing specific examples of biogeochemical cycles, viz., the carbon, oxygen, and nitrogen cycles some terms must be defined: cycle, biogeochemical cycles, dynamic equilibrium, and steady state equilibrium. Many agree with the dictionary definition of cycle as “a single complete execution of a periodically repeated phenomenon.” Biogeochemical cycles, then, involve movement of elements back and forth between the physical (nonliving or abiotic) and living (biotic) components of the earth, considered to be a closed system (nothing enters or leaves the system) in most cases. Parts of the cycle function as a dynamic equilibrium, where reversible changes occur within a closed system as predicted by Le Chatelier’s Principle (see Chemical Equilibrium). For example, the carbonate-bicarbonate interaction in the oceans serves to maintain seawater at a relatively constant pH.
There is some movement of the same atoms back and forth within the balanced system. On the other hand, portions of the cycle function as a steady state equilibrium, not always functioning according to Le Chatelier’s Principle, because there is only movement in one direction (irreversible change), and the system is considered to be open. A burning candle illustrates this definition. Since the number of carbon atoms from the wax (or oxygen atoms from the air) is the same as the number leaving in the CO 2 gas produced, we have a steady state of equilibrium. The number of atoms of carbon (or oxygen) input equals the same number being output, so an equilibrium exists, though here they can never be the same atoms, since all CO 2 is lost to the atmosphere (an open system here). An understanding of the difference between dynamic and steady state equilibria is essential to understand the differing effects each input (or output) flux or change may have on a reservoir of an element.
Carbon Cycle
Carbon is one of few elements found in all three “spheres” of the earth’s crust: lithosphere (solid), hydrosphere (liquid), and atmosphere (gas). In the lithosphere it can be found as coal, petroleum, limestone, and other carbonate rocks; in the hydrosphere in shells, coral reefs, and hydrogen carbonate ions dissolved in water; in the atmosphere primarily as CO 2 .
The carbon cycle is the most important of the biogeochemical cycles; it plays the major role in energy transfer within the biosphere. Photosynthesis, primarily an energy storage mechanism for green plants, traps carbon dioxide from the atmosphere and stores energy from the sun in the form of carbohydrates. In essence, CO 2 becomes fixed, or transformed into a more useful, solid form (carbohydrates) by the action of living things:
6CO 2 (g) + 6H 2 O(g) ® C 6 H 12 O 6 (s) + 6O 2 (g)
Carbohydrates can be converted into proteins, fats, and other complex organic compounds. Animals, unable to perform photosynthesis, must depend on green plants to provide their food, incorporating the carbon components from food into many different carbon compounds in the animal body. In the same way, carbon is transferred and reused by carnivores who eat herbivores or omnivores. Life would cease if all carbon remained locked in the cells and tissues of plants and animals. However, carbon is returned to the atmosphere as CO 2 through several pathways. First, and most direct, is cellular respiration, in which living plants and animals break down (oxidize) complex carbon compounds to release chemical potential energy for life processes and smaller carbon compounds, mostly CO 2 :
C 6 H 12 O 6 (s) + 6O 2 (g) ® 6CO 2 (g) + 6H 2 O(g)
The CO 2 released is then available to plants for photosynthesis and the cycle continues. Photosynthesis and respiration are generally closely balanced, reverse reactions. However, these processes have seen changes in steady state equilibria over millennia, resulting in vast “sinks” of oxygen (in the atmosphere) and carbon (in the lithosphere) from the cycle.
The second and most abundant release of CO 2 into the atmosphere is the aerobic decomposition of waste products and dead plants and animals by bacteria and certain fungi that obtain energy from the process. Note that oxygen is needed for this process, although anaerobic bacterial decomposition under suitable conditions can “recycle” carbon as methane, CH 4.
Finally, CO 2 is released into the atmosphere by combustion of organic matter, such as wood, grass, and paper (made from wood), which may have removed carbon from the cycle as organic compounds for a short time, as well as fossil fuels, whose carbon has been removed from the cycle a relatively longer time.
