Nitrates are converted into nitrogen by
(a)Ammonifying bacteria
(b)Denitrifying bacteria
(c)Nitrogen fixing bacteria
(d)All of the above

Hint: In forests the bacteria include Alcaligenes, Pseudomonas and bacillus species as well as Actinobacteria especially Streptomyces. They are aggregated in the upper soil layer and around roots and show highest numbers during the cold season.

Complete answer:
In the fifth stage of the nitrogen cycle, nitrogen returns to the air as nitrates are converted to atmospheric nitrogen by bacteria through the process we call denitrification. This leads to an overall loss of nitrogen from soils, because the gaseous sort of nitrogen moves into the atmosphere.

Additional Information: -In order to maneuver through the various parts of the cycle, nitrogen must change forms. In the atmosphere, nitrogen exists as a gas, but in the soils it exists as nitrogen oxide, and nitrogen dioxide, and when used as a fertilizer, can be found in other forms, such as ammonia, which can be processed even further into a different fertilizer, ammonium nitrate.
-The nitrogen cycle is a repeating cycle of processes during which nitrogen moves through both living and nonliving things: the atmosphere, soil, water, plants, animals and bacteria.
-There are five stages within the organic process , and that we will now discuss each of them in turn: fixation or volatilization, mineralization, nitrification, immobilization, and denitrification. In this image, microbes in the soil turn nitrogen gas into what is called volatile ammonia so the fixation process is called volatilization.
-Leaching is where certain sorts of nitrogen (such as nitrate, or $NO_3$) becomes dissolved in water
and leaks out of the soil, potentially polluting waterways.

So, the correct answer is, ‘Denitrifying bacteria.’

Note: -Excess nitrogen can also leach—or drain—from the soil into underground water sources, or it can enter aquatic systems as above ground runo. This excess nitrogen can build up, resulting in a process called eutrophication.
-Eutrophication happens when an excessive amount of nitrogen enriches the water, causing excessive growth of plants and algae.
-Too much nitrogen can even cause a lake to show bright green or other colors, with a “bloom” of smelly algae called phytoplankton