- Intro to biogeochemical cycles
- Biogeochemical cycles overview
- The water cycle
- The water cycle
- The carbon cycle
- The carbon cycle
- The nitrogen cycle
- The nitrogen cycle
- The phosphorus cycle
- Phosphorus cycle
- Eutrophication and dead zones
- Biogeochemical cycles
The nitrogen cycle
The key role of microbes in nitrogen fixation. How overuse of nitrogen-containing fertilizers can cause algal blooms.
- Nitrogen is a key component of the bodies of living organisms. Nitrogen atoms are found in all proteins and .
- Nitrogen exists in the atmosphere as gas. In nitrogen fixation, bacteria convert into ammonia, a form of nitrogen usable by plants. When animals eat the plants, they acquire usable nitrogen compounds.
- Nitrogen is a common limiting nutrient in nature, and agriculture. A limiting nutrient is the nutrient that's in shortest supply and limits growth.
- When fertilizers containing nitrogen and phosphorus are carried in runoff to lakes and rivers, they can result in blooms of algae—this is called eutrophication.
Nitrogen is everywhere! In fact, gas makes up about 78% of Earth's atmosphere by volume, far surpassing the we often think of as "air".
But having nitrogen around and being able to make use of it are two different things. Your body, and the bodies of other plants and animals, have no good way to convert into a usable form. We animals—and our plant compatriots—just don't have the right enzymes to capture, or fix, atmospheric nitrogen.
Still, your and proteins contain quite a bit of nitrogen. Where does that nitrogen come from? In the natural world, it comes from bacteria!
Bacteria play a key role in the nitrogen cycle.
Nitrogen enters the living world by way of bacteria and other single-celled prokaryotes, which convert atmospheric nitrogen——into biologically usable forms in a process called nitrogen fixation. Some species of nitrogen-fixing bacteria are free-living in soil or water, while others are beneficial symbionts that live inside of plants.
Nitrogen-fixing microorganisms capture atmospheric nitrogen by converting it to ammonia——which can be taken up by plants and used to make organic molecules. The nitrogen-containing molecules are passed to animals when the plants are eaten. They may be incorporated into the animal's body or broken down and excreted as waste, such as the urea found in urine.
Prokaryotes play several roles in the nitrogen cycle. Nitrogen-fixing bacteria in the soil and within the root nodules of some plants convert nitrogen gas in the atmosphere to ammonia. Nitrifying bacteria convert ammonia to nitrites or nitrates. Ammonia, nitrites, and nitrates are all fixed nitrogen and can be absorbed by plants. Denitrifying bacteria converts nitrates back to nitrogen gas.
Nitrogen doesn't remain forever in the bodies of living organisms. Instead, it's converted from organic nitrogen back into gas by bacteria. This process often involves several steps in terrestrial—land—ecosystems. Nitrogenous compounds from dead organisms or wastes are converted into ammonia——by bacteria, and the ammonia is converted into nitrites and nitrates. In the end, the nitrates are made into gas by denitrifying prokaryotes.
Nitrogen cycling in marine ecosystems
So far, we’ve focused on the natural nitrogen cycle as it occurs in terrestrial ecosystems. However, generally similar steps occur in the marine nitrogen cycle. There, the ammonification, nitrification, and denitrification processes are performed by marine bacteria and archaea.
The illustration shows the nitrogen cycle. Nitrogen gas from the atmosphere is fixed into organic nitrogen by nitrogen-fixing bacteria. This organic nitrogen enters terrestrial food webs. It leaves the food webs as nitrogenous wastes in the soil. Ammonification of this nitrogenous waste by bacteria and fungi in the soil converts the organic nitrogen to ammonium ion—NH4 plus. Ammonium is converted to nitrit—NO2 minus—then to nitrate—NO3 minus—by nitrifying bacteria. Denitrifying bacteria convert the nitrate back into nitrogen gas, which reenters the atmosphere. Nitrogen from runoff and fertilizers enters the ocean, where it enters marine food webs. Some organic nitrogen falls to the ocean floor as sediment. Other organic nitrogen in the ocean is converted to nitrite and nitrate ions, which is then converted to nitrogen gas in a process analogous to the one that occurs on land.
Some nitrogen-containing compounds fall to the ocean floor as sediment. Over long periods of time, the sediments get compressed and form sedimentary rock. Eventually, geological uplift may move the sedimentary rock to land. In the past, scientists did not think that this nitrogen-rich sedimentary rock was an important nitrogen source for terrestrial ecosystems. However, a new study suggests that it may actually be quite important—the nitrogen is released gradually to plants as the rock wears away, or weathers.
Nitrogen as a limiting nutrient
In natural ecosystems, many processes, such as primary production and decomposition, are limited by the available supply of nitrogen. In other words, nitrogen is often the limiting nutrient, the nutrient that's in shortest supply and thus limits the growth of organisms or populations.
How do we know if a nutrient is limiting? Often, this is tested as follows:
- When a nutrient is limiting, adding more of it will increase growth—e.g., it will cause plants to grow taller than if nothing were added.
