A Love Letter to A. P. Environmental Science Students

It's getting close to Halloween, and the scariest thing for most APES students is the nitrogen cycle, so why don't we tackle that today.  The dreaded nitrogen cycle strikes fear into the hearts of Environmental Science students everywhere because it's so difficult and so necessary to know.  But just like vampires, mummies and werewolves, it gets more interesting the more you understand it (though it still might scare the you-know-what out of you!).  Don't fear - you can do this!

Do you know why I put a picture of this Chicago bean tree (actually a honey locust) here?

First of all: what is nitrogen and why do we care so much about it?  Living things on earth are mostly made of the atoms carbon, hydrogen and oxygen, but proteins need a little nitrogen.  No living thing can live without proteins, therefore nitrogen is necessary for life.  Nitrogen, however, is in short supply, at least in a usable form.  If you think of atoms like Legos, each atom would be a different type of Lego.  Atoms can bond together to make larger structures like proteins, carbohydrates, DNA and oils, which then add up to whole organisms, just like Legos can be put together to make bigger things.  Nitrogen atoms are the special Lego pieces that are rare but that you can't build anything without.  Animals get all their nitrogen from eating protein in food.  When you eat food, your body takes apart the food's Legos (atoms) so that it can put them back together again into the molecules you need (or your body burns the Legos for energy, but the analogy kind of breaks down there).

Usable form?  Take a big, deep breath - it will help calm you down, plus it demonstrates the next concept.  What you just inhaled (air, hopefully) is 78% nitrogen.  OK, exhale.  You just exhaled every bit of nitrogen that you had inhaled, absorbing none of it.  Nitrogen in the air is in the form of N2 (I don't have subscript), or two nitrogen atoms double-bonded to each other.  There is actually a triple covalent bond holding two nitrogen atoms together in N2, which makes them nearly impossible to get apart.  Think of identical Lego pieces so hopelessly stuck together that you have to go get a kitchen knife to separate them.  Your lungs don't have the biological equivalent kitchen knives, so they can't break apart the nitrogen atoms to use them, and all the nitrogen you inhale is useless to you (except that it makes our atmosphere very stable).

The Kitchen Knife Monopoly:  Nature runs a tight kitchen, and it won't let you play with knives.  Only specific soil bacteria are allowed to have knives.  By which I mean only bacteria can fix nitrogen from it's stuck form to separate, useful atoms.  The separate atoms are quickly bonded to hydrogens to make ammonia (NH3).  The name of this process is nitrogen fixation.  It is thought that the ability to fix nitrogen evolved once on this planet when bacteria were pretty much the only critters here.  Everyone else after that found that it was easier to trade with bacteria than to evolve their own way of fixing nitrogen.  It is extremely difficult to fix nitrogen because a lot of energy is required to break the triple bond between two nitrogen atoms.  Bacteria expend a lot of ATP to break the nitrogen, but no one else is going to do it for them, so they just get on with it.

Wheeling and Dealing:  Anyone else who wants fixed nitrogen has to get it from bacteria.  They can wait for the bacteria's waste and absorb that straight from the soil in the form of ammonia, but bacteria aren't very wasteful, and there isn't much nitrogen available this way.  Some organisms have found a better way to get nitrogen: they can strike a deal with the soil bacteria.  Many plants, especially legume-type plants, have a mutualistic symbiosis with the soil bacteria.  Plants provide the ATP and some oxygen in exchange for lots of ammonia.  Legumes even provide specialized little lumps (nodules) on their roots to house the bacteria. 

What About Us?  As I said earlier, we get all our nitrogen from food.  We need protein in our diet because of the nitrogen in protein.  Think of foods that are high in protein.  Did you say meat and eggs?  You're right, but think of vegetarian foods that are high in protein.  Hopefully you said beans, peanut butter and tofu.  When you think about it, you realize that all these foods are legume-type plants, which makes sense because legumes are able to get more nitrogen from housing nitrogen-fixing bacteria in their roots.

Random picture of black beans!

But that's not all is it?  Sorry, no.  When we learn about Environmental Science, we always learn about the full cycle of things - externalities, life-cycle costs, recycling, etc.  We have to finish the story.

Energy From Nitrogen?  Since it takes so much energy to break N2 apart (a reduction reaction for you chemistry folks), it would follow that energy can be had from getting those two nitrogens back together.  In fact, that's just what TNT and fertilizer bombs do is allow a large quantity of nitrogens to reunite - boom!  Bacteria in the soil can take advantage of single-nitrogen molecules and get some energy from oxidizing them in two different ways.  The first way is called nitrification, in which bacteria take ammonia molecules and make nitrite, then nitrate ions for energy.  Extra nitrate ions left in the soil from this process are very easy for plants to absorb and use.  You can think of this as half-oxidizing the nitrogens (sorry, chemistry teachers), and releasing some of the available energy.  Fully oxidizing two nitrogen compounds results in the production of N2, releasing more energy and returning the nitrogen to that useless gas molecule in the atmosphere.  (We just came full circle!)

Still Not Done:  It helps to know the following trivia about the nitrogen cycle too.  (1) Animal waste and dead organisms contain a lot of nitrogen.  Soil bacteria break down that waste and release the nitrogen into the soil as ammonia (in a process called ammonification), which is available for plants to use.  You may have noticed this if you have a dog that pees in spots around the yard that result in greener grass patches.  If your dog always pees in the same spot, you no doubt notice that too much ammonia is toxic.  (2) Agriculture results in nitrogen being absorbed from the soil into plants, then plants being carted away to markets, along with the soil nitrogen (where did all the good Legos go?).  Soil nitrogen must be replenished with chemical fertilizer, manure, growth of legumes or other fertilizer.  (3) Too much nitrogen in a body of water, say from fertilizer runoff or sewage overflows, can cause the plants to overgrow, choke out the sunlight and cause lower-down organisms to die, decompose and use up all the oxygen.  Then everything else in the water suffocates and dies.  This is called eutrophication and is worth understanding very solidly.  (4) Some single-nitrogen compounds can be made from the energy of lightning separating N2 molecules.  Also, humans use the Haber Process the break the N2 bonds to make ammonia for making bombs and fertilizer.  The Haber process uses lots of energy to split the N2.  It uses N2 from the air but gets hydrogens from natural gas (CH4), so making fertilizer is a very fossil-fuel-intensive process.