In is the medium by which sediments—a
In sedimentary cycles the main reservoir is the soil and the sedimentary and other rocks of the earth’s crust. Both involve biotic and abiotic agents, both are driven by the flow of energy and both are tied to the water cycle.
Living organisms, atmosphere and earth maintain between them a circulation of water and moisture, which is referred to as water cycle or hydrologic cycle. As we have already discussed in chapter 10, water forms a very significant factor of environment and without the cycling of water, biogeochemical cycles could not exist, ecosystems could not function, and life could not be maintained.
Water is important for an ecosystem for several reasons—it is the medium by which nutrients are introduced into autotrophic plants; it is an important part of living tissue, either as liquid water or as part of essential organic molecules; it serves as a means of thermal regulation for both plants and animals; it is the medium by which sediments—a prime source of mineral nutrients—are removed from or added to local ecosystems; it covers the great majority of the earth’s surface, and is the dominant feature of all aquatic ecosystems.
The hydrologic cycle is driven by solar energy and gravity. More than 80 per cent of the total insulation that is not lost immediately as electromagnetic radiation goes to evaporate water. The atmospheric water vapor produced by this means can then condense around particles of dust in the atmosphere, often called nucleation particles.
The atmosphere possesses a limited capacity for holding water vapor, thus, the droplets formed by this means are heavy enough to fall as precipitation under the influence of gravity. Eventually, the hydrologic cycle can be defined as an alternation of evaporation and precipitation, with the energy used to evaporate the water being dissipated as heat in the atmosphere as the water condenses.
Distribution of water in earth’s surface:
Water is not evenly distributed throughout the earth. Almost 95 per cent of the total Water on earth is chemically bound into rocks and does not cycle. Of the remainder about 97.3 per cent is in the ocean, about 2.1 Per cent exists as ice in the polar caps and permanent glaciers, and rest is fresh water, present in the form of atmospheric water vapor, ground water, soil water, or inland surface water (Nace, 1967).
Table: 19 1. Distribution of water in the earth’s crust and surface (Clapham, Jr., 1973):
A. Chemically bound water of rocks: Does not cycle
1. Crystalline tocks – 250,000 ? 1017 Kg.
2. Sedimentary rocks – 21,00 ? 1017 Kg.
B. Free water: Moves via hydrologic cycle
1. Oceans – 13,200 ? 1017 Kg.
2. Ice caps and glaciers – 292 ? 1017 Kg.
3. Ground water to a depth of 4000 m. – 83.5 ? 1017 Kg.
4. Freshwater lakes – 1.25 ? 1017 Kg.
5. Saline lakes and inland seas – 1.04 ? 1017 Kg.
6. Soil moisture – 0.67 ? 1017 Kg.
7. Atmospheric water vapour – 0.13 ? 1017 Kg.
8. Rivers – 0.013 ? 1017 Kg.
The rate of cycling of water:
The rate of cycling between surface and atmosphere is very rapid. The amount of water vapor in the atmosphere is sufficient, on the average, turnover the entire earth to a depth of 2.25 cm. But the average annual rainfall for the earth is about 81.1 cm (Furon, 1967) and in some places it ranges upto 1.200cm.
This means that the average turnover time for atmospheric water is about 114 days, or that the equivalent of all the water vapor in the entire atmosphere fall as precipitation and is re-evaporated more than 32 times per year.
Further, the distribution of evaporation and rainfall is quite uneven. If we compare the annual evaporation and precipitation over land and sea, we find that relatively more water precipitates on land than evaporates from land.
This is fortunate from the point of view of terrestrial organisms. Even so, the amount of rainfall on the open ocean is proportionately greater than that on land, taking into account the relative percentages of the earth’s surface covered by land and sea.
Nature of hydrologic cycle:
The hydrologic cycle over the oceans is extremely simple-the water is evaporated from the surface of the ocean and water vapors form the clouds which when cool down precipitate the water as rain fall.
But several routes are open to precipitation that falls on land-direct evaporation, transpiration, entry of water into ground water system and runoff consequently, the routes of hydrologic cycles on land can be divided into following three main categories-the rapidly cycling portion or evapotranspiration, which includes the evaporation and transpiration, the less rapidly cycling water, or surface runoff, and very slowly cycling ground water that seeps into the soil can end up in any one of these three categories.
