So, I recently watched a video regarding the first synthesized fertilizer, and I thought it was pretty interesting that farmland was basically screwed if they didn’t figure out how to get more nitrogen into dirt.
But then I thought about it, I was taught that plants make the bulk of themselves out of carbon, which they absorb from the air in carbon dioxide. Why is the same not true with nitrogen? Our atmosphere is a little more than 2/3rds nitrogen after all.
I tried looking it up, but the result was basically “Because nitrogen in the atmosphere is in a gaseous form” but that wasn’t really helpful.
In: 91
I think this is the first time I’ve seen an ELI5 on the front page where all the answers are wrong!
Yes, atmospheric nitrogen takes a lot of energy to convert, but plants already provide all of that energy to their bacterial and fungal friends in the soil. So that’s not the reason.
The real issue is that photosynthesis and nitrogen fixation are incompatible because of oxygen. Photosynthesis makes oxygen molecules and oxygen molecules destroy the machinery that turns atmospheric nitrogen into usable forms.
There are organisms that do both photosynthesis and nitrogen fixation, but they don’t do it in the same place or at the same time. Some photosynthetic bacteria (like Anabaena) make special cells called heterocysts that fix nitrogen but don’t photosynthesize, while others (like Cyanothece) photosynthesize during the day, then fix nitrogen during the night.
When photosynthesis and nitrogen fixation were first evolving, this incompatibility wasn’t an issue because there wasn’t molecular oxygen in the atmosphere to any appreciable degree. Photosynthesis created it all. Once it had, created an environment that made nitrogen fixation much more challenging.
As u/writingtherongs pointed out, plants COULD have evolved a separate compartment to fix nitrogen in, but that just hasn’t happened in the last ~3 billion years. They’ve always relied on other organisms to provide their fixed nitrogen.
Alfalfa fixes nitrogen through bacterial action – nitrogen fixing bacteria. That’s why crop rotation is done. It adds nitrogen to the soil and feeds the cattle.
Nitrogen in the air is found in a molecule that is like a coconut. Without a tool like a hammer (or the chemical equivalent, which plants don’t have) it’s really hard to break open and get to the good stuff. Nitrogen in the ground is found in a molecule that is like a banana. It’s easy to peel and get to the good stuff.
Nitrogen gas really likes being a gas. You have to put a lot of energy into breaking it apart. Like lightning strike levels of energy. Indeed lightning strikes are responsible for turning a significant amount of nitrogen into ammonia. But, lightning is not the main source of usable nitrogen. Aside from human production of ammonia for fertilizer, bacteria are main fixers of nitrogen.
There are a family of enzymes ([Nitrogenase](https://en.wikipedia.org/wiki/Nitrogenase)) that can be used to reduce the amount of energy required to turn nitrogen gas into something useable by plants. However, this enzyme breaks down in the presence of oxygen. Unlike nitrogen, oxygen gas isn’t particularly happy being a gas all itself and will react with nitrogen compounds quite readily (fertilizer is known for being explosive, and the N in TNT stands for nitrogen). Plants are heavily involved with oxygen, either producing it during the day or consuming it at night. If plants tried to produce this enzyme for themselves it would rapidly break down in the presence of the oxygen being produced by photosynthesis.
As such, nitrogen fixation is left to the bacterial specialists.
I’m sure someone gave you the answer so I want to offer an additional piece of information. Many Nitrogen fixing plants like legumes can get Nitrogen from the air. They have a relationship with bacteria that help fix the nitrogen for them so they can use it. I use them for cover crops and them being able to fix nitrogen is the reason they make such a great cover crop.
So you can think it like this, whenever you have a job you
1) DIY- if it’s a difficult job, one you don’t know, the inertia is more, so you think of alternates than doing it yourself. Breaking N2 Bonds are high energy and skill tasks so Plants try and figure out other ways.
2) Leave it to the forces of nature and do some praying – By gods grace nature is there to help in form of Bacteria, all you have do if offer them sugar as offering and they will fulfill you wishes albeit slowly and steadily. No hurry here…
4) Hire a Professional – sometimes you can’t just rely on nature so you hire professional people to fulfill your needs… AKA leguminous plants… They hire, feed and shelter bacteria and ask them to do their jobs.
4) Buy readymade – A.K.A Fertilizers by humans- .plants are relaxed but we humans are greedy we want all and we want now and the result is Fertilizers.
Hope it helps.
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