Cycle nutrients from dead organisms back to the soil, where it becomes available for plants.

Is it a four letter word? 
If yes, it could be: 
Life, or 
Food. 

If not a four letter, it could be the Trophic Levels (fungi, plant,herbivore, detrivore, omnivore, carnivore, Second carnivore ie Predators that prey on other predators) it all forms a cycle where one dies and becomes decomposted by bacteria (microscopic) broken down so plant life can consume the nutrients, then the detrivores etc come in who eventually get eaten by the carnivores and cycle goes on :) 

The correct answer is c