Nitrogen fixation means:
1. releasing nitrogen from dead and decaying materials.
2. conversion of nitrite to nitrate.
3. conversion of nitrate to nitrite.
4. conversion of nitrogen gas into ammonia.

Conversion of ammonium to nitrate during nitrogen cycle is called as:
1. Nitrogen assimilation
2. Denitrification
3. Nitrification
4. Nitratification

A microbially facilitated process of dissimilatory nitrate reduction that may ultimately produce molecular nitrogen (${\mathrm{N}}_2$) through a series of intermediate gaseous nitrogen oxide products is called as: