eli5: Nitrogen and Nirates


Is there a difference between Nitrogen and Nitrates? Why would I use one over the other when discussing water quality, fertilizers, or agriculture?

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Nitrates are the most raw organically accessible form of nitrogen. Pure nitrogen exists in the atmosphere as a gas and can only be processed by some organisms in the food network. Fertilizers I’d assume use nitrates in their composition.

Nitrogen is an element, it exists in very high quantities in the atmosphere as a gas bonded to itself. We’d call that “Diatomic Nitrogen” and write it down as N2 for short. It’s worth noting that in this form Nitrogen is very stable, meaning it does *not* like to participate in chemistry, you might as well ignore it.

However, living things need nitrogen to live, we use it in the chemistry of our bodies. So how do we “Get” the nitrogen if it’s out there doing everything it can to avoid interacting with us?

You need something to use energy to break apart the N2 in the back into individual nitrogen atoms and then *force* into new molecules that *do* want to work with us chemically. You can get an idea of just how important this ability is for living things, it’s *critical*. Certain organisms, mostly microorganisms, have the ability to do this and we call them “nitrogen fixers” and these organisms have a niche in nature where they usually pair themselves up with plants, the plants give them food and they give the plants this transformed workable nitrogen in a new molecule format called “nitrates” and “nitrites”, those are fancy chemistry words for nitrogen that has been forced together with varying amounts of oxygen atoms. These compounds are very easy for plants to work with and create the chemistry they need to live.

So in our lives, *nitrogen* is usually not that important, it avoids us and we avoid it. However *nitrates and nitrites* are SUPER important for life to exist so we talk about them all the time. We discovered that plants *need* them to survive so we discovered chemical techniques to make them artificially and then put them into the soils to make our crops grow better, this is what we call fertilizer.

It’s important for water because a lot of the nitrites/nitrates *don’t* get taken by plants and run off into the water supply where they can eventually become free food for things like algae which “bloom” (reproduce like crazy) sucking up all the oxygen from the water which kills off the fish and other animals that now suffocate.

It’s important for human water because it’s part of water quality in general, high levels of these chemicals can either be indicators of water pollution (from run off or sewage) and indicate the likely presence of harmful microorganisms, or even just affect the water tastes or behaves in other industrial purposes like brewing beer.

Nitrogen gas is two nitrogen atoms bound tightly together. It takes so much energy to break that bond that there are very few processes in biology that can do it. Living things need bioavailable nitrogen compounds for things as basic as DNA and protein synthesis. This is *mostly* in the form of nitrate, but not exclusively.

For example, if you pee in a bucket and use it to fertilize your garden, the nitrogen is in the form of urea, which plants cannot absorb. Bacteria convert it to ammonia, which is a gas that is easily absorbed by water. Some plants can absorb ammonia directly, but most wait for bacteria to convert it to nitrates and nitrites. But those aren’t the only forms of nitrogen in the soil- earthworms contain a significant amount of bio-available nitrogen. You might purchase bloodmeal to fertilize your garden- dried blood from slaughterhouses. This is protein, a complex mixture of proteins, but it is equivalent to about 12% nitrogen by weight.

This is why it is proper to talk about how much nitrogen a biological system contains. It is technically referring to “fixed nitrogen”, but it is most common just to omit the word “fixed”. All of those biological systems exist at the bottom of an atmosphere with billions of tons of nitrogen gas, but it is so inert that it is irrelevant to most discussions of biological processes. If you just refer to nitrate, you’re talking about one specific chemical form. Talking about “nitrate” doesn’t include ammonia, which becomes nitrate in a few weeks, or urea, which becomes ammonia in a few days, or living flesh, which turns into those things when it dies.