Nitrication

Why does nitrification take place in the soil when ammonium is already in plant available form? Is it because not all of the ammonium is taken up by plants so some of the ammonium in solution happens to get oxidation by autotrophs?

Both ammonium and nitrate serve the same purpose to plants and soil organisms right? They both sources of N just in different forms.

Also, what exactly does fixation and reduction mean? Since different fixation processes seem to render nutrients both plant available and unavailable.

Thanks!

Ammonification is done by heterotrophic microorganisms and convert the amines/a.a. released by bacteria and fungi decomposition to ammonium, which then can be converted to nitrites and nitrates, absorbed by plants, used by microorganisms, adsorb to clay minerals, or released to atmosphere. Nitrification is after ammonification and converts ammonium to nitrate. It has two steps: first, ammonium to nitrite. Then nitrite to nitrate.

Yes, both ammonium (NH+) and nitrate (NO3-) ions are available to plants.

We have discussed three types of fixation processes during the discussion session #3. In this course we mentioned N2 biological fixation (that converts N2 from atmospheric air to plant available forms through activity of N-fixing organisms), NH4+ fixation (which represents trapping of NH4+ ions in the inter layer space of certain phylosillicate minerals, reducing availability of this form of N to plants) and phosphate fixation (a chemical reaction with either Fe/Al or Ca ions which leads to formation of insoluble phosphate compounds that are not available to plants). Hence, the outcomes of these 3 types of "fixation" are different

Do we need to know the equations for ammonification and nitrification?

You need to know to describe those reactions (or any other chemical reaction mentioned in our lectures). Just knowing to write chemical reactions without knowing to describe what is going on is not what we are after in this course.