Nitrogen Cycle — Definition & Diagrams

Nitrogen is one of the key molecules of life. It is one of the main components of nucleic acids, DNA and RNA. The nitrogen cycle describes how nitrogen moves through the biosphere and the different forms it can be found in. The cycle is very important because it impacts Earth's plant growth, our primary producers, and therefore all other living organisms.

Nitrogen is found in different forms, including organic nitrogen, ammonium, nitrite, nitrate, nitrous oxide, nitric oxide, and inorganic nitrogen gas. However, different plants and animals can only use certain forms of nitrogen.

Major Processes in the Cycle:

Fixation

Overall reaction: $\color{#50BF14}{\text{N}}$$\rightarrow$$\color{#50BF14}{\text{NH}}$

The atmosphere is the largest source of nitrogen, but gaseous nitrogen ($\color{#50BF14}{\text{N}}$) is not able to be used by most organisms. In order to be taken up by plants, atmospheric nitrogen is converted into nitrate ($\color{#50BF14}{\text{NO}}$). The first step of this process is called nitrogen fixation and turns $\color{#50BF14}{\text{N}}$ into ammonia ($\color{#50BF14}{\text{NH}}$).

Atmospheric nitrogen makes its way into the soil through precipitation. Once in the soil, fixation is done mainly by very specific bacteria. With the help of an enzyme called nitrogenase, these organisms combine the atmospheric nitrogen with hydrogen and form ammonia ($\color{#50BF14}{\text{NH}}$).

This process is also done by farming industries to enrich the soil and increase crop yield.

Ammonification

Overall reaction: $\text{Organic nitrogen}$$\rightarrow$$\color{#50BF14}{\text{NH}}$

Organic nitrogen is found in animal waste as well as in the remains after an animal dies. This nitrogen can be converted by bacteria and fungi into ammonium ($\color{#50BF14}{\text{NH}}$). This process is called ammonification.

Nitrification

Overall reaction: $\color{#50BF14}{\text{NH}}$$\rightarrow$$\color{#50BF14}{\text{NO}}$$\rightarrow$$\color{#50BF14}{\text{NO}}$ or $\color{#50BF14}{\text{NH}}$$\rightarrow$$\color{#50BF14}{\text{NO}}$$\rightarrow$$\color{#50BF14}{\text{NO}}$

Although ammonia can be absorbed by some plants, it often undergoes another conversion first. During nitrification, ammonia from nitrogen fixation and ammonium from ammonification can be converted to nitrites ($\color{#50BF14}{\text{NO}}$) and then nitrates ($\color{#50BF14}{\text{NO}}$). These new compounds can be used by plants to make other organic compounds useful to other organisms. This is done by special bacteria.

Once nitrates are formed, they can be absorbed, or assimilated, by plants. The nitrogen, protein, and other compounds made by plants are later consumed by other organisms. When those organisms die, the organic matter can undergo ammonification yet again.

Denitrification

Overall reaction: $\color{#50BF14}{\text{NO}}$$\rightarrow$$\color{#50BF14}{\text{N}}$

Nitrates can be assimilated by plants, or they can be converted and released back into the atmosphere. This process is called denitrification and reduces nitrates in the soil to nitrogen gas. This is done by bacteria and is an anaerobic process, meaning it doesn't require oxygen. Denitrification is a normal part of respiration for these unique organisms because they can use nitrate as an electron acceptor when oxygen is not available. The product is nitrogen gas, which enters the atmosphere and restarts the cycle.

The Marine Ecosystem

The marine ecosystem also helps cycle nitrogen. The general process is the same, but here are a few differences:

Nitrogen makes its way into water through precipitation, runoff, or directly from the atmosphere. The nitrogen gas can't be used by the phytoplankton, which need ammonia or nitrate to produce organic matter, much like terrestrial plants.

In the ocean, cyanobacteria perform nitrogen fixation to make ammonia. The ammonia then sinks to the lower levels of the water where it can be converted by bacteria into nitrite and nitrate. These specific bacteria can only function in the dark, low levels of water.

Nitrate can be returned to the upper levels of water by mixing and upwelling. There, it can be taken up by phytoplankton.

Some nitrogen compounds settle to the ocean floor and eventually form sedimentary rock. Nitrogen-filled sedimentary rock moves from the ocean floor onto land over a very long period of time through a process called geological uplift. Once on land, the nitrogen is slowly released as the rock undergoes weathering.

Nitrogen pollution has recently become an issue due to human activities. The problem comes mainly from waste being dumped into the water sources.

How Is Nitrogen Moved Through the Environment?

The nitrogen cycle is a biogeochemical cycle consisting of different forms of nitrogen.

All organisms require nitrogen to make amino acids which form proteins. Nitrogen gas makes up the majority of the Earth's atmosphere (78%). Turning this gas into a usable form is a complicated process which requires the help of some very specific organisms.

Nitrogen-fixing bacteria that live on the roots of plants or freely in the soil are responsible for all natural nitrogen fixation. These bacteria have a symbiotic relationship with the plants because they help each other recycle products back into the ecosystem.

(Video) Nitrogen and Phosphorus Cycles: Always Recycle!

by CrashCourse

Nitrogen is constantly being transported between organisms and the environment. In this video, Hank from Crash Course covers the nitrogen and phosphorus cycles.

Summary:

Nitrogen makes up about 78% of the atmosphere. That's a large amount of nitrogen, but we are not able to us it because the nitrogen in the atmosphere exists as a molecule with a triple bond. That's a very, very strong bond.

Thankfully, we have unique little bacteria buddies that are more than happy to break this bond for us because they have specific enzymes to do the job. They turn gaseous nitrogen into nitrates in the soil. This process is called nitrogen fixation. Plants are then able to absorb the nitrates. Animals eat the plants and also get in on the nitrogen action. When organisms die, their nitrates return to the soil and another type of bacteria turns the nitrates into nitrogen gas. The whole cycle then repeats.