Nitrogen cycle

Nitrification is the oxidation of ammonia to nitrate in the soil; it takes place in two steps.
Step 1: ammonia is oxidised to nitrite by $Nitrosomonas$ and $Nitrococcus$: $2NH_3 + 3O_2 \rightarrow 2NO_2^- + 2H^+ + 2H_2O$
Step 2: nitrite is oxidised to nitrate by $Nitrobacter$: $2NO_2^- + O_2 \rightarrow 2NO_3^-$
These nitrifying bacteria are chemoautotrophs.
Reduction of nitrates to free nitrogen is denitrification ($Pseudomonas$, $Thiobacillus$); conversion of free nitrogen to nitrogen compounds is nitrogen fixation; conversion of proteins into ammonia is ammonification.
Hence nitrifying bacteria oxidise ammonia to nitrates.

The figure is the nitrogen cycle, showing atmospheric $N_2$, the soil nitrogen pool ($NH_3 \rightarrow NO_2^- \rightarrow NO_3^-$) and the biomass.
A is the arrow from nitrate in the soil back to atmospheric $N_2$: reduction of nitrate to nitrogen by bacteria such as $Pseudomonas$ and $Thiobacillus$ is denitrification.
B is the arrow from decaying biomass to $NH_3$: decomposition of the organic nitrogen of dead plants and animals into ammonia is ammonification.
C receives nitrate/ammonia from the soil by 'uptake': these are the plants.
D is the group that obtains its nitrogen from plants and returns it to the decaying biomass: the animals.
Hence A – denitrification, B – ammonification, C – plants, D – animals.