Rhizobia or rhizobe group of bacteria participate in fixation of atmospheric nitrogen, they reside in nodules of the plant roots and take amino acids from plants which they provide back to plants along with newly fixed atmospheric nitrogen. Plants would be incapable of utilizing atmospheric nitrogen without these bacteria that carry special enzyme nitrogenase for this fixation which is carried out by a reduction reaction formig nitrites and nitrates. Nitrogen, the most abundant element of the atmosphere would be unavailable to plants without reduction through these organisms. You can imagine how plants would grow without rhizobia and why it is not a good idea to wipe these organisms out!
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