| In view of the predominant effect of fungi on the dynamic change and structural characteristics of humic substance based on the microbial utilization of cellulose and lignin, in this research, the methods of imitated soil and shake-flask liquid culture were adopted to study the effects of cellulose and lignin utilized by different types of fungi(Trichoderma viride, Aspergillus niger and Penicillium) on the quantity and formed sequence of HS fraction in Black soil and the characteristic of microbial residue itself formed from the different microbial growth periods and the changing law in numbers of their extracted humic-like substance in the pure liquid culture.The relevant conclusions were as below:(1) Cultured in Black soil, the different microbial communities exerted both impacts from quantity and quality upon humic fractions isolated from the Black soil mixed with wheat straw. In the culture process, the total organic carbon (TOC) of Black soil was consumed continuously, in which most of WSS component (water soluble substances) was primarily utilized. First, the tested microbial communities synthesized HA inside their cells, and then which was decomposed and transformed into FA outside the microbial cells. The advantageous role of fungi in promoting the synthesis of HA and decomposition of FA was the most. The microbial culture was conducive to disintegrate the inert component (humin, Hu), which could also prompt the transition of humic-like substances from wheat straw into true humic substances.(2) As two exogenous carbon sources, the cellulose and lignin were added in the Black soil separately, of which TOC and WSS could be consumed gradually by fungi and mixed strains. The cellulose itself contained a large amount of organic molecules whose properties were similar to FA component, but which was little found in the lignin sample. The cellulose utilized by Penicillium was helpful to enhance the amount of FA in the Black soil, but the opposite rule was shown if the lignin was selected. The soil mixed with cellulose or lignin transformed by Aspergillus niger was all in favour of the consumption of soil FA. The C content of FA in soil mixed with cellulose was increased by Trichoderma viride and mixed strains, however, whose amount was decreased if the lignin was served as substrate. Cellulose utilized by fungi and mixed strains, the transformed rule between HA and FA was consistent with the Lignin Theory. However, the Black soil mixed with lignin affected by Trichoderma viride and Aspergillus niger, whose rule conformed to the Polyphenol Theory. The degradation and oxidation process occurred simultaneously in Black soil mixed with lignin cultured by Penicillium, which was beneficial to the mutual transformation between FA and HA continually. The influence rule of mixed strains on Black soil mixed with lignin was also in line with the Lignin Theory. Cellulose added could increase the C contents of different components of Hu in soil, but there were little organic molecules resembled to HMi (Iron combined with humin) and HMr (Non-soluble humin) existed in lignin sample. Cellulose used by different treatments could enhance the amount of HMi in soil dramaticlly, but when lignin served as substrate instead of cellulose, whose quantity could be decreased a little. Whether the cellulose or lignin was chosen, Trichoderma viride could effectively enhance the proportion of humic-extracted acid in soil. Furthermore the amount of HMr component in soil was all decreased regardless of substrate type.(3) Cultured in nutrient solution, when the corn straw served as C source, the C contents in both cell metabolites and microbial residues had a loss compared with CK. If the C source was replaced with sawdust, the C content in cell metabolites was inceased but its content in microbial residues was decreased, among which the role of Penicillium was the most. The aliphatic degree of residue derived from corn straw was enhanced by Aspergillus niger, Penicillium and mixed strains, of which the effect of Penicillium was the most, and the first two treatments were also helpful to the degradation of microbial residue derived from sawdust. Under the same condition, the residue formed from corn stalk, whose WSS component had a net loss at the end of culture, whereas the opposite law was shown if the sawdust served as C source. Whether the corn stalk or sawdust served as available substrate, the changing rule of different treatments on the forming sequence between HLA and FLA in their residues conformed to the Polyphenol Theory that showed that FLA was formed first and then tansformed into HLA. In this process, when the corn stalk served as C source, the role of Trichoderma viride was the most, but as the C source provided by sawdust, the mixed strains had the most significant effect. The Hu component in the residue derived from corn stalk could be transformed effectively, of which the Trichoderma viride had the most advantage, while the accumulation of inert component (Hu) was larged if the sawdust was applied under the same condition.(4) When the cellulose and lignin samples served as C sources in culture fluid respectively, all the inoculated treatments could promote the loss of C content in cell metabolites, but enhance the accumulation of microbial residue. The mixed strains and Trichoderma viride shown their advantage over others in conversion efficiency of cellulose and lignin, respectively. Compared with the pure cellulose and lignin samples, all the N-containing compounds in the microbial residues cultured by different treatments could be increased. As C source, the cellulose was helpful to enhance the aromatization degree and reduce the amount of O-containing functional groups in their residues, but the transformed rule of lignin was contrary, which experienced the process of oxidation and degratation. In the culure process, the cellulose utilized by Trichoderma viride and Aspergillus niger were helpful to the formation of humic-like structure in their microbial residues, both of them could also promote the polymerization of HLA molecules. However, the oxidation, dehydrogenation and decomposition happened in the HLA extracted from the residues formed by Penicillium and mixed strains in the cellulose fluid. The existence of phenol in the FLA indicated it was an indispensable, intermediate product of the formation process of FA fraction. In the matter of Hu-like fraction extracted from residue, its oxidation, dehydrogenation and degradation was also prompted by Penicillium, whereas the effect of Trichoderma viride was opposite. In utilizing N-containing compounds, the advantage of mixed strains was more than the other mono-fungi treatments, among which the effect of Aspergillus niger was the least and it could accumulate lots of amino compounds in its Hu-like fraction. The highest oxidation and strongest aliphatic degree were shown in the residue formed by Penicillium in the lignin fluid, but there was still a gap compared with humic substance, while the remaining three treatments could contribute to the oxidative degradation of lignin directly and transformed their residues into the humic-like substances. The residue formed by Trichoderma viride in the lignin culture fluid was the most similar to humic-like substances. Taken the Aspergillus niger for example, the residue formed from cellulose, whose maturity degree of organic matter was worse. However, when the lignin was adopt as C source, the hydrophobicity, stability of organic C and resistance to decomposition of residue had more advantage over cellulose. The cellulose utilized by Penicillium could reduce the amount of FLA in residue, while the accumulation of this component was took place if the C source was provided by lignin, and the remaining three treatments were all helpful to enhance the amounts of FLA in their residues regardless of substrate type. In the matter of HLA extracted from the residue, the cellulose utilized by Trichoderma viride and Penicillium could enhance the amount of HLA, but when the lignin applied, the HLA components were consumed effectively by them. Regardless of cellulose or lignin served as substrate, Aspergillus niger and mixed strains could remarkably consume the HLA components isolated from their residues.Whether they were cultured in soil or culture fluid, there were some differences existed in their own cells of Trichoderma viride, Aspergillus niger and Penicillium, after the utilization of two substrates (cellulose or lignin), they had a driving effect on the formation of HS in Black soil or humic-like substance extracted from the microbial residue. The research could reveal the significant role of fungi in promoting the formation of humic substance and provide a theoretical basis for seeking a more suitable substrate as C source. The research could not only work out the problem about the effects of two substrates utilized by different types of fungi in the soil or liquid culture conditions on the characteristics of HS (or humic-like substance), but also be of great significance in illuminating the puzzles about the formed pathway or characteristic of different HS fractions and the dominant role of fungi in the formation of HS. |
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