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You_Stole_My_Hot_Dog t1_jcsbaxb wrote

Remember the first rule of evolution (or at least should be the first rule): evolution does not perfect things, it just finds solutions that are good enough.

With that being said, plants simply can’t use nitrogen in its gaseous form. Instead, they can only use ammonium or nitrates. These are continuously produced by bacteria in the soil and taken up by plants through their roots (if you’re curious, look up “nitrogen fixation cycle”).

In an ideal world, plants would just be able to take nitrogen straight out of the air, convert it for their needs, and never worry about having enough. But like it said, evolution uses whatever works. What likely happened was that as plants were first evolving, there were already plenty of bacteria fixing the nitrogen in the soil. So there was no selection pressure to evolve a new nitrogen-obtaining mechanism, when they could simply uptake it in the roots with the rest of the water/minerals they were up taking. Simple, and it works. Not perfect, but good enough.

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RadRiverOtter t1_jcseb4z wrote

Just adding a little to this. Nitrogen is often the limiting factor in an ecosystem. Certain plants, mostly Legumes, evolved to form a symbiosis with rhyzoba bacteria that are able to convert gaseous nitrogen into usable NH3. These plants are generally "pioneer species"; popping up in disturbed or nitrogen deficient regions. Once these plants finish their life cycle and break down they increase the overall usable nitrogen in the ecosystem. More fascinating are the incredible traits that many taxa have evolved in adaptation to nitrogen deficient soils that don't use rhyzoba. One example being the several convergent carnivorous plants. Or plants that trick animals into defecating on them.

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Allfunandgaymes t1_jcy0ejj wrote

Yep! As a gardener, I frequently sow sweet peas and vetch (both legumes) into my lawn and unused beds to generate nitrogen. I simply let these plants decompose in place once the season is over. This is how you build healthy soil, in addition to the sporadic addition of compost.

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nicknoxx t1_jcspmj7 wrote

Nitrogen fixing plants can use gaseous nitrogen and deposit it in the soil.

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BloodshotPizzaBox t1_jctm11i wrote

I assume that a big factor here is that the nitrogen in the air isn't "freely available" in the same sense that it is in nitrates in the soil. It's stuck to other nitrogen atoms, and N2 has a huge bond energy compared to the nitrogen-hydrogen bond in stuff like ammonium ions.(To digress, the fact that nitrogen atoms so desperately wants to bond with other nitrogen atoms is what makes nitrates so reactive in things like explosives. The formation of N2 releasing a lot of energy is the flip side of breaking N2 requiring a lot of energy.)

As to the reason why plants need nitrogen (the element as opposed to the gas) in the first place: it's a major component of chlorophyll. I mean, probably other reasons, but definitely that one.

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