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Nitrogen Cycle

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The nitrogen cycle follows the movement of nitrogen in its different forms. Nitrogenous gas makes up 80% of the atmosphere and most nitrogen enters into the ecosystem via nitrogen-fixing bacteria, but many organisms still struggle with nitrogen availability because they cannot use it in that form. The process of making nitrogen accessible is called nitrogen fixation. Many forms of bacteria are capable of this. Many of these bacteria are found in the soil but some species have a symbiotic relationship with microbes which are capable of fixation. Other key processes include nitrification, denitrification, and ammonification.

Outside of the carbon, oxygen and hydrogen and one of the most important elements for living creatures is nitrogen. So the nitrogen cycle is vital for organisms, it's used and is a key component in amino acids, nucleic acids pretty much it's what we need and that's one of the reasons why in organic fertilizers that farmers put on their fields, ammonia is one of the most common elements in it which contains that nitrogen. Now 80% of our atmosphere is nitrogen and gas so you'd think well we can just get nitrogen by breathing it in. However nitrogen gas is useless in most cases I inhale a nitrogen gas I exhale the exact same amount of nitrogen gas. Some of it can get into my blood stream but not so much I can't absorb it, it's not in a useful form.

That's why we need this process called nitrogen fixation so most of the nitrogen that's in our ecosystem gets there via these nitrogen fixing bacteria. They convert that atmospheric nitrogen gas into ammonia and into ammonium the ion. Ammonia winds up volatizing as a gas if they don't use it and that's why when it hits water converts into ammonium. Now there's a number of other processes involved in the nitrogen cycle, one is called nitrification which is converting the ammonium into nitrites and then ultimately into nitrates NO3 minus. Denitrification is the reverse of this essentially it's converting that NO3 minus into ultimately nitrogen gas which is returned ultimately to the atmosphere.

Ammonification takes organic forms of nitrogen for example in amino acids or proteins and it converts that organic nitrogen into ammonium. Now let's take a look at this diagram over here which shows you the complexity of the nitrogen cycle. So here we see the atmospheric nitrogen, it gets incorporated by nitrogen fixing bacteria many of whom live root nodules in a symbiotic mutualistic relationship with plants like legumes like peanuts. Well there's other soil fixing, or nitrogen fixing soil bacteria that are producing more of this ammonia and converting through ammonification into NH4. Now plants can assimilate this ammonium and take it in and then we can it from the plants when we eat them. Decomposers can get it from plants when they decompose them.

Now nitrification however, is the most common way to incorporate nitrogen for the plants and that's done by again various bacteria in the soil that do this process of converting the ammonium into nitrites which are NO2 minus and ultimately into NO3 minus, and then through again the process of assimilation they're pulled in. Elsewhere in the soil you could have denitrifying bacteria that return the nitrogen to the atmosphere completing and restarting the nitrogen cycle.