How the Nitrogen Cycle Works
The nitrogen cycle is a crucial biogeochemical cycle that recycles the element nitrogen (N2) into its various usable forms. It is very similar to other cycles, such as the water and oxygen cycles. As such, the nitrogen cycle is extremely important in sustaining Earth's plentiful ecosystems. Nitrogen by itself is actually quite inert (does not react), so it has to be converted into forms that organisms can make use of, such as ammonium (NH4).
But before we get into the nitty gritty, let's define a biogeochemical cycle.
A biogeochemical cycle is a process where chemical elements or molecules move throughout the earth essentially recycling the element/molecule that goes through the cycle. Once a cycle starts, it eventually returns to it's starting position, completing a circle wherein the element/molecule reverts to the form it began in. If we take apart the name, we find that biogeochemical cycles involve biological, geological, and chemical factors. The nitrogen cycle is a special kind of biogeochemical cycle called a nutrient cycle. This type of cycle moves essential elements between both living and nonliving matter. An an example, an animal takes in nitrogen, then expels it into the environment, where it eventually makes its way back into another animal.
We'll begin nitrogen's journey in the atmosphere, but remember, this is a cycle. You could start or end at any point, although the atmosphere is likely where the cycle began in the first place.
Where it Occurs:
Everywhere! The nitrogen cycle is a vital portion of the world's ecosystem, just as important as the oxygen, carbon, phosphorus, and water cycles. As a cycle, it moves throughout nearly everything on the planet. It happens in plants, animals, bacteria, the atmosphere, water, anywhere you can imagine!
In fact, the water cycle is one of the few cycles involving a molecule instead of just a single element.
Take a deep breath. Feel all that oxygen flowing into your lungs? Well you shouldn't, because actually, about 80% of what you just inhaled is nitrogen! That's right, almost 80% of the entire world's atmosphere is nitrogen, which makes it a pretty important element, huh?
Nitrogen, which generally comes in pairs, hence the "2" in N2, exists as a gas in the atmosphere. The problem is, most organisms can't actually use nitrogen gas for any biological functions that keep them alive! And what about all that wonderful nitrogen you just inhaled? Well that went right out when you exhaled. So how do we actually get our nitrogen? In order for humans and really anything else to use nitrogen, it has to be altered to a different form.
Psst. Don't forget, while most diazotrophs are bacteria, some archaea are too! What's an archaea, you ask? Check the Terms to Know list at the bottom of the page!
In order to use atmospheric nitrogen, organisms must first "fix" it into a more usable form. And who can we thank for fixing our broken nitrogen? Why, bacteria of course!
Precipitation (rain, snow, etc...) deposits atmospheric nitrogen into the soil, where bacteria known as diazotrophs work their magic. These diazotrophs contain an enzyme called mo-nitrogenase that allows them to combine one nitrogen atom with either three or four hydrogen atoms to create ammonia (NH3) or ammonium (NH4+). The diazotrophs, which can live freely or with another organism in a symbiotic relationship, can then convert ammonia and ammonium into organic compounds essential for their survival. Many diazotrophs undergo symbiotic relationships with plants, such as legumes. This allows them to exchange their ammonia or ammonium for the plant's nutrients, such as carbohydrates. In this way, usable nitrogen is passed to plants.
Hint: It's also good to know that lightning can actually fix nitrogen as well. The enormous energy from lighting is enough to split a pair of nitrogen atoms, allowing the atoms to form nitrites. However, this method of fixation is relatively rare.
Nitrification is a two-step process that converts ammonium first into nitrites (NO2-) and second into nitrates (NO3-) so that nitrogen can be easily absorbed by plant roots. More helpful bacteria, such as Nitrosomonas carry out this process. These bacteria are known as nitrifying bacteria, because they are able to remove the four hydrogens of ammonium and replace them with two oxygen atoms, transforming ammonium into nitrite. Other nitrifying bacteria, such as Nitrobacter, add another oxygen to nitrite to create nitrate. It's important that nitrites become nitrates, because nitrites are toxic to plants. By the way, most nitrifying bacteria live freely in the soil instead of symbiotically with plants.
So What's the Point?
Obtaining usable nitrogen is crucial to building many biological structures, including amino acids, which make protein, DNA, and RNA.
