When a plant dies, most of the nitrogen in the plant's tissues is:

The Nitrogen Returns into the Soil (Nitrogen Cycle)

Explanation:

After a plant dies it's the job of a decomposer to carry along the nitrogen back to the surface so that it may continue to cycle along, until it eventually returns back to the decomposer through the death of another plant. This part of the cycle is small compared to the whole Nitrogen cycle.

atp is used for energy.

allowed for the prediction of the properties of missing elements.