How do plants relate to the nitrogen cycle?
Plants with the help of bacteria fix free nitrogen and use it to grow.
More specifically, nitrogen is a crucial component in photosynthesis, and this allows the plant to grow in size. Plants absorb nitrogen after bacterias have denitrified it. Otherwise, they could not use it because nitrogen is created from denitrified nitrate.
The atmosphere is full of nitrogen, but they are all in the form of nitrate, which plants or animals cannot use.
The cycle is: Atmosphere -> bacteria -> plants -> atmosphere