Evapotranspiration includes evaporation and transpiration. Evaporation refers to water that is evaporated directly from any surface other than a plant, such as a lake, soil surface, or animal skin.
In most cases, the main effects of direct evaporation are to moderate the temperature of local area and to allow the hydrologic cycle to continue. In some ecosystems, evaporation also leads to a concentration of salts in the water of soil which may be a critical environmental factor.
Transpiration is water that evaporates from the surface of leaves of plants. Transpiration acts to move the biogeochemical cycles for all mineral nutrients that enter the food chain via the roots of plants.
2. Surface runoff:
If transpiration is related to the mechanism of nutrient uptake, the gross movement of soluble and solid particles in the ecosystem is accomplished largely by runoff. Nutrients that have accumulated in sediments or soils can be eroded by streams and removed altogether from a local ecosystem, or soluble nutrients may be carried by soil into surface water, where they are removed from the area.
Streams may carry sediment particles which can be chemically altered through additional weathering so that the nutrient elements they contain may be utilized by organisms. Finally moving water acts as an agent of erosion which removes soil and allows weathering of the underlying rock to make their nutrients available to plants.
3. Ground water:
Ground water is water that saturates either sediment or rock below the water table. In general, it is not trapped by plants for transpiration and it is too deep to be directly evaporated from the soil surface, it is an exceedingly important reservoir for water which moves from one phice to another under the influence of gravity.
The area where the net water movement is from the surface into the ground water system is termed a catchment area; areas where ground water reaches the surface and runs off are termed springs. A rock body through which ground water flows is called an aquifer. A well drilled into an aquifer that has sufficient hydrostatic pressure to force water up into it is called an artesian well.
The hydrologic cycle on land thus includes cvapotranspiration of water from earth’s surface and leaf surface > formation of clouds > precipitation > surface runoff + accumulation of water as ground water > return of water to sea via streams or direct evaporation and cloud formation, and so on.
The global water cycle:
The atmosphere, oceans and masses form a single gigantic water system that is driven by solar energy. The presence and movement of water in any part of the system affects the presence and movement in all other parts. In fact, the atmosphere has great significance in world’s water system. At any one time the atmosphere holds no more than a 10- to 11- day supply of rainfall in the form of vapor, clouds and ice crystals.
Thus the turnover of water molecules is rapid. Because the source of water in the atmosphere is evaporation from land and sea, there are global differences in the amount of evaporation and the amount of moisture in the atmosphere at any given point. Evaporation at lower latitudes is considerably greater than higher latitudes, showing the greater heat budgets produced by the direct rays of the sun. Evaporation is greater over ocean than over land.
Thus, evaporation, precipitation, detention, and transportation maintain a stable water balance on the earth. Consider the amount of water that falls on the earth in terms of 100 units (Fig. 19.2). On the average, 84 per cent of water is lost from the ocean by evaporation, while only 77 per cent of water is gained by it from precipitation. Land areas loss 16 per cent of water by evaporation and gain 23 per cent of it by precipitation.
Water runoff through rivers from land to oceans makes up 7 per cent, which balance the evaporative deficit of the ocean. The remaining 7 per cent of water circulate as atmospheric moisture.
The gaseous geochemical cycles are of following types:
The oxygen cycle:
Oxygen the by-product of photosynthesis is involved in the oxidation of carbohydrates with release of energy, carbon dioxide and water. Its primary role in biological oxidation is that of a hydrogen acceptor.
The breakdown and decomposition of organic molecules proceeds primarily by dehydrogenation. Hydrogen is removed by enzymatic activity from organic molecules in a series of reactions and is finally accepted by the oxygen, forming water.
Though oxygen is necessary for life, but being very active chemically, molecular O2 may be toxic to living body cells. Therefore, for the protection from toxic effects of molecular O2, cells possess the cellular organelles called peroxisomes which mediate oxidative reactions resulting in the production of hydrogen peroxide which in turn is used through the mediation of other enzymes as an acceptor in oxidizing other compounds.
The major supply of free oxygen which supports life occurs in the atmosphere. There are two significant sources of atmospheric oxygen. One is the photo dissociation of water vapor in which most of the hydrogen released escapes into outer space.
