It's been a while since we touched on the Earth's biogeochemical cycles.  This time we're going to go over one that tends to be easily overlooked.  We're pretty familiar with water and to a degree we even deal with the carbon cycle as a major planetary concern.  This time, though, let's zone in on nitrogen.  This super abundant element is always right at our fingertips, but only a fraction of life's chemical makeup is nitrogen.  While it may make up a small part of us, it is absolutely essential to all living things.

Let's dive in!

By Wiskirchensl - Own work, CC BY-SA 4.0, Link

Why do we need nitrogen?

The human body is composed of a number of elements.  We're mostly water, carbon, and hydrogen, but nitrogen plays a key role in our makeup.  It makes up about 3% of our body composition and is the backbone to our genetics.  Nitrogen is necessary to create amino acids that compose the proteins we run on as well as the nucleic acids that define our genes.  Suffice to say, without nitrogen, life as we know it would not exist.  Without nitrogen, we cannot grow nor can we reproduce.​

Where is nitrogen?

Okay, so it's pretty important.  But at the top of this page, we mentioned that it's all around us.  Why would we spend so much time thinking about where it comes from?​

Nitrogen gas makes up about 78% of our atmosphere, but most living things can't actually use it in this form

While it's true that nitrogen is all around us (it makes up roughly 78% of Earth's atmosphere), our atmospheric nitrogen is unusable by humans.  In fact, it's unusable by most life on Earth.  We do get nitrogen from the atmosphere, it just finds its way to us by way of, you guessed it, a biogeochemical cycle!

Bacteria in our soils can infuse hydrogen into nitrogen gas to weaken the bond and break it down.

​Thanks bacteria!

Nitrogen fixation

So we have this air brimming with nitrogen gas, and we can breathe it in all day, but our bodies can't use it in this state.  The way nitrogen gas exists, it is bound too tightly.  So tightly that, as mentioned above, most living things can't break it.  Luckily we have some unlikely friends on this planet, and they live underground: soil bacteria!

This may sound like a stretch of the imagination, but, really, all of our cycles truly depend on the ecosystems of this planet all working together to ensure life is possible.  There are bacteria in our soils that are able to take the nitrogen gas they come in contact with and push a bit of hydrogen into it.  This morphs the nitrogen gas into ammonium.  This process is known as nitrogen fixation, and it's primarily because of this phenomena that plants are able to pick up the cycle from here.​

We should say, one of two things can happen from here.  The bacteria can further break down the ammonium into nitrates which it uses to power itself, and plants will take some of those nitrates to reduce to nitrite ions for their growth.  Alternatively, plants with ammonium-compatible root bacteria can break down the ammonium themselves.  In both of these cases, it's the nitrate form of nitrogen that plants finally work with to construct their amino and nucleic acids.​

How nitrogen moves

It likely comes as no surprise how the rest of the planet gets its nitrogen: eating plants.  Or eating things that eat plants.  Since the nitrogen is already constructed into the acids life is accustomed to working with, we're all able to benefit from that construction simply by ingesting the finished project.  Thankfully, for us humans, getting this element is as easy as eating our vegetables.

From here, nitrogen is expelled from the organism that is housing it.  This can be by way of urination, death, etc.  Once the waste reaches the soil, it is again bacteria that is capable of taking nitrogen in its waste state and converting it back to nitrogen gas by way of a process called denitrification.  This is generally done in oxygen-deprived areas, where the nitrogen waste can substitute oxygen in the bacteria's respiratory functions.  They essentially exhale nitrogen back into the atmosphere.​

Nitrogen is a key ingredient in the creation of amino and nucleic acids that all Earth life needs. Thankfully plants handle that part, we just need to eat them!

And that's about it!  This may sound like a fairly closed loop, but it's also important to know how we, as humans, have come in contact with this cycle (because of course we do).  When humans discovered that nitrogen was key in plant growth, we naturally wanted to harness it to expedite that process.  What do we use to this day to help plants grow?  Oh right, fertilizer.​

Nitrogen-rich fertilizers help our agriculture, but have massive effects on the environments around them too

Ways we impact the nitrogen cycle

That's right, one of fertilizer's key components is nitrogen in the form of nitrates ready for plants to use.  Unfortunately, overuse of fertilizer leads to an abundance of nitrogen runoff into our soils, groundwater, and even our oceans, impacting each a bit differently.

In our soils, the high volume of nitrates ends up being confusing to plants as well as the bacteria that would typically break down nitrogen gas into this form to foster its own growth.  This can lead to changes in plant behavior as well as an over-abundance of ammonia and ammonium in the soil which can actually be toxic to plants at high levels.  In our groundwater, this impacts humans more directly, as our bodies aren't accustomed to nitrogen in the nitrate form.  We're used to having it ready-made into the acids we need. This nitrogen-infused water making it into our bodies can start reactions within us which can result in a few notable types of cancer.​

And while we didn't get too much into how marine ecosystems interact with this cycle, they too use nitrogen from the atmosphere and break it down in similar ways.  When our fertilizers runoff into the ocean, they can result in ocean acidification.  One of the clearest examples of this is the oceanic dead zone at the mouth of the Mississippi River, which is the final destination for a ton of agricultural waste in the Midwestern United States.  The excessive amounts of nitrogen-rich fertilizer hitting the ocean created an algal bloom-induced hypoxia, or extremely oxygen-deprived marine environment.  Nothing is capable of living in this area.  Nothing.

While this is alarming, understanding how much the balance our planet impacts life on it is really important.  Little tilts in that balance can cause a ripple effect that we, and all life on Earth, have to deal with for generations.  Being able to recognize that we're consumers of these cycles is key.  We directly benefit from these environmental functions and seeing that can help us move forward in ways that let us keep being a part of this wonderful planet for eons to come.​

~ And, as always, don't forget to keep wondering ~

Sources
* Nitrogen Stats:
Cox, Arthur N., ed. (2000), Allen's Astrophysical Quantities (Fourth ed.), AIP Press, pp. 258–259, ISBN 0-387-98746-0

* Nitrogen Fixation:
Postgate, J. (1998). Nitrogen Fixation (3rd ed.). Cambridge: Cambridge University Press.

* Mississippi River Dead Zone:
"Dead Zone | 1 Mississippi". 1Mississippi.Org, 2020, https://1mississippi.org/dead-zone/. Accessed 14 June 2020.​

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