Which organisms convert nitrogen into a form that is usable by

humans

there are two or three types or methods in which they change according to the class you study so this a answer from a 8th grader (there are difference in classes)

Nitriates are a form of nitrogen that is usable by plants.

It is assimilated into plant tissue as protein. The nitrogen is passed through the food chain by animals that consume the plants, and then released into the soil by decomposer bacteria when they die.

Nitrification is the conversion of ammonia (NH3) to nitrate (NO3). It is usually performed by soil living bacteria, such as nitrobacter. This is important because plants can assimilate nitrate into their tissues, and they rely on bacteria to convert it from ammonia to a usable form.

denitrifying bacteria convert nitrogen into ammonia which is a form of nitrogen that is useful to humans. Bacteria "fixes" gaseous nitrogen into nitrogen usable by plants in the form of nitrate.