The nitrogen content of the biosphere remains constant due to:

Nitrifying bacteria

How are organisms like Pseudomonas and Thiobacillus of great significance in the nitrogen cycle?

The process of conversion of $2 NO_{3}^{-} \rightarrow 2 NO_{2}^{-} \rightarrow 2 NO \rightarrow N_{2}O \rightarrow N_{2}$ is called and is carried out by:

$A$: Ammonia is oxidized first to nitrite and then to nitrate by the action of $Nitrosomonas$ and $Nitrobacter$ respectively.
$R$: Nitrite thus formed is absorbed by leaves.

Study the nitrogen cycle process shown below and select the option that correctly identifies the four blanks $A, B, C,$ and $D$ in the context of nitrogen transformation.