Question

what breaks down plants and animal remains to form humus​

Answer 1

Bacteria and other microorganisms.

Answer 2

Organic matter break down is a biological process because it is the soil organisms (microorganisms, earthworms, microarthropods, ants beetles etc) that perform the chemical and physical changes. ... The first organic compounds to be broken down are those that are easy to break down, incluing amino acids and sugars.