The nutrient with a key role in biological N-fixation is

The nutrient with a key role in biological N-fixation is Molybdenum.
Nitrogen fixation is a chemical process by which molecular nitrogen (N₂), with a strong triple covalent bond, in the air is converted into ammonia (NH₃) or related nitrogenous compounds.
The conversion of N₂ into ammonia occurs at a metal cluster called FeMoco, an abbreviation for the iron-molybdenum cofactor. Molybdenum-dependent nitrogenase is the most commonly present nitrogenase.