- If a non-limiting nutrient is instead added, it won't have an effect—e. g., plants will grow to the same height whether the nutrient is present or absent.
For example, if we added nitrogen to half the bean plants in a garden and found that they grew taller than untreated plants, that would suggest nitrogen was limiting. If, instead, we didn't see a difference in growth in our experiment, that would suggest that some other nutrient than nitrogen must be limiting.
Nitrogen and phosphorus are the two most common limiting nutrients in both natural ecosystems and agriculture. That's why, if you look at a bag of fertilizer, you will see it contains a lot of nitrogen and phosphorus.
Human activity affects cycling of nitrogen.
We humans may not be able to fix nitrogen biologically, but we certainly do industrially! About 450 million metric tons of fixed nitrogen are made each year using a chemical method called the Haber-Bosch process, in which is reacted with hydrogen——at high temperatures. Most of this fixed nitrogen goes to make fertilizers we use on our lawns, gardens, and agricultural fields.
In general, human activity releases nitrogen into the environment by two main means: combustion of fossil fuels and use of nitrogen-containing fertilizers in agriculture. Both processes increase levels of nitrogen-containing compounds in the atmosphere. High levels of atmospheric nitrogen—other than —are associated with harmful effects, like the production of acid rain—as nitric acid, —and contributions to the greenhouse effect—as nitrous oxide, .
Also, when artificial fertilizers containing nitrogen and phosphorus are used in agriculture, the excess fertilizer may be washed into lakes, streams, and rivers by surface runoff. A major effect from fertilizer runoff is saltwater and freshwater eutrophication. In this process, nutrient runoff causes overgrowth, or a "bloom," of algae or other microorganisms. Without the nutrient runoff, they were limited in their growth by availability of nitrogen or phosphorus.
Eutrophication can reduce oxygen availability in the water during the nighttime because the algae and microorganisms that feed on them use up large quantities of oxygen in cellular respiration. This can cause the death of other organisms living in the affected ecosystems, such as fish and shrimp, and result in low-oxygen, species-depleted areas called dead zones.
Want to join the conversation?
- I heard that lightning also helps in nitrogen fixation so is it teue or not?(25 votes)
- during lightning the high temperature and pressure in the air , convert nitrogen into its oxides which dissolve in water to give nitric and nitrous acids.these are used by various life forms. so its true(19 votes)
- Can ammonia be directly converted into atmospheric Nitrogen? Or does it have to be first nitrites or nitrates and then converted into atmospheric Nitrogen?
Thanks to anyone who answers this question.(10 votes)
- It has to go through the process of becoming a nitrite, then a nitrate, before it can become atmospheric Nitrogen. All parts of the cycle are needed, and you cannot skip a step of the cycle.(9 votes)
- If an animal consumes a plant that contains nitrogen, is the animal getting the nitrogen from the proteins in the plant?(4 votes)
- Yes, around half of the nitrogen in a plant is incorporated into proteins. These will be broken down to amino acids (or small peptides) during digestion and absorbed.
A significant amount will also be incorporated into nucleic acids (RNA and DNA), which will also be broken down and absorbed.
- What kind of cycle is the nitrogen cycle? Gaseous or sedimentary?(4 votes)
- Both! As you see from the diagram, the nitrogen cycle goes both airborne and underground!(2 votes)
- what is the formula for oxygen and hydrogen carbon dioxied and nitrates?(5 votes)
- Why is it that normal Nitrogen is "N" while Nitrogen in the atmosphere is "N2"?(2 votes)
- because nitrogen atoms tend to bond with other nitrogen atoms to form nitrogen molecules, which are N2.(6 votes)
- how does amino acids fit into this(3 votes)
- In general, the nitrogen cycle has five steps:
Nitrogen fixation (N2 to NH3/ NH4+ or NO3-)
Nitrification (NH3 to NO3-)
Assimilation (Incorporation of NH3 and NO3- into biological tissues)
Ammonification (organic nitrogen compounds to NH3)
Denitrification(NO3- to N2)
Nitrogen assimilation is the formation of organic nitrogen compounds like amino acids from inorganic nitrogen compounds present in the environment. Organisms like plants, fungi and certain bacteria that cannot fix nitrogen gas (N2) depend on the ability to assimilate nitrate or ammonia for their needs.
- why are the dead zone full of nitrogen and phosphorus , and why do we still us nitrogen rich soils?(3 votes)
- Nitrogen and phosphorous from agricultural runoff are the primary culprits, but sewage, vehicular and industrial emissions, and even natural factors also play a role in the development of dead zones.
Increased nutrient concentrations lead to blooming of algae which cause depleted oxygen levels. Ending up with eutrophication.
- Is nitrogen and phosphorus the cause of eutorfication ? Or is one the cause and not both ?(3 votes)
- Before the atmosferic nitrogen is transfered to ammonia, is it transfered to nitrogen fixing
bacteria (legume root) and ammonium first.(3 votes)