The other source is photosynthesis, active only since life began on earth. Because photosynthesis and respiration are cyclic, involving both the release and utilization of oxygen, one would seem to balance the other, and no significant quantity of oxygen would accumulate in the atmosphere.
However, at some time in the earth’s history the amount of oxygen introduced into the atmosphere had to exceed the amount used in the decay of organic matter and that tied up in the oxidation of sedimentary rocks.
Part of the atmospheric oxygen represents that portion remaining from the unoxidized reserves of photosynthesis—coal, oil, gas, and organic carbon in sedimentary rocks. The amount of stored carbon in the earth suggests that 150?1020g of oxygen has been available to the atmosphere, over 10 times as much as now present, 10 ? 1020g (Johnson, 1970).
The main non-living (a biotic) oxygen pool consists of molecular oxygen, water, and carbon dioxide, all intimately linked to each other in photosynthesis and other oxidation-reduction reactions, and all exchanging oxygen with each other. Oxygen is also biologically exchangeable in such compounds as nitrates and sulphates utilized by organisms that reduce them to ammonia and hydrogen sulphide.
The cycling of oxygen is very complex (Fig. 19.3). As a constituent of CO2, it circulates freely throughout the biosphere. Some carbon dioxide combines with calcium to from carbonates. Oxygen combines with nitrogen compounds to from nitrates, with iron to ferric oxides, and with many other minerals to form various other oxides. In these states oxygen is temporarily withdrawn from circulation.
In photosynthesis the oxygen freed is split from the water molecule. This oxygen is then reconstituted into water during plant and animal respiration. Some part of the atmospheric oxygen that reaches the higher levels of the troposphere is reduced to ozone (O3) by high energy ultraviolet radiation.
The carbon cycle:
The carbon being a basic constituent of all organic compounds and a major element involved in the fixation of energy by photosynthesis is so closely tied to energy flow that the two are inseparable. The source of all the fixed carbon both in living organisms and fossil deposits is carbon dioxide CO2, found in the atmosphere and dissolved in the waters of the earth.
During photosynthesis, carbon from atmospheric CO2 is incorporated into the production of the carbohydrate, glucose, C6H13O2, that subsequently may be converted to other organic compounds such as polysaccharides (sucrose, starch, cellulose, etc.), proteins and lipids.
All the polymeric organic compounds containing carbon are stored in different plant-tissues as food and from them the carbon is passed on to the tropic levels of herbivores or phytoparasites, or retained by the plant until it serves as food for decay organisms (viz., decomposers). Some of the carbon is returned to the atmosphere (or the enveloping aqueous medium) in the form of a byproduct of plant respiration, in which, a considerable portion of glucose is oxidized to yield CO2, H2O and energy as follows:
1/6C6H12O6 + O2 > CO2 + H2O + Energy
The CO2 which is released as the by-product of plant respiration is again used by plants in photosynthesis. This is the basic carbon cycle which is simple and complete. Decomposing microorganisms are important in breaking down dead material with the release of carbon back into the carbon cycle (Fig. 19.4).
Similarly, carbon taken up by herbivores or phytoparasites may travel a number of routes. It may be incorporated into protoplasm (assimilation) and stored until the organism dies, where upon it is utilized by decomposers; it may be released through animal respiration; it may serve as live food for other organisms or finally it may be stored in the environment as CO2. Similar fates await carbon at the carnivore tropic levels.
In fact, all the carbon of plants, herbivores, carnivores and decomposers is not respired but some is fermented and some is stored. The carbon compounds that are lost to the food chain after fermentation, such as methane arc readily oxidized to carbon dioxide by inorganic reactions in the atmosphere.
As for the storage of carbon in sediments, just as deposition works to store initials, erosion may uncover them, and inorganic chemical weathering of rock can oxidize the carbon contained there. Some carbon is permanently stored in sediments and not uncovered by weathering; it may be replaced by carbon dioxide released from volcanoes and other similar examples of intense geological activity.
In modern age man has greatly increased the rate at which carbon is passing from sedimentary form to carbon dioxide. The combustion of fossil fuels is a significant means of recycling sedimentary carbon much faster than natural weathering.
