Assimilation is the process by which plants and animals incorporate the
NO3- and ammonia formed through nitrogen fixation and nitrification.
Plants take up these forms of nitrogen through their roots, and incorporate
them into plant proteins and nucleic acids. Animals are then able to utilize
nitrogen from the plant tissues.
Nitrification
Nitrification is the process that converts ammonia to nitrite and then to nitrate and is an important step in the global nitrogen cycle. It is a two-step process in which NH3/ NH4+ is converted to NO2- by the soil bacteria called ammonia-oxidizers such as Nitrosopumilus, Nitrosospira etc. Ammonia-oxidizing bacteria have been found to be abundant in oceans, soils, and salt marshes, also.
For complete nitrification, both ammonia oxidation and nitrite oxidation must occur.
Ammonia- and
nitrite-oxidizers also play a very important role in wastewater treatment
facilities by removing harmful levels of ammonium that could lead to the
pollution of the receiving waters. Ammonia- and nitrite-oxidizers help to
maintain healthy aquaria by facilitating the removal of potentially toxic
ammonium excreted in fish urine.
Anammox
Traditionally,
all nitrification was thought to be carried out under aerobic conditions, but
recently a new type of ammonia oxidation occurring under anoxic conditions was
discovered. Anammox (anaerobic ammonia oxidation) is carried out by anammox
bacterium like Brocadia anammoxidans. Anammox bacteria oxidize
ammonia by using nitrite as the electron acceptor to produce gaseous nitrogen.
Anammox bacteria were first discovered in anoxic bioreactors of wasterwater
treatment plants but have since been found in a variety of aquatic systems,
including low-oxygen zones of the ocean, coastal and estuarine sediments,
mangroves, and freshwater lakes. In some areas of the ocean, the anammox
process is considered to be responsible for a significant loss of nitrogen.
Ammmonification
When an organism
excretes waste or dies, the nitrogen in its tissues is in the form of organic
nitrogen (e.g. amino acids, DNA). Various fungi and prokaryotes then decompose
the tissue and release inorganic nitrogen back into the ecosystem as ammonia in
the process known as ammonification. The ammonia then becomes available for
uptake by plants and other microorganisms for growth.
Denitrification
Denitrification is the process that converts nitrate to nitrogen
gas, thus removing bioavailable nitrogen and returning it to the atmosphere.
Dinitrogen gas (N2) is the ultimate end product of denitrification,
but other intermediate gaseous forms of nitrogen exist. Some of these gases,
such as nitrous oxide (N2O), are considered greenhouse gasses,
reacting with ozone and contributing to air pollution.
Denitrification is
an anaerobic process, occurring mostly in soils and sediments and anoxic zones
in lakes and oceans. Some denitrifying bacteria include species in the
genera Bacillus, Paracoccus, and Pseudomonas.
Denitrification
is important in that it removes fixed nitrogen (i.e., nitrate) from the
ecosystem and returns it to the atmosphere in a biologically inert form (N2).
Wetlands provide a valuable place for reducing excess nitrogen levels via
denitrification processes. This is particularly important in agriculture where
the loss of nitrates in fertilizer is detrimental and costly. Denitrification
in wastewater treatment plays a very beneficial role by removing unwanted
nitrates from the wastewater effluent, thereby reducing the chances that the
water discharged from the treatment plants will cause undesirable consequences
(e.g., algal blooms).
Ecological Implications of Human Alterations to the Nitrogen Cycle
1. Many human activities have a significant impact
on the nitrogen cycle. Burning fossil fuels, application of nitrogen-based
fertilizers, and other activities can dramatically increase the amount of
biologically available nitrogen in an ecosystem.
2. Nitrogen availability often limits the primary
productivity of many ecosystems, hence, large changes in the availability of
nitrogen can lead to severe alterations of the nitrogen cycle in both aquatic
and terrestrial ecosystems. Industrial nitrogen fixation has increased
exponentially since the 1940s, and human activity has doubled the amount of
global nitrogen fixation
3. In terrestrial ecosystems, the addition of
nitrogen can lead to nutrient imbalance in trees, changes in forest health, and
declines in biodiversity. With increased nitrogen availability there is often a
change in carbon storage, thus impacting more processes than just the nitrogen
cycle.
4. In agricultural systems, fertilizers are used
extensively to increase plant production, but unused nitrogen, usually in the
form of nitrate, can leach out of the soil, enter streams and rivers, and
ultimately make its way into our drinking water.
5. Much of the nitrogen applied to agricultural and
urban areas ultimately enters rivers and nearshore coastal systems. In
nearshore marine systems, increases in nitrogen can often lead to anoxia (no
oxygen) or hypoxia (low oxygen), altered biodiversity, changes in food-web
structure, and general habitat degradation. One common consequence of increased
nitrogen is an increase in harmful algal blooms. Toxic blooms of certain types
of dinoflagellates have been associated with high fish and shellfish mortality
in some areas.
6. Alterations to the nitrogen cycle may lead to an
increased risk of parasitic and infectious diseases among humans and wildlife.
7. Increases in nitrogen in aquatic systems can
lead to increased acidification in freshwater ecosystems.
Nitrogen is the most important nutrient in regulating primary
productivity and species diversity in both aquatic and terrestrial ecosystems.
Microbially-driven processes such as nitrogen fixation, nitrification, and
denitrification, constitute the bulk of nitrogen transformations, and play a
critical role in the fate of nitrogen in the Earth's ecosystems.
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