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It is my (very basic) understanding that neither plants nor animals utilize the nitrogen in the atmosphere. Humans do not make use of atmospheric nitrogen through respiration and plants do not extract nitrogen from the air, but rather from the soil. First of all, am I correct in this understanding?

If I'm right so far, then what role (if any) does the nitrogen in our atmosphere play, biochemically speaking?

I understand that it plays a significant physical role, contributing to air pressure, allowing light to permeate, allowing liquids to exist on the surface, burning up incoming meteors thus protecting life, and basically being a physical gas that is not oxygen or carbon dioxide thus keeping the concentration of those gasses low. But I'm interested in the biochemical use of atmospheric nitrogen if any. So, is nitrogen a necessary atmospheric component for life, in terms of its chemical reactions with living things? Or is the atmospheric nitrogen essentially unused in the chemistry of life?

Wyck
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    Nitrogen is a necessary part of amino acids which are building blocks for proteins. Animals get this nitrogen from eating plants, plants get them from symbiotic microorganisms and those germs get nitrogen from the atmosphere. So we do need atmospheric nitrogen in order to make meat. – slebetman Nov 16 '22 at 00:06
  • https://en.wikipedia.org/wiki/Nitrogen_fixation – Maximilian Press Nov 17 '22 at 20:22
  • @slebetman. Nicely put, but there is also the Haber process (industrial N2 fixation) without which agriculture as we know it would be very different. As the quoted wikpedia article puts it: "With average crop yields remaining at the 1900 level the crop harvest in the year 2000 would have required nearly four times more land and the cultivated area would have claimed nearly half of all ice-free continents, rather than under 15% of the total land area that is required today" – user338907 Nov 18 '22 at 19:09

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To get soil nitrogen in the first place, nitrogen fixation is necessary which takes atmospheric N2 and converts it into biologically useful forms.

Nitrogen fixation is performed by bacteria and archaea. You may occasionally hear about nitrogen fixing plants, especially peas and beans in an agricultural context, but these involve symbiotic relationships with bacteria that do the actual work.

