Answer:
Denitrification = bacteria convert nitrates into nitrogen gas.
Denitrification occurs when nitrogen is removed from the soil and this happens when bacteria concert nitrites back to Nitrogen gas.
Nitrification = Bacteria convert ammonia to nitrites and nitrates.
Nitrification is the process by which bacteria concert ammonia made from ammonification into useful organic compounds like nitrites.
Nitrogen fixation = bacteria break down waste products and return nitrogen to the soil.
Nitrogen fixation is when bacteria return nitrogen to the soil by breaking down waste products that fall into the soil.
Ammonification = bacteria convert atmospheric nitrogen to ammonia.
During ammonification, bacteria converts nitrogen compounds to Ammonia or Ammonium.
Assimilation = Plants and animals take in and use nitrogen compounds.
Assimilation then occurs when plants take in these nitrogen compounds and use them. Animals will get these compounds by eating the plants.