The carbon cycle is a two component cycle: between the atmosphere and lithosphere and between the atmosphere and hydrosphere, with the atmosphere functioning as the common reservoir. One study, using C-14, has estimated that a CO 2 molecule’s residence time in the atmosphere is 10-15 years on average. It is important to note that vast amounts of carbon in the oceans are removed from biological processing as relatively unusable sediments, one of the major “carbon sinks.”
Although it has been estimated that 2.5 x 10 12 tons of CO 2 cycle through the oceans and atmosphere about once every 300 years, carbon is locked up as insoluble carbonates, either as calcareous sediments or in the form of calcite, CaCO 3 , or dolomite, CaMg(CO 3 ) 2 , a much longer time within the ocean. A second long-range carbon sink consists of the fossil fuels, primarily found in the lithosphere.
Approximately one part in 10,000 of all organic substances have been removed from the carbon cycle over the ages, primarily as deep ocean sediments. These ocean sediments becomeburiedinclay andsandandare thusremovedfromthe oxygenneededfordecay. Methane (CH 4 , natural gas) is produced when the appropriate anaerobic bacteria are present.Otherwise the solid carbonaceous material is buried deeper,with proportionate increases in pressure and temperature, until disproportionation reactions begin, producing more methane and longer chain hydrocarbons (petroleum).
On land, woody plants from the Carboniferous Era&emdash;a warm, moist period occurring approximately250-350millionyearsago&emdash;underwent asomewhatsimilarmetamorphosis. This woody material was composed of approximately 0-16% protein, 20-55% cellulose, 10-35% lignin, and approximately 10% inorganic salts by mass. Much of the protein was incorporated into body structures of aerobic and anaerobic microorganisms initially converting the material to peat, the first step in coal formation. Likewise, cellulose, a complex carbohydrate composed of long glucose strands, was converted to CO 2 and H 2 O by similar metabolic processes. Most of what remained was lignin, a complex, three dimensional polymer more resistant to bacterial degradation than cellulose. Lignin accumulated in stagnant swamps under water, eventually compacting to form peat.
Further sedimentation and crustal upheavals gradually caused geologic metamorphism as peat was subjected to varying amounts of heat and pressure, driving off additional CO 2 and H 2 O. As a result, deposits composed largely of carbon, with small quantities of hydrogen, oxygen, nitrogen, iron, and sulfur, were left behind as the substance we know as coal. The rank of a coal indicates how nearly it approaches both the composition and structure of graphite (Figure 7).
Figure 7. Structure of Graphite
For example, under extreme conditions of temperature and pressure, anthracite coal forms when peat deposits become extensively aromatized, eventually condensing into polycyclic systems with very high carbon content, often as much as 95%, and with about 3% H and 2% O. The aromatic ring systems in anthracite may contain in excess of 30 condensed rings, developing structures that become more and more like graphite. The structures inFigures7and 8illustratetherelationship betweengraphiteandanthracite coal. Figure 9 shows that the structure of bituminous coal, a more open structure produced by less heat and pressure, contains fewer polyaromatic systems, indicating that bituminous coal is not as far along in the coalification process as anthracite coal.
Diamond, the hardest natural substance, is a product of this coal-forming process under the most extreme conditions. This tetrahedral, almost pure allotropic form of carbon, is produced from coal material at extremely high temperature and pressure deep within the earth. In fact, diamonds are usually found near areas of igneous intrusion near tectonic plate boundaries.
Variation in color in diamonds is associated with mineral impurities. There is much concern today about the increasing amounts of CO 2 released to the atmosphere by the use of fossil fuels and the “greenhouse effect” this use may be producing. As shown in Figure 11, the major source of atmospheric CO 2 is not respiration by terrestrial plants and animals or the burning of fossil fuels, as might be expected, but fungal and microbial decomposition of dead organic matter. Extrapolating back from known data, scientists have found that CO 2 levels in the atmosphere seem to have maintained a fairly constant average at about 265 ppmv ± 30 ppmv (parts per million by volume) since life appeared on earth,with the exception of approximately the last 150 years. This relatively constant level over time seems to have been due to a CO 2 HCO 3 – equilibrium established between the atmosphere and the ocean surface. It should be noted, however, that there seems to have been some large with climate changes in the past. glaciers show that CO 2 levels dropped (15,000-30,000 yrs ago). This raises the cold climate (and hence the reduction concentration level (cause unknown) is disagreement among the experts, concentrations and worldwide temperature.
As noted above, CO 2 levels since constant until the
last 150 years. This burning of fossil fuels during the Industrial
the concentration of CO 2 in the atmosphere the Industrial
Revolution, and is currently Many hypothesize this increase should
called the “greenhouse effect,” resulting In a true
greenhouse, solar energy (visible the enclosure. Plants and other
materials it in the form of lower energy infrared transmits visible
light, absorbs this infrared radiation and prevents its escape from
the greenhouse (or automobile when the windows are up on a sunny
day). As a result, the air becomes warm and is prevented from rising
out of the greenhouse by the glass. The atmospheric “greenhouse
effect” is a partial misnomer, since there is no barrier in the
atmosphere (like the glass) to prevent heat convection or wind
cooling. However, several gases, the most important of which are H
2 O and CO 2 , absorb much of the long
wavelength infrared radiation re-emitted from the earth’s
surface, causing the atmosphere to be warmer than it would be if all
infrared radiation were re-radiated into space. Without the natural
“greenhouse effect” the average temperature of earth would
be around the freezing point of H 2 O. As Figure 12
illustrates, an “atmospheric window” of infrared
wavelengths around 10,000 nm are not absorbed, re-radiation into
space and prevents the planet from overheating. 
A good illustration of this effect (primarily caused by the “greenhouse” gas, H 2 O) can be easily seen when comparing day/night temperature fluctuations in arid and humid areasof theplanet.Inarid locations,thermalenergy(infrared radiation)emittedbythe warmed surface, is rapidly lost to space since there is no water vapor to absorb it, and night time temperatures comparedtodaytimetemperatures are very cold. In humid areas, water vapor absorbs some of the emitted thermal energy (infrared radiation), moderating the nighttime heat loss and producing less fluctuation in day/night temperatures (Figure 13). It seems plausible to theorize that the steady increase of CO 2 in the atmosphere since the Industrial Revolution began to gradually cause greaterabsorptionofre-emittedinfraredradiationfrom thesurfaceofthe planet, with a resulting increase in global temperature. The effects of this increase could be disastrous.
Many are aware that global warming, more rapid at the poles than equator, might result in a catastrophic rise in sea leve as polar ice caps melt. However, few realize the possible effects on foodproduction.Continuing increases in averageworldwidetemperature will shift wheat-growing zones towards the poles, away from areas of fertile to poorer land. Rates of desertification should increase in certain areas of the earth as a result of shifting rainfall patterns. Deep ocean upwellingofnutrients willbereducedas surfacewaterswarm,producing widespread climatic changes that will decrease biological productivity.
Although increased concentrations of CO 2 should enhancephotosynthetic rates in plants, there appears to be no evidence that there is increasing storage of carbon in the biota or detritus of the earth. Temperature data from two separate studies have shown that, during a sizeable part of the period since 1950, worldwide temperatures have actually decreased. In addition, there appears to be certain periods during each year when CO 2 levels are highest and lowest. As Figure 14 illustrates, peaks correspond to periods of low CO 2 demand (fall/winter) and troughs correspond toperiods of high photosynthetic demand (spring/summer). It seems safe, therefore, to conclude that CO 2 levels do play a role in world climate, but that there are undoubtedly other factors that seem to be mitigating the effect, primary among them the interactions of the atmosphere-hydrosphere interface.
Figure 13. “The Greenhouse effect.”
Figure 14. Buildup of carbon dioxide in the atmosphere.
