Which of the following is present in the maximum amount in the atmosphere?

Most of the plants obtain or absorb nitrogen from the soil in the form of

Denitrification is carried out by bacteria $......$ and $......$.

Identify $X$ and $Y$ in the given nitrogen cycle chart.

Which bacteria convert ammonium salts into nitrite?

$A$: 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.