The nitrogen content of the biosphere remains constant due to:

Fishing and Guano birds assist the cycling of

Match the organisms given in Column $I$ to their functions given in Column $II$ and choose the correct option.

Column $I$	Column $II$
$A$. Thiobacillus	$1$. Free-living nitrogen-fixing cyanobacteria
$B$. Nitrosomonas	$2$. Denitrification
$C$. Nostoc	$3$. Free-living aerobic nitrogen-fixing bacteria
$D$. Azotobacter	$4$. $NH_{3}$ to Nitrite

Codes:
$A \quad B \quad C \quad D$

The bacteria which convert $NO_3^- \rightarrow$ free $N_2$ are called as:

The process of conversion of nitrogen gas into ammonia is called $P$,and the process of decomposition of organic nitrogen in plants and animals into ammonia is called $Q$.

Non-biological nitrogen fixation is: