g a solid organic waste (generic formula: C) is degraded by CO2 by microbial processes. Does this mean C is getting oxidized

Yes, it does.

Explanation:

In the microbial process, the waste works as a fuel for the microorganism, it uses its fuel to produce its energy like the animals use glucose as fuel. So, the reaction is an aerobic respiration process, the fuel reacts with oxygen gas to produce energy, carbonic gas, and water.

Thus,

C + O2 - -> CO2 + H2O + energy

The oxygen is a good oxidizer agent, it means that in a redox reaction it intends to reduce (gain electrons) meanwhile the other reactant intends to lose electrons, and so be oxidized. So, C is being oxidized in this process.