Oxygen Cycle
The oxygen available to plants and animals has been produced over the past two billion years by photosynthesis in plants, primarily phytoplankton in the oceans. The oxygen produced by the earliest atmospheric organisms is locked away from the atmosphere in the banded iron formations deposited 2.3 x 10 9 years ago as Fe 2+ was oxidized to Fe 3+ .
Oxygen was also used up in the oxidation of reduced atmospheric gases, like NH 3 , CH 4 , and H 2 S. However, only with the evolution of protein molecules needed for oxygen transport did aerobic photosynthetic organisms spread through the hydrosphere, precipitating the remaining Fe 3+ in seawater as iron(III) oxide, Fe 2 O 3 , and releasing oxygen as O 2 to the atmosphere. As an ozone layer developed, photoautotrophs (organisms capable of photosynthesis) began to range onto the land, producing larger and more varied populations, increasing the O 2 levels further. Oxygen atoms (in CO 2 ) from the atmosphere become internalized as part of living organisms in carbohydrates or other organic substances. These are returned, along with additional oxygen atoms (O 2 ) from the atmosphere, as CO 2 during the respiration process. This cycle from photosynthesis to respiration and back to photosynthesis is continuous. Oxygen is needed in respiration to break down carbohydrates in which carbon has been stored within the biota, either directly from photosynthesis (plants) or by acquisition from other organisms (animals). In this way oxygen is depleted from the atmosphere as carbon is released back to it. In certain cases where carbon was trapped away from oxygen (coal and petroleum formations), the oxygen released originally is not used in respiration, causing a surplus of O 2 to build up in the atmosphere. This build-up is believed to be the source of the present reservoir of approximately 10 15 tons of oxygen in the atmosphere. There is some concern today about depleting available oxygen by the burning of fossil fuels or the destruction of the tropical rain forests. However, J. Calvin Giddings (see References) proposes that only 1 part in 60 of the O 2 in the atmosphere would be consumed if all of the worldwide reserves of recoverable fossil fuels were burned. Likewise, he says that catastrophic destruction of the ocean’s phytoplankton and land plants would produce “no serious dent in the O 2 supply, but it would terminate the ultimate source of all food and lead to rapid extinction of life.” It is important to note that the 10 15 tons of oxygen in the atmosphere today represents only 1/21 of the oxygen evolved over geologic time. The rest has been used to oxidize H 2 S, SO 2 , NH 3 , and CO emitted by volcanoes over the ages and solid iron and sulfur in the crust. Since this process continues today, the slow erosion of the global oxygen content must continue to be met by photosynthesis and organic sediment formation. This is one reason why students should be concerned about pollution of the oceans and rapid deforestation occurring worldwide. Though not immediately threatening, these processes cannot but impact the “oxygen reservoir” in time.
Nitrogen Cycle
In this cycle, free atmospheric nitrogen (N 2 ), is converted into soluble nitrogen compounds, then incorporated into plants and animals, and then returned to the atmosphere through organic decomposition. Because the triple bond of molecular nitrogen, (N = N), is so strong, most living things cannot use it directly. Atmospheric nitrogen must first be converted into a usable form by a process called fixation. Actually the term “fixation” applies to any gaseous substance that is condensed into a more usable form. (Thus atmospheric carbon dioxide is “fixed” by photosynthesis into the more usable form glucose.) The fixation of nitrogen can occur in three ways: biologically, atmospherically, and industrially. Most natural nitrogen fixation occurs when nitrogen from the atmosphere is introduced into the soil where it is converted to ammonia compounds by soil bacteria such as Rhizobium japonicum. This process occurs on the roots of leguminous plants such as alfalfa, beans, and clover. (In Asia, nitrogen fixation occurs in rice paddies as a result of certain blue-green algae.) The next step in nitrogen fixation is called “nitrification” and occurs when chemosynthetic bacteria oxidize ammonia compounds into nitrites (NO 2 – ) and nitrates (NO 3– ), enriching the soil for plants, which use soluble nitrates to form plant proteins. When these plant proteins are eaten by animals, the nitrogen is removed as amino acids and used to build animal protein. When plant and animal matter decay, other bacteria break down amino acids from these organisms or their waste products and release ammonia compounds. The nitrogen cycle is completed when anaerobic bacteria break down nitrates and other nitrogen compounds in the soil to release free nitrogen back into the atmosphere. Thus, the nitrogen cycle is dependent at each step upon microorganisms and could not proceed without them. Nitrogen fixation occurs atmospherically when lightning and fires cause temperatures high enough to form oxides of nitrogen. These oxides are washed to earth by rain as nitrous (HNO 2 ) and nitric acids (HNO 3 ), adding the nitrate ion to both the soil and the waters of the earth. Nitrogen fixation occurs industrially using the Haber process that combines nitrogen and hydrogen to
form ammonia.
