Nitrogen Cycle

Introduction

Nuclear fusion was responsible for the formation of nitrogen in the hearts of stars. Nitrogen-containing gases were thrown across the Universe as old stars burst. Nitrogen gas was the primary component of the Earth’s atmosphere when it was formed.

The current composition of the Earth’s atmosphere is roughly 78 percent nitrogen, 21% oxygen, and 1% other gases. Because too much oxygen can be detrimental to cells, this is an optimal balance. Furthermore, oxygen is combustible. In its gaseous state, nitrogen, on the other hand, is inert and harmless. Plants and animals, on the other hand, are unable to utilize nitrogen gas in their cells.

Nitrogen

Nitrogen,its chemical symbol is N. It is a colorless and odorless element. In fact, nitrogen is the most prevalent gaseous element in the Earth’s atmosphere. 

It is a naturally occurring element that is required for both plant and animal development and reproduction. All living things, including humans, require nitrogen.

It is essential for plant growth: without it, plants cannot survive, resulting in minimal food yields; nevertheless, too much nitrogen can be hazardous to plants. While nitrogen is necessary for food production as it is one of the important components of proteins, too much nitrogen may be damaging to the environment.

Nitrogen Forms

Comprehending how nitrogen interacts chemically in the environment may be important in understanding the complexities of nitrogen loading to coastal waterways. nitrogen is a chemical element that may react with itself or with other elements to form various compounds. 

For example, nitrogen gas, N2, is formed when two nitrogen atoms create a chemical connection. Only a particular type of bacteria, as well as industrial fertilizer manufacturing, can “fix” this mostly inert molecule into physiologically active nitrogen compounds. Fertilizer manufacturing currently outnumbers natural nitrogen fixation in terms of supplying N2 to the biosphere.

Steps in Nitrogen Cycle 

The various steps required for cycling of  nitrogen in the nature are given below-

1. Nitrogen Fixation

Bacteria convert nitrogen gas from the environment into ammonia during the nitrogen fixation process.Nitrogen-fixing bacteria, often known as “diazotrophs,” have an enzyme called “nitrogenase” that combines nitrogen and hydrogen atoms. The reaction for nitrogen fixation is: 

 N2+ 6H+ + 6e- →  2NH3

Ammonia is a nitrogen containing molecule that dissolves in water and interacts more easily with the enzymes of other organisms

Surprisingly, the enzyme nitrogenase can only work in the absence of oxygen. As a result, organisms that employ it have had to create oxygen-free chambers in which to fix nitrogen.

Rhizobium nodules located in the roots of nitrogen-fixing legume plants are common examples of nitrogen-free compartments. The stiff shell of these nodules prevents oxygen from entering the spaces where Rhizobium bacteria convert nitrogen gas into ammonia.

2. Nitrification

A variety of soil bacteria assist in nitrification, which converts ammonia to nitrate, a type of nitrogen that plants and animals may utilize. This involves two processes, each of which is carried out by a distinct species of bacterium.

To begin, soil bacteria like Nitrosomonas and Nitrococcus convert ammonia to nitrogen dioxide. Nitrobacter, a kind of soil bacteria, then adds a third oxygen atom to produce nitrate.

These bacteria do not convert ammonia for plants and animals out of altruism. They are “chemotrophs,” meaning they get their energy from volatile molecules. They gain energy for their own life activities by metabolizing nitrogen and oxygen together.

The process may be thought of as an approximate (and considerably less efficient) counterpart of mammalian cellular respiration, in which energy is extracted from carbon-hydrogen bonds and oxygen is used as an electron acceptor, resulting in carbon dioxide at the conclusion of the process.

Nitrates, the end result of this critical chain of bacterial activities, may be synthesized artificially and are a key component in many soil fertilizers. Such fertilizer is also referred to as “nitrate fertilizer.” By pouring nitrates into the soil, such fertilizers allow plants to develop swiftly without relying on the rate at which nitrogen-fixing microorganisms execute their duties. 

For example the chemical reaction which converts ammonia to nitrite is given as:

NH3 + O2   →  NO2−  +  H+

Further the chemical reaction which convert nitrite to nitrate is given as:

 2 NO2− + O2  →  2 NO3−

Therefore the second reaction step is a nitrification of the nitrogen cycle.

