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What is Nitrogen Cycle – Definition
For processes that participate in the cycling of nitrogen through the biosphere constitute the nitrogen cycle. Nitrogen is very much important for the living organisms. Nitrogen forms an essential part of amino acids which make up proteins and DNA. Nitrogen is essential for all living cells.
Explain Nitrogen Cycle Steps
- Nitrogen fixation
Microorganisms play a major role in all four processes of the nitrogen cycle.
Air which is about 78% nitrogen gas is the major reservoir of nitrogen. Most organisms cannot use nitrogen in this (gas) form.
Plants must secure their nitrogen in a fixed form that is incorporated in compounds such as nitrate ions, ammonia, urea, carbon dioxide etcetera. Animals secure their nitrogen and all other compounds from plants or animals that have fed on plants.
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 plants.
- Industrial fixation – N2+H2 →NH3
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 to 8 percent of the total nitrogen fixed.
Under great pressure at a temperature of 6000C and with the use of a catalyst (that fast the reaction) atmospheric nitrogen and hydrogen usually derived from natural gas or petroleum can be combined to form ammonia (NH3).
N2+H2 →NH3→ Ammonia
Ammonia can be used directly as a fertilizer but most of it is further processed to urea and ammonium nitrate (NH4NO3).
The ability to fix nitrogen is found only in certain bacteria.
- Some live in a symbiotic relationship with plants of the legume family (for example soybeans, alfalfa).
- Some established symbiotic relationships with plants other than the legume (For example- alders).
- Some nitrogen-fixing bacteria live free in the soil.
- Nitrogen-fixing cyanobacteria are essential for 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 passed through food webs just as carbohydrates do. At each trophic level, our 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 to nitrites.
- Bacteria of the genus Nitrobacter oxidize nitrites to nitrates.
These two groups or 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 legumes in addition to fixing atmospheric nitrogen also perform nitrification by converting some of their organic nitrogen to nitrites and nitrates. These reach the soil when they shed their leaves.
The three processes nitrogen fixation, nitrification, and Decay describe so far removed the 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 the soil and in aquatic sediments where conditions are in aerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in the air respiration. Thus they close the nitrogen cycle.
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