Why do plant use nitrogen in their own form?

Nitrogen is important in producing chlorophyll in plants, that’s what makes them green. nitrogen also acts as a component of amino acids for the plant. these amino acids are considered the “building blocks” of proteins, and without proteins, plants wouldn’t be able to survive.

as for HOW plants use nitrogen, they get it from the soil rather than directly from the air.

Decrease most likely because plants produce oxygen.