Small portion of carbon, especially in the sea, is found not as organically fixed carbon but as carbonate (CO3), especially calcium carbonate (CaCO3). CaCO3 is very commonly used for shell construction by such animals as clams, oysters, some protozoa, and some algae. Carbon dioxide reacts with water to from carbonate in the following three step reaction:
The precise amount of each of these constituents in the water depends on the pH of the water. Organisms such as clams can combine bicarbonate or carbonate with calcium dissolved in the water to produce calcium carbonate. After the death of the animals, this calcium carbonate may either dissolve or remain in sedimentary form.
Carbonic acid Bicarbonate Carbonate The precise amount of each of these constituents in the water depends on the pH of the water. Organisms such as clams can combine bicarbonate or carbonate with calcium dissolved in the water to produce calcium carbonate. After the death of the animals, this calcium carbonate may either dissolve or remain in sedimentary form.
Certain control mechanisms are inherent in the carbon cycle. The rate of carbon utilization is dependent on its availability. If excessive amounts of carbon are taken up in any one phase of the cycle, other phases of activity may be inhibited or slowed down.
For example, if the pH of water is alkaline, more carbon is tied up in a carbonate and less is in solution. This removal of carbon in solution would upset the equilibrium established between the atmospheric and the dissolved CO2 and the new effect would be a movement of CO2 into solution until equilibrium was reached.
Peculiarities of carbon-cycle:
Though carbon-cycle exhibits basic similarity with other biogeochemical cycles, yet it is unusual in that the organic phase is not essentially a complete cycle within itself. The organic (biotic) and atmospheric (a biotic) phases, however, are so closely intertwined that the rapid cycling typical of the organic phase is present.
The multiplicity of paths along which carbon can flow is typical of biogeochemical cycles in general, and provides a well- system with adequate feedback mechanisms to insure an adequate supply of the carbon.
It is significant that all phases of the cycle yield carbon dioxide at some time, and carbon dioxide is the raw material for them Thus, despite its relative low concentration in the atmosphere (0.03 per cent), carbon in a form in which it can be used by living organisms is virtually always present.
The nitrogen cycle:
Nitrogen is an essential constituent of different biologically significant organic molecules such as amino acids and proteins, pigments, nucleic acids and vitamins. It is also the major constituent of the atmosphere, comprising about 79 per cent of it.
The paradox is that in its gaseous state, N2, abundant though it is unavailable to most life. Before it can be utilized it must be converted to some chemically usable form.
To be used biologically, the free molecular nitrogen has to be fixed and fixation requires an input of energy. In the first step molecular nitrogen, N2, has to be split into two atoms: N2 > 2N. The free nitrogen atoms then must be combined with hydrogen to form ammonia, with the release of some energy:
2N+3H2 > 2NH3
This fixation comes about in two ways. One is by high-energy fixation such as cosmic radiation, meteorite trails, and lightning that provide the high energy needed to combine nitrogen with oxygen and hydrogen of water.
The resulting ammonia and nitrates are carried to the earth in rain water. The second method of nitrogen-fixation which contributes about 90 per cent of fixed nitrogen of earth is biological. Some bacteria, fungi, and blue-green algae can extract molecular nitrogen from the atmosphere and combine it with hydrogen to form ammonia.
Some of this ammonia is excreted by the nitrogen-fixing organism, and thus becomes directly available to other autotrophs. Some of these nitrogen-fixing organisms may be free-living, either in the soil (e.g., bacteria-Azobacter and Clostridium) or in water (e.g., blue-green algac-Nostoc, Calo- thrix and Anabaena) and produce vast quantities of fixed nitrogen.
In other cases, certain symbiotic bacteria of genus Rhizobium, although unable to fix atmospheric nitrogen themselves can do this when in combination with cells either from the roots of legumes (e.g., peas, beans, clover and alfalfa) and of other angiosperms such as Alrtus, Ceanothus, Shepherdia, Elaeagnus and Myrica, or from the leaves of African genera of Rubiaceae and Pavetta (Fig. 19.5).
The bacteria invade the roots or leaves and stimulate the formation of root-nodules or leaf-nodules, a sort of harmless tumor. The combination of symbiotic bacteria and host cells remains able to fix atmospheric nitrogen and for this reason legumes are often planted to restore soil fertility by increasing the content of fixed nitrogen.