N 2 (g) + 3H 2 (g) 
2NH 3 (g)
Ammonia is one of the top industrial chemicals produced in this country each year. A large amount of manufactured ammonia is converted to nitric acid in the Ostwald process and is then made into inorganic nitrates, such as NH 4 NO 3 ,and used for fertilizer (see Industrial Inorganic Chemistry).
Nitrogen Oxides of Interest
Dinitrogen oxide (N 2 O), a relatively unreactive oxide of nitrogen is produced by certain microbes. It was commonly used as an anesthetic known as “laughing gas.” This compound reacts in the stratosphere to produce nitrogen gas and atomic oxygen.
N 2 O(g) + hv 
N 2 (g) + O(g)
Atomic oxygen can react with more N 2 O to produce nitrogen and oxygen gas.
N 2 O(g) + O(g) 
N 2 (g) + O 2 (g)
Nitrogen monoxide (NO), and nitrogen dioxide (NO 2 ) usually appear together in the atmosphere and are often expressed as NO x . NO enters the atmosphere from natural sources, including biological processes and lightning. Large amounts enter the air from pollutants, especially as the products of fossil fuel combustion. Until recently, over half of NO x in the atmosphere was produced by the automobile. High temperatures, such as those associated with lightning, are required to form NO.
N 2 (g) + O 2 (g) 
2NO(g)
Further reaction of NO with oxygen produces NO 2 .
2NO(g) + O 2 (g) 
2NO 2 (g)
Nitrogen dioxide can undergo many reactions that involve oxygen atoms, oxygen gas, and ozone. NO 2 is ultimately removed from the air as organic nitrogen, nitrates, or nitric acid. These compounds form major sources of nitrogen for biological nitrogen cycles. The equation below shows how rain converts nitrogen dioxide to nitric acid, a component of acid rain.
3NO 2 (g) + H 2 O(l) 
2HNO 3 (aq) + NO(g)
NO and NO 2 differ in their properties and reactivity. NO is a colorless, odorless gas; NO 2 is a reddish-brown, toxic gas that causes severe respiratory problems in animals (including humans) and even death in high concentration. NO 2 is one of the major components of “photochemical smog.” Photochemical smog occurs when ultraviolet light fuels the reaction of nitrogen dioxide with unburned hydrocarbons from exhaust of automobiles to produce ozone and other irritating chemicals causing health hazards in urban areas. NO 2 can also damage some metals and fade fabric dyes. (For additional information refer to the Photochemistry and Industrial Inorganic Chemistry modules.) Since nitrogen oxide and nitrogen dioxide can also react with ozone, there is concern that adding more NO to the atmosphere might deplete the ozone layer.
NO(g) + O 3 (g) 
NO 2 (g) + O 2 (g)
The ozone layer protects us from damaging ultraviolet radiation by absorbing certain wavelengths of light (240-300 nm) and preventing it from reaching earth. Supersonic planes, which travel in the stratosphere, emit considerable amounts of NO x during flight. As the number of supersonic flights increase, there is concern that the amount of ozone in the stratosphere might be significantly diminished.