Many plants have bacterial symbionts that fix nitrogen for them in exchange for carbohydrates. The triple bonded nitrogen diatomic atom is not accessible directly to plants as they do not have the enzymes to break that triple bond. Some bacteria do.
The basic component of chlorophyll is nitrogen. Chlorophyll is the " workbench " ( so to speak ) on which plants make the food they need and the vegetative matter that composes them.
So, when these plants die that nitrogen is released from the plant material into decomposers and eventually back into the atmosphere where it becomes diatomic again. Then back into plants to complete the cycle.