Nitrates are transformed into nitrogen by

The conversion of ammonia into nitrate by microorganisms is called:

Choose the correct pair:

Column-$I$	Column-$II$
$I$. Nitrosomonas	Nitrite to nitrate
$II$. Thiobacillus	Denitrification
$III$. Nostoc	Free-living nitrogen fixer
$IV$. Azotobacter	Anaerobic nitrogen fixer

Plants usually absorb Nitrogen in the form of nitrates because Nitrogen is . . . . . . .

The nitrogen content of the biosphere remains constant due to:

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 $Q$.