Function 2 : BIOGEOCHEMICAL CYCLES
In ecosystems flow of nutrients is cyclical. The nutrients cycle from dead remains of organisms released back into the soil by detrivores which are absorbed again i.e. nutrient absorbed from soil by the root of green plants are passed on to herbivores and then carnivores.
This recycling of the nutrients is called biogeochemical or nutrient cycle (Bio = living, geo = rock, chemical = element). The transfer of matter involves biological, geological and chemical processes; hence the name. These cycles facilitate the transfer of matter from one form to another and from one location to another on planet earth.
It is a circuit or pathway by which a chemical element (or molecule) moves through both biotic and abiotic compartments of an ecosystem. Abiotic factors - water (hydrosphere), land (lithosphere), and air (atmosphere); the living factors of the planet can be referred to collectively as the biosphere. All chemical elements occurring in organisms are part of biogeochemical cycles.
It thus provides a clear demonstration of the harmonious interactions between organisms and their environment, both biotically and abiotically.
The entire earth or biosphere is a closed system i.e. nutrients are neither imported nor exported from the biosphere. There are two important components of a biogeochemical cycle
(1) Reservoir pool
Though components of the biogeochemical cycle are not completely lost, they can be held for long periods of time in one place. This place, called a reservoir is a place or region or location where a biogeochemical element is in its highest concentration. Eg) Coal deposits that are storing carbon for a long period of time, atmosphere/rocks which stores large amounts of nutrients.
Influx : difference between the amount of elements entering a reservoir and the amount leaving the reservoir.
(2) Cycling pool /exchange pool/ compartments of cycle
They are relatively short storages of carbon in the form of plants and animals. Eg) plants and animals, which temporarily use carbon in their systems and release it back into a particular reservoir. Carbon is held for a relatively short time in plants and animals when compared to coal deposits. The amount of time that a chemical is held in one place is called its residence time.
Note: Generally, reservoirs are abiotic factors while exchange pools are biotic factors
Elements transported in the biogeochemical cycles are categorized as:
Micro elements - elements required by living organisms in smaller amounts. Eg) boron used mainly by green plants, copper used by some enzymes and molybdenum used by nitrogen-fixing bacteria.
Macro elements - elements required by living organisms in larger amounts. Eg) carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur
Importance of biogeochemical cycles
1. They enable the transformation of matter from one form to another. This transformation enables the utilization of matter in a form specific to particular organisms.
Eg) Nitrogen (N2), despite its abundance in the atmosphere, often is the most limiting nutrient for plant growth, as most plants can only take up nitrogen in two solid forms: ammonium ion (NH4+) and the ion nitrate (NO3-). So, biogeochemical cycles enable the provision of elements to organisms in utilizable forms.
2. They enable the transfer of molecules from one locality to another.
Eg) Some elements like N2 are highly concentrated in the atmosphere, but some of the atmospheric N2 is transferred to soil through the N2 cycle
3. These cycles facilitate the storage of elements. Elements carried through the biogeochemical cycles are stored in their natural reservoirs, and are released to organisms in small consumable amounts.
Eg) Through N2 cycle and with the help of N2 fixing bacteria, green plants utilize N2 in bits though it is abundant in the atmosphere.
4. They assist in functioning of ecosystems, ie proper functioning in a state of equilibrium. Whenever any imbalances occur, the ecosystem through the biogeochemical cycles restores to the equilibrium state. The adjustment is such that the disturbing factor is eliminated.
5. Biogeochemical cycles link living organisms with living organisms, living organisms with the non-living entities and non-living entities with non-living entities. This is because all organisms depend on one another and, the biotic and abiotic component of the ecosystem are linked by flow on nutrients engineered by the biogeochemical cycles.
6. They regulate the flow of substances. As the biogeochemical cycles pass through different spheres, the flow of elements is regulated since each sphere has a particular medium and the rate at which elements flow is determined by the viscosity and density of the medium. Therefore elements in the biogeochemical cycles flow at differing rates within the cycle and this regulates the flow of the elements in those cycles.
It is the time a particular element or molecule of a substance being carried in the biogeochemical cycle takes to make one complete cycle. It may range from several days to millions of years.
Eg) water droplet of average size may stay in the atmosphere for about ten days before precipitation, carbon atoms may reside in the earth crust for the age of the Earth.
The speed of the cycles depends on the medium in which the molecule being cycled is and the surrounding conditions. So, climatic conditions have a significant impact on the biogeochemical cycles.
Cycles that involve molecules or ions in a gaseous state are generally shorter than cycles that involve solid or liquid state transfer because of the slow rate at which molecules move through the lithosphere.
