Different pathways in promoting humic acid formation of multi-source biochar addition between fiber and livestock manure type solid waste composting.

This study selected fruit wood biochar (FWB), walnut shell biochar (WSB) and corncob biochar (CB) to compare and explore the humic acid (HA) formation mechanism differences added to the Chinese herbal residue (CHR) and Sheep manure (SM) composting. The results showed that the addition of multi-source biochar could improve carbon stability. Organic matter (OM) loss rates of CK, FWB, WSB and CB during SM composting were 25.3 %, 22.8 %, 20.8 % and 25.3 %, respectively, and that of CK, FWB, WSB and CB during CHR composting were 62.2 %, 47.9 %, 41.0 % and 43.3 %. Meanwhile, biochar addition promoted the HA formation, especially WSB in SM and CB in CHR. Moreover, the prediction results of the random forest model indicated that biochar could enhance the metabolic activity of bacterial communities closely related to HA formation during SM and CHR composting due to its porous structure. Finally, the structural equation model verified that CB with larger total pore volume promoted the conversion of macromolecular OM in CHR to form HA by stimulating microbial activity, while WSB with stronger adsorption capacity formed HA by polymerizing mall molecular components solidified in SM. This study provided theoretical and technical support for biochar to promote humification during different types materials composting.