Which process does not contribute to the microbial breakdown of a herbicide?

The process of evaporation of solvent does not contribute to the microbial breakdown of a herbicide. Microbial breakdown involves biological processes where microorganisms such as bacteria, fungi, and other microbes degrade chemical compounds within the environment.

In contrast, evaporation is a physical process that involves the transition of a substance from a liquid to a gas phase and does not involve biological activity or transformation of the herbicide itself. The other options, like oxidation by sunlight, action by microorganisms, and chemical transformation, all engage in processes that lead to the breakdown or alteration of the herbicide’s structure, thereby making them relevant contributors to its microbial degradation.