3. Assimilation

The generated nitrates in the soil are taken by the plants via their root system during this process. Nitrates are found in plants, which are ingested by consumers and then pass through the food chain and enter the food web. Nitrate and other nitrogen molecules are absorbed by assimilation. nitrogen compounds are required for the production of important biomolecules.

4. Ammonification

We have now transferred nitrogen from the atmosphere to the cells of plants and animals.

Because there is so much nitrogen in the atmosphere, it may appear that the process could finish there – however the supply of nitrogen in the atmosphere is not endless. Keeping nitrogen inside plant and animal cells would produce massive changes in our soil, environment, and ecosystems.

Fortunately, this is not the case. In a healthy ecosystem like ours, wherever energy has been expended in the creation of an organic compound, another form of life is ready to recover that energy by breaking those chemical bonds.

Soil microorganisms degrade dead plants and animals in a process known as “ammonification.” During the process, these decomposers convert amino acids and nucleic acids into nitrates and ammonia, which are then released back into the soil.

Plants and nitrifying bacteria may take up the ammonia there again. Alternatively, the denitrification process can turn ammonia back into atmospheric nitrogen.

5. Denitrification

Anaerobic bacteria can convert nitrates back into nitrogen gas in the last phase of the nitrogen cycle.

This process, like the conversion of nitrogen gas to ammonia, must take place in the absence of oxygen. The chemical reaction for this:

2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

As a result, it is frequently found deep in the earth or in moist conditions where dirt and muck keep oxygen at bay.

Denitrification is a crucial process in some ecosystems because it prevents nitrogen molecules in the soil from piling up to harmful levels.Thiobacillus denitrificans, Micrococcus denitrificans are denitrifying bacteria.

Nitrogen Cycle in Marine Ecosystem

The nitrogen cycle process happens in the marine environment in the same way that it does in the terrestrial ecosystem. The only difference is that it is carried out by sea microbes rather than humans.

As sediments are compacted over extended periods of time and form sedimentary rock, nitrogen-containing chemicals fall into the ocean. These sedimentary rocks have moved to land as a result of geological uplift. It was previously unknown that these nitrogen-containing sedimentary rocks provide an important source of nitrogen. However, subsequent studies have shown that the nitrogen from these rocks is released into the plants as a result of weathering.

Significance of Nitrogen Cycle

• Nitrogen is a necessary component of life as we know it. Its particular chemical bonding characteristics enable it to form structures such as DNA and RNA nucleotides, as well as the amino acids that makeup proteins. These compounds would not be able to exist without nitrogen
• The earliest nucleotides and amino acids are assumed to have developed spontaneously in the early Earth’s volatile environment when energy sources such as lightning strikes could drive nitrogen and other atoms to react and build complex structures
• This mechanism may have spontaneously created self-replicating organic molecules, but in order for life to reproduce and evolve, it needed to find out how to produce these nitrogen compounds on demand
• Today, “nitrogen fixers” are creatures that can convert atmospheric nitrogen gas into nitrogen compounds that other organisms may utilize to generate nucleic acids, amino acids, and other chemicals. These nitrogen fixers are such an important element of the ecosystem that agriculture would be impossible without them
• Ancient people recognised that if nitrogen-consuming crops were not alternated with nitrogen-fixing crops, their fields would fallow and be unable to support growth. To make the soil more fertile, most artificial fertilisers now contain life-giving nitrogen molecules as their principal constituent

Conclusion

Nitrogen is plentiful in the atmosphere, but it is useless to plants and animals until it is transformed into nitrogen compounds. Nitrogen-fixing bacteria are critical in converting atmospheric nitrogen into nitrogen molecules that plants can utilise. Through their roots, the plants receive usable nitrogen molecules from the soil. These nitrogen molecules are then utilized in the cell to produce proteins and other substances. Animals absorb nitrogen through ingesting nitrogen-containing plants or other animals. Humans ingest the proteins from these plants and animals, and the nitrogen is subsequently assimilated into our system. During the last phases of the nitrogen cycle, bacteria and fungi aid in the decomposition of organic waste, allowing nitrogenous chemicals to be dissolved into the soil and utilized by plants anew.