Humus composition of mineral–microbial residue from microbial utilization of lignin involving different mineral types

This study explored the mineral contribution of lignin to humus (HS) formation through the change of HS composition in microbial–mineral residue (MMR). The liquid shake flask culture method was adopted to collect the MMR formed through the microbial utilization of lignin in the presence of goethite, bayerite, δ-MnO2, kaolinite, and montmorillonite. The carbon (C) contents of humic-like acid (HLA), fulvic-like acid (FLA), and humin-like (HLu) in MMR, represented as CHLA, CFLA, and CHLu, respectively, coupled with the ΔlogK of the HLA alkali-soluble extract and CHLA/CFLA ratio were analyzed at 10, 30, 60, and 110 d. In terms of improving HLA aggregated on minerals, the following rule was observed: goethite > bayerite > montmorillonite > kaolinite ≈δ-MnO2. Goethite was most likely to adsorb organic molecules with a high degree of polymerization. Compared with kaolinite and montmorillonite, goethite, bayerite, and δ-MnO2 were more helpful for decreasing the molecular weight and the degree of HLA condensation. Goethite, δ-MnO2, and montmorillonite presented the greatest advantages in enhancing the relative proportions of CHLA, CFLA, and CHLu, respectively, in MMR. In MMR formed in the presence of kaolinite, goethite, and bayerite, CHLA was decreased by 14.8%, 12.0%, and 5.8%, respectively, at the end of culture, whereas the CHLA associated with δ-MnO2 was increased by 12.0%. δ-MnO2 contributed the most to the conversion of CFLA to CHLA. Due to expandability and a much greater adsorption capacity, montmorillonite was most beneficial to the accumulation of CHLu.