The Nitrogen Cycle

The Nitrogen Cycle

The nitrogen cycle is the cycling of nitrogen between organisms and the environment. Without nitrogen, proteins and nucleic acids cannot be produced. Nitrogen composes nearly 79% of the earth's atmosphere, the most abundant gas on the earth. Nitrogen gas, reacts chemically only under unique conditions. As a result, nitrogen is most often utilized by organisms in the form of the nitrate ion (NO3) found in nitrate salts. Only organisms capable of making their own food can use nitrates to generate proteins, nucleic acids, and other nitrogen compounds. Atmospheric nitrogen can be converted into the nitrates, the usable form of nitrogen. The process is called nitrogen fixation: the conversion of nitrogen gas into nitrates and ammonium ions, which can then be used by plants.

All organisms produce wastes and eventually die. In bacterial decay, proteins are slowly broken broken down into ammonia compounds, then to nitrites, and finally back to nitrates. The nitrates can then reenter the cycle. At various stages in the decay process, denitrifying bacteria (soil bacteria that reduce nitrates or nitrites to gaseous nitrogen and some nitrous oxide) can break down nitrites and nitrates, releasing nitrogen gas back into the atmosphere.

The Nitrogen Cycle:

All life requires nitrogen-compounds, e.g., proteins and nucleic acids.
Air, which is 79% nitrogen gas (N₂), is the major reservoir of nitrogen.
But most organisms cannot use nitrogen in this form.
Plants must secure their nitrogen in "fixed" form, i.e., incorporated in compounds such as:
- nitrate ions (NO₃^-)
- ammonia (NH₃)
- urea (NH₂)₂CO
Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants).

Four processes participate in the cycling of nitrogen through the biosphere:

nitrogen fixation
decay
nitrification
denitrification

Microorganisms play major roles in all four of these.

Nitrogen Fixation

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

Atmospheric Fixation

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 5-8% of the total nitrogen fixed.

Industrial Fixation

Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH₃). Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium nitrate (NH₄NO₃).

Biological Fixation

The ability to fix nitrogen is found only in certain bacteria.

Some live in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa).
Some establish symbiotic relationships with plants other than legumes (e.g., alders).
Some nitrogen-fixing bacteria live free in the soil.
Nitrogen-fixing cyanobacteria are essential to 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.

Below is a picture of root nodules on clover

Decay

The proteins made by plants enter and pass through food webs just as carbohydrates do. At each trophic level, their 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.

Nitrification

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 NH₃ to nitrites (NO₂^-).
Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO₃^-).

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.

Denitrification

The three processes above remove 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 soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.

Thus they close the nitrogen cycle.

Are the denitrifiers keeping up?

Agriculture may now be responsible for one-half of the nitrogen fixation on earth through

the use of fertilizers produced by industrial fixation
the growing of legumes like soybeans and alfalfa.

This is a remarkable influence on a natural cycle.

Are the denitrifiers keeping up the nitrogen cycle in balance? Probably not. Certainly, there are examples of nitrogen enrichment in ecosystems. One troubling example: the "blooms" of algae in lakes and rivers as nitrogen fertilizers leach from the soil of adjacent farms (and lawns). The accumulation of dissolved nutrients in a body of water is called eutrophication.

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