The breaking down of a substance into two or more simpler substance is

decomposition

Explanation:

Decomposition is the process of metabolic degradation of organic matter into simple organic and inorganic compounds. The decomposition can involve inner changes of the decaying body together with the action of endogenous and exogenous organisms. The simple organic and inorganic compounds are released into the environment to provide nutrients. Decomposers can be incorporated into nitrogen and carbon cycles to act upon organic matter.