Nodule bacteria may fix as much as 50 to 100 kilograms of nitrogen per acre per year, and free soil bacteria as much as 12 kilograms per acre per year. Further both free soil bacteria (Azobacter, and Clostridium) produce ammonia as the first stable product and like the symbiotic bacteria, they required molybdenum as an activator and are inhabited by an accumulation of nitrates and ammonia in soil.
Recently, certain lichens (Collema tunaeforme and Peltigera ritfescens) were also implicated in nitrogen fixation (Henriksson, 1971). Lichens with nitrogen-fixing ability possess nitrogen-fixing blue green species as their algal component.
Nitrogen fixed by symbiotic and non-symbiotic micro-organisms in soil and water is one source of nitrogen. Another source is organic matter. The nitrogenous wastes and carrion of animals are degraded by the detritus organisms; nitrogen is converted to the amino form (e.g., L-Alanine). The amino group (- NH2) is liberated from organic molecules to form ammonia; this process is called domination. Certain specific bacteria, most notably of the genus Nitrosomonas, can oxidize ammonia to nitrite (NO2) by the reaction.
2NH3 + 3O2 > 2NO2– + 2H2O + 2H+
This reaction takes place in the soil, in lake or sea water or sediments, and whenever ammonia is being released and oxygen is present. As fast as nitrite is produced, other bacteria, such as Nitrobacteria, can combine nitrite with oxygen to form nitrate (NO3) by the reaction:
2NO2– + O2 > 2NO3–
Both of these ractions which are performed by two nitrifying bacteria — Nitrosomonas and Nitrobacter are the parts of a single biological process called nitrification. In nitrification process, thus ammonia is oxidized to nitrate and nitrite yielding energy. This, energy is used by the bacteria to make their organic materials directly from carbon dioxide and water. Nitrate can be taken up by autotrophs at the beginning of food chain.
Under certain circumstances, nitrate is either not produced in the nitrogen cycle or it is degraded before it can be utilized by autotrophs. Degradation of nitrate is called denitrification, and may be important when oxygen concentration is low. Denitrifying bacteria such a Pseudomonas can use the energy of the nitrate ion to drive their metabolism, and in so doing, they break the nitrate down to nitrite, ammonia, or molecular nitrogen:
CH12O6+12NO3 > 12NO2+6CO2 + 6H2O
C6H2O6 + 8NO2 > 4N2+2CO2 + 4CO3– + 6H2O
C6H12O6+3N03– > 3NH3+6CO2 + 3OH-
If denitrification is significant in an ecosystem nitrite is transitory and is also degraded into either ammonia or molecular nitrogen.
Cycling of nitrogen in the ecosystem:
The sources of inputs of nitrogen under natural conditions (Fig. 19 6) are the bacterial fixation of atmospheric nitrogen, addition of inorganic nitrogen in rain from such sources as lightning fixation and fixed “juvenile” nitrogen from volcanic activities, ammonia absorption from the atmosphere by plants and soil, and nitrogen accretion from windblown aerosols, which contain both organic and inorganic forms of nitrogen.
In terrestrial ecosystems, nitrogen, largely in the from of ammonia and nitrates is taken up by plants, which convert it into amino acids and proteins. Animals (primary macro-consumers) may eat the plants and utilize the amino acids from the plant proteins in the synthesis of their own proteins and other cellular constituents. When animals and plants die, the decay bacteria convert the nitrogen of their proteins and other compounds into ammonia.
Animals excrete several kinds of nitrogen-containing wastes-urea, uric acid, creatinine, and ammonia and the decay bacteria converts these wastes to ammonia. Ammonia may be lost as gas to the atmosphere, may be acted upon by nitrifying bacteria, or may be taken up directly by plants.
The nitrates may be utilized by plants, immobilized by microbes, stored in decomposing humus, or leached away. This material is carried to streams, lakes, and eventually the sea, where it is available for use in aquatic ecosystems. There nitrogen is cycled in a similar manner, except that the large reserves contained in the soil humus are largely lacking.
