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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?

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    $\begingroup$ 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. $\endgroup$
    – slebetman
    Commented Nov 16, 2022 at 0:06
  • $\begingroup$ en.wikipedia.org/wiki/Nitrogen_fixation $\endgroup$ Commented Nov 17, 2022 at 20:22
  • $\begingroup$ @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" $\endgroup$
    – user338907
    Commented Nov 18, 2022 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.

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    $\begingroup$ 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? $\endgroup$ Commented Nov 15, 2022 at 5:17
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    $\begingroup$ @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. $\endgroup$
    – Bryan Krause
    Commented Nov 15, 2022 at 5:57
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    $\begingroup$ @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. $\endgroup$ Commented Nov 15, 2022 at 10:11
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    $\begingroup$ @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. $\endgroup$ Commented Nov 15, 2022 at 14:20
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    $\begingroup$ @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. $\endgroup$ Commented Nov 18, 2022 at 7:25
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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.

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    $\begingroup$ 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. $\endgroup$
    – David
    Commented Nov 15, 2022 at 13:01
  • $\begingroup$ They can be taken from nitrate which is readily accessible and the most natural form for nitrogen in the lithosphere. $\endgroup$ Commented Nov 15, 2022 at 14:22
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    $\begingroup$ And how does the nitrate arise? $\endgroup$
    – David
    Commented Nov 15, 2022 at 15:50
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    $\begingroup$ 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. $\endgroup$ Commented Nov 15, 2022 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.

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