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Plants must have nitrogen to grow. According to the answer to this question, there are no plants that can fix their own nitrogen (without the help of bacteria).

Plants get their nitrogen in the form of nitrates (NO3-) or ammonia (NH4+). Nitrates and ammonia get into the soil through

  • lightning (causing N2 and O2 to combine to form NO which then reacts with atmospheric water and is brought to earth by rain)
  • nitrogen-fixing bacteria associated with leguminous plants
  • fixed nitrogen from dead plants/animals (ammonification), urine, etc.
  • ammonifying and other bacteria

The answer also states that it seems to be possible to engineer plants to be able to fix nitrogen.

So one could think plants could have evolved to be able to fix nitrogen, but did not. Maybe it just did not happen based on it's probability to happen.
Is there any other explanation?

Maybe the ability to fix nitrogen would have a harmful effect on plants? Or fixation requires a complex set of related mutations? Are there enough areas with soil depleted of nitrates to exert a selection pressure for nitrogen-fixing plants?

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  • $\begingroup$ Edited your question a bit. Please feel free to roll back if you feel the edit detracts from your intent. :-) $\endgroup$
    – anongoodnurse
    Feb 22, 2015 at 6:17

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That is an interesting question. But rises some line of thought by analogy, e.g. why people won't evolve to produce their own ATP without mitochondria? (I mean of course Eukaryotas in general). I don't see any reason why multicellular organism cannot incorporate bacterial functionality either as happened with mitochondria, or as just separate cell type, that will provide desirable enzymatic reactions.

The answer, no matter how unsatisfactory, might be simple: because it so happened. We definitely can create experimental conditions lacking N-fixing bacteria as well as varying amounts of nitrates, nitrites etc. And then produce and screen line of mutants for ability to fix nitrogen without symbiotic partners.

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    $\begingroup$ I don't think it would be an unsatisfactory answer to conclude it could have happened either way, with some probability, and the random outcome was it did not - without any known mechanisms influencing the probability. $\endgroup$
    – Volker Siegel
    Feb 22, 2015 at 13:06
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    $\begingroup$ i feel that in biology there are many thing that are just are. but such an answer is always insufficient to my taste. for example, why DNA is winded that way and not opposite, or others. in case of nitrogen, maybe, it was easier for two different specialized organisms to live together rather than to one of them acquire some special traits. $\endgroup$
    – aaaaa says reinstate Monica
    Feb 22, 2015 at 13:11
  • $\begingroup$ DNA is winded in the Type B double helix because it is usually the lowest energy conformation available. There is almost always a good reason why something happens, it may not be known for nitrogen fixation, but the reason probably exists. See en.wikipedia.org/wiki/… $\endgroup$
    – March Ho
    Feb 22, 2015 at 15:16
  • $\begingroup$ I meant right- versus -left-handedness of the helix, or chirality of glucose (bacteria eats only D-Glu) $\endgroup$
    – aaaaa says reinstate Monica
    Feb 22, 2015 at 17:01
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The most obvious answer to your question is that the enzyme responsible for nitrogen fixation is inactivated by oxygen. So photosynthesis, which produces oxygen, rules out nitrogen fixation, and, as you know, plants photosynthesise.

However, they could have evolved specialized cells for nitrogen fixation, which do not photosynthesise. In fact, some colonial cyanobacteria, which photosynthesise, form such specialized cells called heterocysts.

So why haven't plants evolved such specialized cells? I think a fundamental answer to this question would invoke the evolutionary theory of specialization which says that specialization in one thing at the cost of the ability to do something else is selected for when the trade-off is convex, i.e. when you can be really good at only one thing, and being able to do both implies you are actually quite bad at both.

I hope this sets you off to find the fundamental reason you are looking for.

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    $\begingroup$ Interesting answer. Can you please add some references to it? $\endgroup$
    – Chris ♦
    Apr 1, 2015 at 17:49
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    $\begingroup$ Nitrogen fixation occurs in some species of Cyanobacteria, which would be impossible if your hypothesis is correct. $\endgroup$
    – March Ho
    Apr 1, 2015 at 18:15
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    $\begingroup$ @MarchHo, doesn't the second sentence of the second paragraph of my answer give an reason why it is possible? I.e they create anaerobic circumstances in specialized cells that do not photosynthesize, where N2 fixation takes place, thus separating the two processes? $\endgroup$
    – Daan
    Apr 2, 2015 at 11:14
  • $\begingroup$ @Chris, I was taught at uni about heterocysts and the inhibition of N fixation by O2. I don't have references ready for it. You can read about it at Wikipedia, but I wouldn't refer to Wikipedia to substantiate claims. The evolutionary theory I got from Adaptive Dynamics, see e.g. fig 2.2 of this book. If I have time I will add refs. $\endgroup$
    – Daan
    Apr 2, 2015 at 15:07
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I believe that evolution is driven by the needs of the organism to adapt not by the need to adapt for human beings consumption. meaning that some nitrogen gets back to the soil through the effect of nature e.g. fixation of nitrogen during storms and when fecal matter decomposes in soil and these I assume provide enough nitrogen for normal plant growth and so the plant is not "forced" to adapt to say a deficiency in nitrogen but for humans I assume because of agriculture the land gets "exhausted" and so we need fertilisers but plants have not evolved becuase I assume there was no pressing need for it. That is my own thinking assuming I understood your question

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