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.

nucleic acid molecules are incredibly complex, containing the code that guarantees the four different nucleotide units comprise dna, the nucleic acid of interest to the genealogist. and one of four nitrogen-containing molecules called nucleotides

hypoxemia refers to a below usual concentration of oxygen in the blood, mainly in the arteries. hypoxemia is an indication of an issue associated with circulation or breathing and may lead to various symptoms like shortness of breath.  

the condition is identified by determining the level of oxygen in the blood sample obtained from an artery. it can also be predicted by determining the oxygen saturation of the blood with the application of a pulse oximeter.  

the normal arterial oxygen is about 75 to 100 mm hg. the values underneath 60 mm hg is generally considered with the requirement for supplemental oxygen. the normal pulse oximeter readings generally range between 95 to 100 percent. the values under 90 percent are regarded as low.  

in the given case, the oxygen saturation is measured at 93 percent, hence, there is no need to administer oxygen, that is, one should withhold oxygen.