Comparison of the effects of self-assembly and chemical composition on humic acid mineralization

•Microbial mineralization of humic acids is related to their self-assembly.•Self-assembled samples were more readily degradable than disassembled samples.•Mineralization of self-assembled samples did not vary with chemical composition.•Self-assembled samples did not show preferential mineralization.

The purpose of this study was to quantitatively assess the effect of organic matter self-assembly on its resistance to microbial mineralization. Humic acids isolated from leonardite, two peats and a mineral soil were used as organic matter samples because they provide a broad range of variability in terms of origin and the nature of their organic components. Using a benzene-methanol extraction the original humic acid samples were disassembled into humic components and a humic-lipid composite. The composite was further disassembled by using an alkaline aqueous extraction into humic amphiphilic and lipid components. Mixtures that reproduced the composition of self-assembled samples were prepared by mixing the solid individual fractions in the exact proportions that they were present in the original material. The original humic acid and their corresponding mixtures were added as the sole carbon source in separate aerobic cultures containing a microbial consortium isolated from a mineral soil. After incubation for 125 days mineralization of the self-assembled samples was shown to be higher by as much as 70% compared to their corresponding physical mixtures. The extent of mineralization of the self-assembled samples was not correlated to chemical composition base on the carbon-type distribution or hydrophobicity index derived from 13C solid-state NMR spectra. Mineralization of the physical mixtures and fractions did vary with chemical composition and was accompanied by preferential mineralization of alkyl carbon. These results suggest the microbial mineralization of humic acids is related to their self-assembly.