Life in the water contributes organic matter and dead organisms that undergo deposition and subsequent release of ammonia and ultimately nitrates. In aquatics ecosystem atmospheric nitrogen is fixed by numerous blue-green algae.
Under natural conditions nitrogen lost from ecosystems by gentrification, volatilization, leaching, erosion, windblown aero- sols, and transportation out of the system is balanced by biological fixation and other sources. Both chemically and biologically.
Comparison of nitrogen-cycle and carbon-cycle:
A comparison of the nitrogen and carbon cycles points out some obvious differences between them. Although the atmospheric phase clearly constitutes the main reservoir for both elements in the total environment, atmospheric nitrogen exists in a very different form from that in which it is taken up by autotrophs.
But it constitutes a source from which losses sustained by the organic phase can be made up. Also, there are more inorganic chemical reactions external to the food chain in the nitrogen cycle than in the carbon cycle. Despite these differences, both the nitrogen and carbon cycles are examples of finely tuned circuits which meet demands for the needed elements through several pathways, each controlled by effective self-regulating feedback loop.
Mineral elements required by living organisms are obtained initially from inorganic sources. Available forms occur as salts dissolved in soil water or lakes, streams, and seas. The mineral cycle varies from one element to another, but essentially it consists of two Phases; the salt-solution phase and the rock phase. Mineral salts come directly from the earth’s crust by weathering.
The soluble salts then enter the water-cycle. With water they move through the soil to streams and lakes and eventually reach the sea, where they remain indefinitely. Other salts are returned to the earth’s crust through sedimentation. They become incorporated into salt beds, silts and limestone’s; after weathering them again enter the cycle.
Plants and animals, the living components of the ecosystems, fulfill their mineral requirements from mineral solutions in their environments. Other animals acquire the bulk of their minerals from plants and animals they consume. After the death of living organisms the minerals are returned to the soil and water through the action of the organisms and process of decay.
There are different kinds of sedimentary or mineral cycles, depending on the kinds of elements, but following two cycles are very significant for a ecosystem.
Sulphur, like nitrogen, is an essential part of protein and amino acids and is characteristic of organic compounds. It exists in a number of states—elemental sulphur, S, sulphides, sulphur monoxide, sulphite, sulphates. Of these three are important in nature: elemental sulphur, sulphides and sulphates.
The sulphur cycle (Fig. 19.7) is both sedimentary and gaseous (i.e., it includes gaseous phase and sedimentary phase). The sedimentary phase of sulphur cycle is long-termed and in it sulphur is tied up in organic and inorganic deposits.
From these deposits, it is released by weathering and decomposition and is carried to terrestrial and aquatic ecosystems in a salt solution. Atmospheric (gaseous) phase of sulphur-cycle is less pronounced and it permits circulation on a global scale.
Sulphur enters the atmosphere from several sources—the combustion of fossil fuels, vilcanic eruption, the surface of the oceans and gases released by decomposition. Initially sulphur enters the atmosphere as hydrogen sulphide, H2S, which quickly oxidizes into another volatile form, sulphur dioxide, SO2. Atmospheric sulphur dioxide, soluble in water, is carried back to earth in rainwater as weak sulphuric acid, H2SO4.
Whatever its source, sulphur in a soluble form, mostly as sulphate (S04-) is absorbed through plant roots, where it is incorporated into certain organic molecules, such as some amino acids (e.g., cystine) and proteins. From the producers the sulphur in amino acids is transferred to the consume animals, with excess being excreted in the faeces.
Excretion and death carry sulphur in living material back to the soil and to the bottoms of ponds, lakes, and seas where the organic material is acted upon by bacteria of detritus food chain. Within the detritus food chain, the sulphydryl group (—SH) of amino acids (e.g., L-cysteine) is separated from the rest of the molecule as hydrogen sulphide (H2S) by most decomposing bacteria as a normal part of the degradation of proteins. In an aerobic environment, the hydrogen sulphide is oxidized to sulphate by bacteria specially adapted to perform this conversion:
H2S+202 S04– + 2H
The sulphate produced then can be reused by the autotrophs. In anaerobic environments, such as bottom of certain lakes, it is impossible to oxidize sulphide by this means, because the process of oxidation requires oxygen. But if infra-red radiation is present in these environments, there are photosynthetic bacteria that can use it to manufacture carbohydrates and oxidize sulphide either to elemental sulphur or to sulphate:
6CO2 + 12H2S+hv > C6H12O6 + 6H2O + 12S
6CO2 + 12H2O + 3H2S + hv > C6H12O6 + 6H2O+3SO4– + 6H+
Elemental sulphur can also be utilized by other bacteria to form sulphate. If oxygen is present, the reaction is quite rapid.
