Elements and inorganic compounds that sustain life tend to circulate in the earth's biosphere in regular paths from the atmosphere to the lithosphere (soil) or hydrosphere (water) into living things and then back into these environments. These are called biogeochemical cycles.Thus, all of the elements that function in animals or plants follow some of cyclic paths. The following are the important cycles of materials found in ecosystems.
1. Hydrologic or Water Cycle - water is collected, purified and distributed on the earth's fixed supply of water.
2. Carbon - Oxygen Cycle - depends on photosynthesis and respiration
3. Nitrogen Cycle - relies heavily on bacteria
4. Phosphorus Cycle - depends on the weathering of rock
THE WATER CYCLE
The Water Cycle (also known as the hydrologic cycle) is the journey water takes as it circulates from the land to the sky and back again. The Sun's heat provides energy to evaporate water from the Earth's surface (oceans, lakes, etc.). Plants also lose water to the air (this is called transpiration). The water vapor eventually condenses, forming tiny droplets in clouds. When the clouds meet cool air over land, precipitation (rain, sleet, or snow) is triggered, and water returns to the land (or sea). Some of the precipitation soaks into the ground. Some of the underground water is trapped between rock or clay layers; this is called groundwater. But most of the water flows downhill as runoff (above ground or underground), eventually returning to the seas as slightly salty water.
The movement of carbon, in its many forms, between the biosphere, atmosphere, oceans, and geosphere is described by the carbon cycle, illustrated in the adjacent diagram. The carbon cycle is one of the biogeochemical cycles. In the cycle there are various sinks, or stores, of carbon (represented by the boxes) and processes by which the various sinks exchange carbon (the arrows).
We are all familiar with how the atmosphere and vegetation exchange carbon. Plants absorb CO2 from the atmosphere during photosynthesis, also called primary production, and release CO2 back in to the atmosphere during respiration. Another major exchange of CO2 occurs between the oceans and the atmosphere. The dissolved CO2 in the oceans is used by marine biota in photosynthesis.
Two other important processes are fossil fuel burning and changing land use. In fossil fuel burning, coal, oil, natural gas, and gasoline are consumed by industry, power plants, and automobiles. Notice that the arrow goes only one way: from industry to the atmosphere. Changing land use is a broad term which encompasses a host of essentially human activities. They include agriculture, deforestation, and reforestation.
The adjacent diagram shows the carbon cycle with the mass of carbon, in gigatons of carbon (Gt C), in each sink and for each process, if known. The amount of carbon being exchanged in each process determines whether the specific sink is growing or shrinking. For instance, the ocean absorbs 2.5 Gt C more from the atmosphere than it gives off to the atmosphere. All other things being equal, the ocean sink is growing at a rate of 2.5 Gt C per year and the atmospheric sink is decreasing at an equal rate. But other things are not equal. Fossil fuel burning is increasing the atmosphere's store of carbon by 6.1 Gt C each year, and the atmosphere is also interacting with vegetation and soil. Furthermore, there is changing land use.
The carbon cycle is obviously very complex, and each process has an impact on the other processes. If primary production drops, then decay to the soil drops. But does this mean that decay from the soil to the atmosphere will also drop and thus balance out the cycle so that the store of carbon in the atmosphere will remain constant? Not necessarily; it could continue at its current rate for a number of years, and thus the atmosphere would have to absorb the excess carbon being released from the soil. But this increase of atmospheric carbon (in the form of CO2) may stimulate the ocean to increase its uptake of CO2 .
THE NITROGEN CYCLE
All life requires nitrogen-compounds, e.g., proteins and nucleic acids.
Air, which is 79% nitrogen gas (N2), is the major reservoir of nitrogen. But most organisms cannot use nitrogen in this form. Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as:
- nitrate ions (NO3−)
- ammonia (NH3)
- urea (NH2)2CO
Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants). Four processes participate in the cycling of nitrogen through the biosphere:
- nitrogen fixation
Microorganisms play major roles in all four of these.
The nitrogen molecule (N2) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy. Three processes are responsible for most of the nitrogen fixation in the biosphere:
- atmospheric fixation by lightning
- biological fixation by certain microbes — alone or in a symbiotic relationship with some plants and animals
- industrial fixation
The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth.
Atmospheric nitrogen fixation probably contributes some 5– 8% of the total nitrogen fixed.
Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium nitrate (NH4NO3).
The ability to fix nitrogen is found only in certain bacteria and archaea.
Some live in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa).
Some establish symbiotic relationships with plants other than legumes (e.g., alders).
Some establish symbiotic relationships with animals, e.g., termites and "shipworms" (wood-eating bivalves).
Some nitrogen-fixing bacteria live free in the soil.
Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies.
Biological nitrogen fixation requires a complex set of enzymes and a huge expenditure of ATP. Although the first stable product of the process is ammonia, this is quickly incorporated into protein and other organic nitrogen compounds.
