Golly, please help me understand this nitrogen cycle (the terrestrial one). I skimmed through some materials (e.g. 1, 2) but still don't understand.

What I made of all of of it: first it's "fixed" by converting nitrogen from air to ammonia (nitrogen fixation), then it's converted to nitrites and then nitrates (nitrification), then faeces and dead bodies shed their organic nitrogen which is converted into ammonium (ammonification) (I'm only interested in how nitrogen becomes bioavailable so I skip the denitrification phase).

The articles say all the three are bioavailable, but Wikipedia says ammonia, at least in its gaseous form, is toxic to plants. So a step of converting ammonia to ammonium was missed, wasn't it? And if ammonia is available enough then what does nitrification have to do with it? I read somewhere (I don't remember where, I closed the tab apparently) that ammonium is good for some plants but generally they, plants, prefer nitrates.

Could you give me an article that confirms and, hopefully, explains it? An intelligible explanatory article from an authoritative source would be helpful


1 Answer 1


The first step in the nitrogen cycle is atmospheric nitrogen (N2) and water (H2O) lightning will make (N2) and (H2O) combine into ammonia (NH3) and nitrate (NO3) this will combine with water and fall to the ground.

Step Two is where nitrate can be used by plants directly and the ammonia can be used by bacteria (nitrosomonas)that converts ammonia into nitrite,this nitrite will be converted by an other bacteria (nitrobacter) into nitrate that can be used by plants directly.

A part of the nitrate will be converted to free nitrogen (N2) by denitrifying bacteria,this process can only happen in an oxygen free environment (wetlands and similar environments) it can reduce the nitrate content with up to 50% in water.

Step three is when plants and the animals that live of the plants die and breaks down into ammonia and other waste products (this is where many explanations of the nitrogen cycle usually starts). The waste products gets converted into ammonia by bacteria and the ammonia gets converted to nitrite and the entire cycle starts all over again.

Legumes have a symbiotic relationship with some bacteria that can fixate nitrogen (N2) https://aces.nmsu.edu/pubs/_a/A129/




The rest is from my memory.

  • $\begingroup$ Thanks. Is ammonia bioavailable? $\endgroup$ Commented Feb 22, 2022 at 23:38
  • $\begingroup$ @SergeyZolotarev The link to the legumes has a little about ammonia being made bioavaliable to plants by bacteria but for other plants ammonia needs to be dissolved in water and only in trace amounts as far as i know. $\endgroup$ Commented Feb 23, 2022 at 4:32

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