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Essay on the Nitrogen Cycle

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Here is an essay on the nitrogen cycle.

Nitrogen is one of the important elements in biological compounds, mainly of nucleic acid and protein and, therefore, it is essential for life. Atmosphere contains about 78% nitrogen, but free nitrogen cannot be utilised by most of the organisms, except a few blue green algae and some bacteria.

The fixation of nitrogen takes place by physical, chemical and biological means. Small amounts of N 2 are converted to ammonia by electrical discharges in atmosphere i.e., by physical force, and settle down on earth by rain. About 30 million metric tonnes of N 2 are pro­duced by industry however about 200 million metric tonnes of nitrogen are fixed every year by biological organisms.

The biological N 2 fixation takes place by a few organisms like:

1. Symbiotic Bacteria:

Rhizobium, Brady- rhizobium, Frankia.

2. Free-Living Bacteria:

Azotobacter, Azomonas, Dersia etc.

3. Blue Green Algae:

Nostoc, Anabaena, etc., fix nitrogen inside heterocyst where oxy­genic photosynthesis does not occur.

In nitrogen cycle, free N 2 gas of atmosphere is converted into ammonia or oxidised to nitrate at different stages. Blue green algae and N 2 fixing bacteria play a significant role in converting the atmospheric gaseous nitrogen into organic nitrogenous compounds and, finally, to nitrate, which is soluble in water.

Nitrates are absorbed and utilised by the plants for the synthesis of amino acids vis-a-vis proteins. The plants are consumed by herbivores. When the plants and animals die, they are decomposed by, bacteria, thus the N 2 becomes released in the atmosphere.

The N 2 cycle (Fig. 2.34) thus consists of the following steps:

1. Nitrogen fixation,

2. Ammonification,

3. Nitrification,

4. Denitrification, and finally, and 

5. Release of gases in the atmosphere.

Biological Nitrogen Fixation:

The major share of nitrogen fixation is occupied by biologi­cal N 2 fixation. The biological N 2 fixation of atmospheric nitrogen depends on the nitroge­nase enzyme system, composed of nitrogenase and nitrogenase reductase. Nitrogenase is very much sensitive to O 2 and it becomes irreversibly inactivated on exposure even at low concentra­tion of O 2 . In leguminous plants, N 2 fixation takes place in root nodules, where the enzyme is protected by red pigment leghemoglobin.

The plants are eaten by herbivores and fixed nitrogen goes into their body and after their death, it becomes decomposed and mixed with the soil.

The nitrogen may be fixed in the soil by the following means:

(a) Physical fixation takes place by light­ning and thereby N 2 comes down in soil through rain.

(b) There is huge industrial production of N 2 fertiliser, these are mixed with the soil during cultivation.

(c) Farmyard manure is one of the major sources of N 2 .

(d) Major share of N 2 is contributed by the biological fixers.

Ammonification:

In this process nitrogen in organic matter of dead plants and animals is con­verted to ammonia and amino acids.

Urea is applied in the field as additional nitrogen source. The microbial decomposition of urea causes the release of ammonia which is returned to atmosphere or may go to neutral aqueous environments as ammonium ions.

Nitrification:

This is the process of conver­sion of ammonia to nitrate. It takes place in two steps. Ammonia is first oxidised to nitrous acid by Nitrosomonas, Nitrospira and Nitrococcus. The molecular O 2 acts as electron acceptor in this step. In the second step, nitrous acid becomes oxidised and converted into nitric acid, mostly by Nitrobacter. Proper aeration in the soil is essential for the availability of oxygen.

Denitrification:

During this process, nitrate is converted to molecular nitrogen by the diffe­rent bacteria like Bacillus cereus, Pseudomonas aeruginosa etc. and thus, N 2 goes back into the atmosphere. This is called dissimilatory nitrate reduction. This takes place during depletion of oxygen in the medium.

In assimilatory nitrate reduction process nitrite is converted into ammonia.

The nitrate added to the soil is reduced to NH 3 by plants and fermentative bacteria rather than to nitrogen by denitrifying bacteria.

Related Articles:

• Biological Nitrogen Cycle: (With Diagram) | Ecosystem
• 4 Main Phases of Nitrogen Cycle (With Diagram) | Ecosystem | Biology

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Nutrient Cycles ( AQA A Level Biology )

Revision note.

Biology & Environmental Systems and Societies

Nutrient Cycles

• These nutrients are then returned to the environment when organisms produce waste or die and decompose
• This is due to the waste products and dead organisms being digested (decomposed) by microorganisms
• The products of this decomposition are available to plants as nutrients in the soil
• These plants can then sustain organisms in higher trophic levels (consumers)
• This means these nutrients are constantly being cycled in ecosystems