The proteins made by plants enter and pass through food webs just as carbohydrates do. At eachtrophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. The final beneficiaries of these materials are microorganisms of decay. They break down the molecules in excretions and dead organisms into ammonia.
Ammonia can be taken up directly by plants — usually through their roots. However, most of the ammonia produced by decay is converted into nitrates. This is accomplished in two steps:
Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites (NO2−).
Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3−).
These two groups of autotrophic bacteria are called nitrifying bacteria. Through their activities (which supply them with all their energy needs), nitrogen is made available to the roots of plants.
Many soils also contain archaeal microbes, assigned to the Crenarchaeota, that convert ammonia to nitrites. While more abundant than the nitrifying bacteria, it remains to be seen whether they play as important a role in the nitrogen cycle.
Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification — converting some of their organic nitrogen to nitrites and nitrates. These reach the soil when they shed their leaves.
The three processes above remove nitrogen from the atmosphere and pass it through ecosystems.
Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere.
Once again, bacteria are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.
Thus they close the nitrogen cycle.
Are the denitrifiers keeping up?Agriculture may now be responsible for one-half of the nitrogen fixation on earth through
the use of fertilizers produced by industrial fixation
the growing of legumes like soybeans and alfalfa.This is a remarkable influence on a natural cycle.
Are the denitrifiers keeping up the nitrogen cycle in balance? Probably not. Certainly, there are examples of nitrogen enrichment in ecosystems. One troubling example: the "blooms" of algae in lakes and rivers as nitrogen fertilizers leach from the soil of adjacent farms (and lawns). The accumulation of dissolved nutrients in a body of water is called eutrophication.
Phosphorus enters the environment from rocks or deposits laid down on the earth many years ago. The phosphate rock is commercially available form is called apatite. Other deposits may be from fossilized bone or bird droppings called guano. Weathering and erosion of rocks gradually releases phosphorus as phosphate ions which are soluble in water. Land plants need phosphate as a fertilizer or nutrient.
Phosphate is incorporated into many molecules essential for life such as ATP, adenosine triphosphate, which is important in the storage and use of energy. It is also in the backbone of DNA and RNA which is involved with coding for genetics.
When plant materials and waste products decay through bacterial action, the phosphate is released and returned to the environment for reuse.
Much of the phosphate eventually is washed into the water from erosion and leaching. Again water plants and algae utilize the phosphate as a nutrient. Studies have shown that phosphate is the limiting agent in the growth of plants and algae. If not enough is present, the plants are slow growing or stunted. If too much phosphate is present excess growth may occur, particularly in algae.
A large percentage of the phosphate in water is precipitated from the water as iron phosphate which is insoluble. If the phosphate is in shallow sediments, it may be readily recycled back into the water for further reuse. In deeper sediments in water, it is available for use only as part of a general uplifting of rock formations for the cycle to repeat itself.
Human influences on the phosphate cycle come mainly from the introduction and use of commercial synthetic fertilizers. The phosphate is obtained through mining of certain deposits of calcium phosphate called apatite. Huge quantities of sulfuric acid are used in the conversion of the phosphate rock into a fertilizer product called "super phosphate".
Plants may not be able to utilize all of the phosphate fertilizer applied, as a consequence, much of it is lost form the land through the water run-off. The phosphate in the water is eventually precipitated as sediments at the bottom of the body of water. In certain lakes and ponds this may be redissolved and recyled as a problem nutrient.
Animal wastes or manure may also be applied to the land as fertilizer. If misapplied on frozen ground during the winter, much of it may lost as run-off during the spring thaw. In certain area very large feed lots of animals, may result in excessive run-off of phosphate and nitrate into streams.
Other human sources of phosphate are in the out flows from municipal sewage treatment plants. Without an expensive tertiary treatment, the phosphate in sewage is not removed during various treatment operations. Again an extra amount of phosphate enters the water.
Sulfur is one of the components that make up proteins and vitamins. Proteins consist of amino acids that contain sulfur atoms. Sulfur is important for the functioning of proteins and enzymes in plants, and in animals that depend upon plants for sulfur. Plants absorb sulfur when it is dissolved in water. Animals consume these plants, so that they take up enough sulfur to maintain their health.Most of the earth's sulfur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments. Sulfur can also be found in the atmosphere. It enters the atmosphere through both natural and human sources. Natural recourses can be for instance volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms. When sulfur enters the atmosphere through human activity, this is mainly a consequence of industrial processes where sulfur dioxide (SO2) and hydrogen sulphide (H2S) gases are emitted on a wide scale.When sulfur dioxide enters the atmosphere it will react with oxygen to produce sulfur trioxide gas (SO3), or with other chemicals in the atmosphere, to produce sulfur salts. Sulfur dioxide may also react with water to produce sulphuric acid (H2SO4). Sulphuric acid may also be produced from demethylsulphide, which is emitted to the atmosphere by plankton species.All these particles will settle back onto earth, or react with rain and fall back onto earth as acid deposition. The particles will than be absorbed by plants again and are released back into the atmosphere, so that the sulfur cycle will start over again.