Complex sentence: One part is the biomass of living microorganisms (bacteria), another one partly decomposed or fresh but dead parts of plants and animals and strongly decomposed but stable organic compunds, also refered to as humus.
Split into simpler sentences: One part is the biomass of living microorganisms (bacteria), another one partly decomposed or fresh but dead parts of plants and animals. Furthermore SOM consits of strongly decomposed but stable organic compunds, also refered to as humus.