Of the five elements that are the most important constituents of living things, the only one that requires the action of microorganisms to enter the living system is

The ecosystem consists of both biotic (plants, animals) and abiotic (temperature, soil, pH) factors.

Nitrogen is inert; it cannot be used by plants unless it is broken down into simple nitrite or nitrate forms. This is why some microorganisms such as Nitrosomonas and Nitrobacter convert nitrogen into nitrite and nitrate, which can be used by the plant.

Thus, due to the inert nature of nitrogen, action of microorganisms is required for it to enter the living system.