Which organism convert nitrogen into a form that is unsable by humans?

Certain bacteria, such as those in the genera Rhizobium and Azotobacter, convert atmospheric nitrogen (N₂) into ammonia (NH₃) through a process called nitrogen fixation. This ammonia can then be further transformed into nitrites (NO₂⁻) and nitrates (NO₃⁻) by other bacteria in the soil. While these nitrates can be taken up by plants, which humans consume, nitrogen itself in its gaseous form is not usable by humans without this conversion process.