Some of the important Biogeochemical cycles :
Nitrogen is a very important element in that it is part of both proteins (present in the composition of the amino acids that make those proteins) & nucleic acids, such as DNA and RNA (present in nitrogenous bases). Our atmosphere contains nearly 79% of nitrogen but it can’t be used directly by the majority of living organisms.
Nitrogen cycles from gaseous phase to solid phase then back to gaseous phase through the activity of a wide variety of organisms. Cycling of nitrogen is vitally important for all living organisms. There are five main processes :
NITROGEN FIXATION : Nitrates can then be used by plants or animals Involves conversion of gaseous nitrogen into Ammonia, a form in which it can be used by plants. Atmospheric nitrogen can be fixed by the following three methods
Atmospheric fixation: Lightening, combustion and volcanic activity help in the fixation of nitrogen.
Industrial fixation: At high temperature (400°C) and high pressure (200 atm.), molecular nitrogen is broken into atomic nitrogen which then combines with hydrogen to form ammonia (Haber-Bosch process)
Bacterial fixation: There are two types of bacteria-
- Symbiotic bacteria e.g. Rhizobium in the root nodules of leguminous plants.
- Free-living or symbiotic e.g. Nostoc, Azobacter, Cyanobacteria
can combine atmospheric or dissolved nitrogen with hydrogen to form ammonia.
NITRIFICATION: The conversion of ammonia to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria.
In the primary stage of nitrification, the oxidation of ammonium (NH4+) to nitrites (NO2-), is performed by ammonium oxidizing bacteria (AOB) represented by the "Nitrosomonas" species.
The second reaction is oxidation of nitrite (NO2-) to nitrate (NO3-) by nitrite-oxidizing bacteria (NOB), represented by the “Nitrobacter” species. It is important for ammonia to be converted to nitrates or nitrites because ammonia gas is toxic to plants.
ASSIMILATION: In this process nitrogen fixed by plants is converted into organic molecules such as proteins, DNA, RNA etc. These molecules make the plant and animal tissue.
AMMONIFICATION: Living organisms produce nitrogenous waste products such as urea and uric acid. These waste products as well as dead remains of organisms are converted back into inorganic ammonia by the bacteria. This process is called ammonification. Ammonifying bacteria help in this process.
DENITRIFICATION: Reduction of nitrates back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions (in oxygen free medium), eg) waterlogged soils.
The denitrifying bacteria use nitrates in the soil to carry out respiration and consequently produce nitrogen gas- inert and unavailable to plants. Denitrification is reverse of nitrogen fixation.
Human influences on the nitrogen cycle
Huge increase in transfer of nitrogen into biologically available forms – Due to extensive cultivation of legumes, growing use of the Haber–Bosch process in the creation of chemical fertilizers, and pollution emitted by vehicles and industrial plants
Transfer of nitrogen trace gases from Earth to the atmosphere and from the land to aquatic systems.
Nitrous oxide (N2O) has risen in the atmosphere due to agricultural fertilization, biomass burning, cattle and feedlots, and industrial sources. N2O in the stratosphere breaks down and acts as a catalyst in the destruction of atmospheric ozone.
Nitrous oxide is also a greenhouse gas (GHG) and is currently the third largest contributor to global warming, after carbon dioxide and methane. It is 300 times more potent in its ability to warm the planet, than carbon dioxide.
Ammonia (NH3) in the atmosphere (increasing due to human activities) acts as an aerosol, decreasing air quality and clinging to water droplets, eventually resulting in nitric acid (HNO3) that causes acid rain. Atmospheric ammonia and nitric acid also damage respiratory systems.
The very-high temperature of lightning naturally produces small amounts of NOx, NH3, and HNO3, but high-temperature combustion has contributed to a 6 or 7 fold increase in the flux of NOx to the atmosphere. The higher the temperature, the more NOx is produced.
Ammonia and nitrous oxides are precursors of tropospheric (lower atmosphere) ozone production, which contributes to smog and acid rain, damages plants and increases nitrogen inputs to ecosystems.
Decrease in biodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor species.
Onsite sewage facilities release large amounts of nitrogen into the environment. Microbial activity consumes the nitrogen and other contaminants in the wastewater. But in certain areas, microbial activity is unable to process all the contaminants and wastewater with the contaminants, enters the aquifers.
One health risk associated with drinking water (with >10 ppm nitrate) is the development of methemoglobinemia or blue baby syndrome.
Additional risks of increased availability of inorganic nitrogen in aquatic ecosystems are water acidification, eutrophication of fresh and saltwater systems and toxicity issues for animals. Eutrophication often leads to lower dissolved oxygen levels in the water column, including hypoxic and anoxic conditions, which can cause death of aquatic fauna.
P.S. : OTHER CYCLES WILL BE COVERED IN THE FORTHCOMING ARTICLES