2S + 3O2 + 2H2O > 2SO4– + 4H+
Under anaerobic conditions, elemental sulphur can still be oxidized to sulphate by certain bacteria if nitrate is present:
6NO3 + 5S + 2CaCO3 > 3SO4– + 2CaSO4 + 2CO2 + 3N2
None of these bacterial reactions is unidirectional; under certain conditions, sulphate can also be reduced either to sulphide or to elemental sulphur by bacteria. This series of reactions operating within the organic phase of the sulphur cycle provides a rather tuned mechanism for regulating the availability of sulphur autotrophs.
The sulphur is removed from the organic phase in the form of elemental sulphur which is insoluble and accumulates in sediments.
If iron is present in the sediment, it can combine with sulphide to form iron sulphides, all of which are highly insoluble:
Fe++ + S- > FeS
Fe (ionic) + 2S (ionic) > FeS2 (Ferrous sulphide or pyrite)
FeS2 is highly insoluble under neutral and alkaline conditions and is firmly held in mud and wet soil. Some ferrous sulphide is contained in sedimentary rocks overlying coal deposits. Exposed to the air in deep and surface mining, the ferrous sulphate oxidizes and in the presence of water produces ferrous sulphate and sulphuric acid:
2FeS2 + 702 + 2H2O > 2FeSO4 + 2H2SO4
12FeSO4 + 3O2 + 6H20 > 4Fe2(SO4)3 + 5Fe(OH3)v
In this manner sulphur in pyrite rocks, suddenly exposed to weathering by man, discharges heavy slugs of sulphur, sulphuric acid, ferric sulphate and ferrous hydroxide into aquatic ecosystems. These compounds destroy aquatic life and cause acidic water.
Phosphorus cycle has no atmospheric phase. It occurs naturally in environment as phosphate (PO4-, or one of its analogues, HPO4– or H2PO4), either as soluble inorganic phosphate ions, as soluble organic phosphate (i.e., as a part of a soluble organic molecule), as particulate phosphate (i.e., as part of an insoluble organic or inorganic molecules) or as mineral phosphate (i.e., as part of a mineral grain as found in a rock or sediment).
The ultimate source of phosphate in the ecosystem is crystalline rocks. As these are eroded and weathered, phosphate is made available to living organisms, generally as ionic phosphate.
This is introduced into autotrophic plants through their roots, where it is incorporated into living tissues. From autotrophs, it is passed along the grazing food chain in the same fashion as nitrogen sulphur, with excess phosphate being excreted in the faeces.
An extreme example of faecal phosphate is the tremendous guano deposits built up by birds on the desert west coast of South America. Phosphates can also be released as particulate matter from forest and grassland fires.
In the detritus food chain, as large organic molecules containing phosphate are degraded, the phosphate is liberated as inorganic ionic phosphate. In this form it can be immediately be taken up by autotrophs, or it can be incorporated into a sediment particle, either in the soil of a terrestrial ecosystem or in a sediment of an aquatic ecosystem. The sedimentary phase of phosphorus cycle remains comparatively slow than the organic phase.
Besides phosphorus, there are biogeochemical cycles for all the other nutrients (minerals) used by living organisms, as well as some that are not. Most of them has complete cycles in sedimentary phase. The availability depends on their solubility in water and availability of water as solvent.
Thus, the geochemical cycles of different chemical substances are closed: the atoms are used over and over again. To keep the cycles going does not require new matter but it does require energy, for the energy cycle is not a closed one. Further, the patterns of flow, both of energy and of chemical substances, are of great significance.
The simpler patterns involve energy, as the sources of energy are external to the ecosystem, and flow is unidirectional through it. Chemical substances, on the other hand, are finite and have their orgin inside the ecosystem, thus they must continuously cycle within the system.