Although air is full of nitrogen, it is not available for organisms to readily use.
nitrogen is changed into which substances before being used by living
things?

Nitrogen is changed into AMMONIA, NITRATES AND NITRITES before it can be used by living things.

Explanation:

Nitrogen is the most abundant element in the atmosphere, approximately 78% of the air is made up of nitrogen. Despite the abundance of nitrogen, the element can not be used directly by plants and animals.

Nitrogen fixing bacteria that are found in the soil are the ones responsible for converting atmospheric nitrogen into the forms that can be used by plants. Nitrogen fixing bacteria convert atmospheric nitrogen to ammonia while nitrifying bacteria convert ammonia to nitrates and nitrites.  Plants use ammonia, nitrates and nitrites to make organic molecules.

All living organisms use nitrogen to make various compounds such as DNA, proteins, chlorophyll, etc.

After the death of the living organisms containing the nitrogenous products, dentrifying bacteria decompose the nitrates in their body to atmospheric nitrogen.

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