HS can be separated into humic acid (HA) and fulvic acid (FA). The concentration of carbon of HS (CHS), carbon of HA (CHA) and carbon of FA (CFA) had a similar decreasing trend at the beginning of composting (Fig. 2-D), because of the degradation
of simple organic carbon compounds (such as simple carbohydrates, fats and sugars) which could be easily mineralized and metabolized by microorganisms (Bernal et al. 2009).