Assimilation is basically how usable nitrogen ends up in different organisms. For example, plants can absorb ammonium and nitrates through their roots/ The plants can then extract nitrogen from the ammonium and nitrates, assimilating the usable nitrogen into their cells for use in biological functions.
Now remember how 80% of the air we breathe is nitrogen, but we can't use any of it? Well, because of plants and bacteria, we can! Humans and other animals obtain their nitrogen through assimilation as well. The difference is, while plants absorb the ammonium and nitrates directly from the soil, animals obtain their nitrogen by eating the plants. Standard food chain, you see! Almost all the nitrogen found used in animals can be traced to eating nitrogen-rich plant life.
When animals expel the nitrogen they have consumed or die, the cycle continues by converting the nitrates back into ammonium, hence, ammonification. Animals expel their nitrogen as organic nitrogen through waste, or as their body decomposes after death. Special types of organisms called decomposers break this organic nitrogen down into ammonium, which can then be used in nitrification once more. This means that ammonification can occur before or after nitrification. Many decomposers are fungi, such as mushrooms, and bacteria.
So now that plants, animals, and bacteria have gotten their fill of nitrogen, what happens to the rest of the nitrates? How do we come full circle from atmospheric nitrogen? The answer, simply enough, is that the nitrates turn back into atmospheric nitrogen through a process called denitrification. This process involves helpful denitrifying bacteria, who pretty much reverse the process that nitrifying bacteria go through, converting nitrates into nitrogen gas and releasing it into the atmosphere, thus completing the cycle.
Hint: denitrification occurs under anaerobic conditions, which means it can take place without oxygen.
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The Nitrogen Cycle in Water:
The nitrogen cycle occurs even in the ocean, and plays just as vital a role in the water as it does on land. The main cycle is very similar in the water, but there are a few key differences.
- Nitrogen enters the ocean through precipitation as well, but also through runoff or simply from the atmosphere.
- special bacteria called cyanobacteria fix the nitrogen.
- nitrification is carried out my phytoplankton.
- The water movement causes the movement of nitrogen throughout the ocean, which means nitrogen is not evenly distributed throughout the ocean.
How Do Humans Impact the Nitrogen Cycle?
Human activity has had a drastic effect on the nitrogen cycle in many way. For instance, humans use nitrogen in fertilizers since it's such an essential nutrient for plant life. These chemicals , along with those from pollution by vehicles, industrial facilities, etc... have more than doubled the amount of nitrogen that is annually converted into usual forms. Sound great, right? More usable nitrogen sounds like a fantastic idea! The problem is, the more nitrogen that is converted into organic forms, the more of that nitrogen ends up in places it shouldn't naturally be. Ammonia can runoff into the water, causing eutrophication. Ammonia can also end up in the atmosphere, where it is a leading cause of acid rain. Nitrogen can also return to the atmosphere in the form of nitrous oxide (N2O). Large quantities of nitrous oxide from human activity is the third largest contributor to global warming. Guess it's not such a good thing after all!
For more information, visit the Knowledge Project's info page about the nitrogen cycle.
Terms to Know:
Ammonification: The production of ammonium from the decomposition of organic matter; carried out by decomposers.
Archaea: single celled organisms that differ from bacteria in their metabolic processes; generally live in extreme conditions.
Assimilation: In the nitrogen cycle, the absorption of organic nitrogen by plants and animals.
Bacteria: Single celld organisms that differ from archaea in their metabolic processes; the most common organisms on the planet.
Decomposer: An organism that breaks down organic material.
Denitrification: The process in which bacteria form aatmospheric nitrogen (nitrogen gas) from nitrates.
Diazotroph: Bacteria (and some archaea) that fix nitrogen into a usable form
Enzyme: biological molecules that catalyze, or increase the speed of, biological reactions. Note that enzymes will not cause a reaction to take place if it wouldn't normally, it only causes the reaction to go faster.
Eutrophication: a process where an abundance of nutrients in water causes plantlife (such as algae) to grow excessively, which in turn causes the plants to use much of the oxygen, killing other organisms in the water.
Nitrification: The process in which bacteria in soil and water form nitrites and nitrates from ammonia and ammonium.
Nitrogen Fixation: the conversion of atmospheric nitrogen (nitrogen gas) is converted to ammonia and ammonium.
Symbiotic: a mutual relationship between two organisms wherein each organism provides a benefit to the other.