nitrogen fixation

[arcoiris]

Im almost sure all the options are correct just need to here some opinions
Nitrogen Fixation:
can occur in aquztic systems or in soil bacteria
can occur in root nodules of legumes
may occur as ammonium is converted to nitrate
may occur as nitrogen gas is converted to nitrate
alll ofthe above
thank you

[mith]

I think that while ammonia->nitrite is part of the nitrogen cycle, it's not considered part of fixation.

[Ninking]

Ammonium --> nitrate is called nitrification. It's not fixation 100%.
The nitrogenase complex reduces the N2 gas to ammonium, but not to nitrate. The product of fixation is ammonium. The oxidation of ammonium to nitrite/nitrate happens later and as I already said this process's called nitrification.