Bryan Krause
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    There could be a planet with life based on non-atmospheric nitrogen compounds - but I suppose life would eventually "realize" it could get a whole bunch of energy by "burning" them to get gaseous nitrogen, thus filling the atmosphere with it? – user253751 Nov 15 '22 at 05:17
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    @user253751 I haven't yet found any other planets with life so I'm stuck with this one for observations about how these things work. – Bryan Krause Nov 15 '22 at 05:57
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    @user253751: No, the reduction of nitrates to nitrogen and oxygen is endothermic and does not gain energy by itself. But some lifeforms can use nitrate to oxydise something else and gain a bit of energy this way. – Sir Cornflakes Nov 15 '22 at 10:11
  • @jk-ReinstateMonica And yet, life does finds a way to burn non-atmospheric nitrogen compounds to get energy and nitrogen gas: anammox. https://en.wikipedia.org/wiki/Anammox One can, however, imagine an anaerobic, hydrogen-rich world where nitrogen is mostly in the form of ammonia/ammonium, and perhaps there's no energetically favorable reason to transform it to N2. In such a world, you'd be unlikely to come across nitrate, and if an organism did find or generate some, it would be more energetically rewarding to use it to oxidize hydrogen to form water and ammonia. – Brendan Furneaux Nov 15 '22 at 13:43
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    @BrendanFurneaux: Ammonium Nitrate is a quite unusual compound (even an explosive) consiting of reduced and oxiydised nitrogen in one salt. Of course you can use nitrate to oxydise ammonium and gain energy, this is no contradiction to my previous comment. – Sir Cornflakes Nov 15 '22 at 14:20
  • I'm surprised: Are you saying that without life, there would not be a substantial amount of nitrates on Earth? (I'm not skeptical but merely ignorant ;-). ) – Peter - Reinstate Monica Nov 17 '22 at 16:40
  • @Peter-ReinstateMonica There are other sources, like lightning, and nitrates for fertilizer are made artificially using Haber–Bosch, but yes my understanding is that most nitrate on Earth has a biological origin, though it's also used by biology, so it's quite possible there'd be as much or more nitrate from other sources if there was no biology - this I'm not sure about. – Bryan Krause Nov 17 '22 at 16:54
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    @BryanKrause Nitrate (dissolved in water, not as concentrated nitric acid of course) is thermodynamically favored compared to a blend of oxygen and nitrogen gases in the atmosphere, so in the absence of life and over geological time, we would expect all of the oxygen in the atmosphere to react with nitrogen to form nitrate. However, I expect the nitrate itself would further react to oxidize other compounds dissolved in the ocean. But the elephant in the room is that the free oxygen was generated by life, and most/all nitrate on Earth is generated by a reaction involving free oxygen. – Brendan Furneaux Nov 18 '22 at 07:25
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    @jk-ReinstateMonica Your statement started with "No", in response to user253751' s question about whether life would "burn" non-atmospheric nitrogen compounds to form nitrogen gas; I provided an example of that actually happening, which is furthermore a major part of the nitrogen cycle on Earth. You focused specifically on nitrate, rather than ammonium, but that's not what the original comment asked about. As for ammonium nitrate being an unusual compound -- as a pure salt, sure. But it's not so strange to find the two ions dissolved in the same water. – Brendan Furneaux Nov 18 '22 at 07:32
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    @BrendanFurneaux Yes, that's a pretty big elephant. I'm a bit suspicious over whether geological time is really long enough, though. Yes, conversion to nitrates is energetically favorable, but breaking that triple nitrogen bond is not trivial. Biological fixation of nitrogen goes through a reducing pathway even, not oxidation, just like with the Haber process. Without knowing too much more about the detailed chemistry, I'm a bit skeptical that the oxidation pathway you propose would occur fast enough over geological time, even with a high (i.e., modern) oxygen atmosphere. – Bryan Krause Nov 18 '22 at 16:21
  • @user338907: By nitrogen I mean elementary nitrogen N$_2$, indeed. – Sir Cornflakes Nov 18 '22 at 20:20
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    @BryanKrause I was expecting things like lightning to be part of the mechanism over geological time; but you are probably right that it would be an extremely slow process. Chances are, in the absence of photosynthesis to replenish it, the atmospheric oxygen would be depleted in many other ways before it had a chance to oxidize much of the nitrogen. – Brendan Furneaux Nov 21 '22 at 14:26
  • Also, birds poop (en masse) was historically a good source for nitrogen rich fertilizer. Like others say, there was historically a big problem gleaming on the horizon about acquiring it and being able to feed quickly increasing population of people, before Fritz Haber (quite controversial character) made a breakthrough! To dive deeper into the whole story, I very recommend this youtube video by Veritasium: "The Man Who Killed Millions and Saved Billions" https://www.youtube.com/watch?v=EvknN89JoWo – Kusavil May 02 '23 at 20:36
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While atmospheric nitrogen is definitely necessary for keeping up the ecosystem of Earth as we know it (as Brian Krause has already pointed out), it is not really a necessity for some life form on earth, especially among the extromophiles species and even complete ecosystems can be found that do not rely on atmospheric nitrogen at all.

Atmospheric nitrogen also helps keeping up the atmospheric pressure, without any nitrogen in the atmosphere, water would boil at about 27°C.

Sir Cornflakes
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    Could you explain how the extremophiles obtain the nitrogen atoms in amino acids etc. if it is not ultimately from molecular nitrogen. At present your answer is just an assertion and for the large number of users who are familiar with extremophiles, the Wikipedia link does not clarify the matter. – David Nov 15 '22 at 13:01
  • They can be taken from nitrate which is readily accessible and the most natural form for nitrogen in the lithosphere. – Sir Cornflakes Nov 15 '22 at 14:22
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    And how does the nitrate arise? – David Nov 15 '22 at 15:50
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    Well, the nitrogen has to be somewhere, and under oxydising conditions nitrate is its stable form. Under reducing conditions it would be ammoniak. Elementary nitrogen is only metastable under either condition. Without biological activity, it vanishes rather quickly. – Sir Cornflakes Nov 15 '22 at 16:14
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You are kindof right, Atmospheric N2 is bound with a triple covalent bond, which is the strongest of all simple diatomic molecules.

If you have a vivarium with helium/argon breathing gas and the soil has a ready supply of dissolved nitrogen for fixation into NH3, then the animals and plants will live just fine for a very long time, but many plants that fix N2 that depend on nitrogen fixation symbiosis may fare worse.

N2 is nearly not soluble in rainwater, so ultimately, all plants and animals that depend on nitrogen in the soil require atmospheric N2.

If there was no airborn N2, the forests would be in trouble because they would yellow and have diminishing reserves of nitrates and nitrites...

Nearly all the biospheres would perish unless you keep on fertilizing the soils, them most plant species would be virtually ok growing hydroponically in an atmosphere without N2.

bandybabboon
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