The nitrogen cycle

• The phosphorous cycle
• The nitrogen cycle shows how nitrogen is recycled in ecosystems
• Plants and animals require nitrogen in order to produce proteins and nucleic acids (DNA and RNA)
• Instead, they rely on certain bacteria to convert the nitrogen gas into nitrogen-containing compounds , which can be taken up by plants
• The nitrogen cycle shows this conversion, as well as how the nitrogen in the nitrogen-containing compounds is then passed between trophic levels or between living organisms and the non-living environment
• There are four key processes in the nitrogen cycle that are carried out by different types of bacteria
• Atmospheric nitrogen gas is converted into nitrogen-containing compounds
• This biological nitrogen fixation is carried out by nitrogen-fixing bacteria such as Rhizobium
• The bacteria convert nitrogen into ammonia, which forms ammonium ions (in solution) that can then be used by plants
• These nitrogen-fixing bacteria are found inside the root nodules (small growths on the roots) of leguminous plants such as peas, beans and clover
• The bacteria have a symbiotic (mutually beneficial) relationship with these plants - the bacteria provide the plants with nitrogen-containing compounds and the plants provide the bacteria with organic compounds such as carbohydrates
• Nitrogen compounds in waste products (e.g. urine and faeces) and dead organisms are converted into ammonia by saprobionts (a type of decomposer including some fungi and bacteria)
• This ammonia forms ammonium ions in the soil
• The ammonium ions in the soil are converted by nitrifying bacteria into nitrogen compounds that can be used by plants, known as nitrates
• Initially, nitrifying bacteria such as Nitrosomonas convert ammonium ions into nitrites
• Different nitrifying bacteria such as  Nitrobacter then convert these nitrites into nitrates
• Denitrifying bacteria use nitrates in the soil during respiration
• This process produces nitrogen gas , which returns to the atmosphere
• This process occurs in anaerobic conditions (when there is little or no oxygen available, such as in waterlogged soil)

The phosphorus cycle

• The phosphorus cycle shows how phosphorus is recycled in ecosystems
• Plants and animals require phosphorus in order to produce certain biological molecules such as phospholipids (for cell membranes), nucleic acids (DNA and RNA) and ATP
• Phosphorus in rocks is slowly released into the soil and into water sources in the form of phosphate ions (PO₄³⁻) by the process of weathering (the slow breaking down and erosion of rocks over time)
• Phosphate ions are taken up from the soil by plants through their roots or absorbed from water by algae
• Phosphate ions are transferred to consumers during feeding
• Phosphate ions in waste products and dead organisms are released into the soil or water during decomposition by saprobionts
• The phosphate ions can now be taken up and used once again by producers or may be trapped in sediments that, over very long geological time periods may turn into phosphorus-containing rock once again

You do not need to learn the Latin names of specific types of nitrogen-fixing bacteria or nitrifying bacteria included in the notes above. You just need to know the processes that these groups of bacteria are responsible for carrying out and why these processes are important in the context of the nitrogen cycle.

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Biosynthesis of Nitrogen-Containing Compounds

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• Mark Morrison &
• Roderick I. Mackie  

Part of the book series: Chapman & Hall Microbiology Series ((CHMBS))

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Nitrogen is essential for growth in all biological systems, and its assimilation into a variety of life-sustaining compounds has been the topic of study for many microbiologists. This chapter focuses on ammonia assimilation and the biosynthesis of amino acids, polyamines, pyrimidines, and purines. Wherever possible, emphasis will be directed toward findings obtained from ruminai and colonic bacteria, although the knowledge base developed for these bacteria is relatively superficial. To overcome these limitations, some discussion pertaining to gramnegative enteric bacteria (Escherichia coli, Salmonella typhimurium [official designation, Salmonella enterica , serovar typhimurium ], and Klebsiella spp.) and gram-positive bacteria (Bacillus subtilis and Clostridium spp.) has been included for the sake of clarity and reference. Readers interested in detailed information concerning the topics covered in this chapter, as well as the biosynthesis of nitrogen-containing vitamins and coenzymes, should refer to the volumes edited by Neidhardt et al. (1996) and Sonenshein et al. (1993), as well as the recent review of nitrogen control in bacteria by Merrick and Edwards (1995). The goals of this chapter are to provide a cohesive overview that complements the well-chronicled field of knowledge developed from these intensively studied species, and to highlight opportunities where further studies of ruminai and colonie bacteria may expand our understanding of these processes.

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Morrison, M., Mackie, R.I. (1997). Biosynthesis of Nitrogen-Containing Compounds. In: Mackie, R.I., White, B.A. (eds) Gastrointestinal Microbiology. Chapman & Hall Microbiology Series. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-4111-0_12

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AQA A Level Essay- The Importance of nitrogen containing compounds

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Nitrogen deposition also reduces biodiversity on a larger scale, research shows

by Wageningen University

It has long been known that nitrogen deposition, where nitrogen is released from the atmosphere into soil or water, can lead to biodiversity loss on a small scale. New research shows that this effect can also be seen in larger areas of several kilometers. The study is based on data from eight European countries, including the Netherlands.

The findings are published in the journal Global Change Biology .

Nitrogen in itself is not a problem. In fact, plants need nitrogen to grow. But the deposition of so-called reactive nitrogen compounds, such as nitrogen oxides (NO x ) and ammonia NH 3 ), in excess can be harmful to nature and lead to the loss of plant species in a given area.

Current directives on nitrogen are aimed at protecting biodiversity . These directives are often based on research in very small areas, of only a few square meters. According to lead researcher Fons van der Plas, much less is known about the effects of nitrogen at larger scales, "We do not know for sure whether the same effect can also be seen at larger scales of a few hectares or even larger, while this could have implications for policy."

The new research shows that the negative effects of nitrogen deposition can indeed also be seen in areas of a few hectares to hundreds of square kilometers. Van der Plas adds, "At high nitrogen levels, we see that consistently the same plant species disappear in multiple areas, such as blue knot and betony. These species, which are characteristic of heathland grasslands, are in sharp decline in the Netherlands and are even on the red list of endangered species."

With this study, the researchers confirm what they hypothesized, that the same rules apply on a larger scale as for small areas. They say this underlines the importance of effective nitrogen directives and policies to preserve biodiversity in natural areas.

The study offers new insights that can contribute to the national debate in The Netherlands on nitrogen and biodiversity.

Journal information: Global Change Biology

Provided by Wageningen University

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