Abstract：Humic substances (HS) are formed by the polycondensation of various precursors, and the precursors are the key to regulating the formation of HS. Whether the Maillard reaction precursors can promote the microbial transformation of lignin and the formation of HS remains to be verified. Given this, the method of liquid shake flask culture was adopted, the lignin culture medium serving as the research object, and the liquid shake flask culture of 120 d was started by adding single and combined solutions of catechol, glucose and glycine, and collected the supernatant (cell metabolites) and precipitate (microbial residues) by the centrifugation method. The properties of the cell metabolites and microbial residues were analyzed, and an in-depth study of the FTIR spectral characteristics of bacterial residues was necessary, which was used to evaluate the contribution of each precursor to the microbial transformation of lignin to HS more systematically. The results showed that: (1) Adding glycine into the lignin culture medium after the liquid shake flask culture was more conducive to the condensation of cell metabolites organic molecules, making their structures more complex, while the treatments involving catechol and the addition of glucose alone could promote the degradation of cell metabolites and make their molecular structures simpler. Compared with CK, exogenous addition of Maillard reaction precursors could promote the TOC content loss of cellular metabolites, and single catechol could make the TOC content of cellular metabolites always higher than other treatments; (2) The addition of Maillard reaction precursors could significantly increase the recovery rate of microbial residues formed from the microbial transformation of lignin, in which the recovery rate of microbial residues formed from the treatment of single catechol was the largest. In contrast, the combined solution of catechol, glucose and glycine had the smallest effect on the improvement of the recovery rate of microbial residues. The combined solution of glucose and glycine could keep the recovery rate of microbial residue at the highest level throughout the culture period. Adding Maillard reaction precursors could result in a smaller increase in the TOC content of microbial residues than CK. However, at the end of the culture, the four treatments involving catechol and the addition of a combination solution of glucose and glycine could enhance the TOC content of microbial residues significantly higher than CK; (3) The microbial residues formed from the microbial transformation of lignin had hydroxyl O—H, asymmetric aliphatic —CH3, symmetrical aliphatic —CH2—, aromaticC═C and polysaccharides, which had similar FTIR characteristics to soil humic acid, but its molecular condensation degree could not reach the complexity of soil humic acid. After the completion of the culture, the hydroxyl content in the microbial residues was increased to different degrees, while the polysaccharide content decreased. The addition of a single glycine and the combined solution of glucose and glycine could increase the aliphatic degree of the microbial residues while the content of polysaccharides decreased. The four treatments involving catechol and a single addition of glucose could further increase the proportion of aromatic C in the microbial residues. In conclusion, the addition of Maillard reaction precursors could reduce the TOC content of cell metabolites, improve the recovery rate of microbial residues, and, at the same time, increase the content of hydroxyl groups and reduce the content of polysaccharides. Different precursors had different effects on the aliphatic and aromatization of microbial residues. The effects of aromatization were different. Four treatments involving catechol and a single addition of glucose could increase the proportion of aromatic C in microbial residues.
[1] Kamimura N, Takahashi K, Mori K, et al. Environmental Microbiology Report, 2017, 9(6): 682.
[2] Torres I F, Bastida F, Hernández T, et al. Soil Biology and Biochemistry, 2014, 75: 156.
[3] Huang P M, Wang M K, Chiu C Y. Pedobiologia, 2005, 49(6): 613.
[4] Wu J Q, Zhao Y, Qi H S, et al. Bioresource Technology, 2017, 244: 1194.
[5] Yavmetdinov I S, Stepanova E V, Gavrilova V P, et al. Applied Biochemistry and Microbiology, 2003, 39(3): 259.
[6] Orlov D S. Substances of Soils and General Theory of Humification. Moscow University Press, Moscow, 1995: 42.
[7] Tan K H. Humic Matter in Soil and the Environment: Principles and Controversies, 2n ed. CRC Press. 2014.
[8] Zhang Z C, Zhao Y, Wang R X, et al. Waste Management, 2018, 79: 464.
[9] Machado W, Franchini J C, Guimarães M D F, et al. Heliyon, 2020, 6(6): e04078.
[10] Stevenson F J. Humus Chemistry Genesis, Composition, Reactions. John Wiley and Sons, New York. 1994.
[11] Wu J, Zhao Y, Zhao W, et al. Bioresource Technology, 2017, 226: 193.
[12] Chen X M, Liu R, Hao J K, et al. Bioresource Technology, 2019, 271: 484.
[13] Dou S, Shan J, Song X Y, et al. Pedosphere, 2020, 30(2): 162.
[14] Wu J Q, Zhao Y, Wang F, et al. Bioresource Technology, 2020, 303: 122927.
[15] ZHANG Yi-feng, DOU Sen, YE Shu-fen, et al（张一枫，窦 森，叶淑芬，等）. Journal of Agro-Environment Science（农业环境科学学报），2019，38（9）：2186.
[16] YIN Xian-bao, DOU Sen, TIAN Yu-xin, et al（尹显宝，窦 森，田宇欣，等）. Journal of Jilin Agricultural University（吉林农业大学学报），2020，42（2）：177.
[17] Navarrete I A, Tsutsuki K, Navarrete R A. Soil Science and Plant Nutrition, 2010, 56(2): 292.
[18] Hardie A G, Dynes J J, Kozak L M, et al. Journal of Molecular Catalysis A: Chemical, 2009, 308: 117.
[19] Niu J Y, Li X F, Qi X G, et al. Bioresource Technology, 2021, 341: 125875.
[20] Zou J M, Huang J Z, Yue D B, et al. Chemical Engineering Journal, 2020, 393: 124734.
[21] Wu J Q, Qi H S, Huang X N, et al. Bioresource Technology, 2018, 269: 172.
[22] Zhang X M, Zong Y, Xu L Q, et al. Chemical Engineering Journal, 2021, 425: 130521.
[23] Watteau F, Villemin G. Bioresource Technology, 2011, 102(19): 9314.
[24] Zhang Y C, Yue D B, Ma H. Chemosphere, 2015, 130: 42.
[25] Baddi G A, Cegarra J, Merlina G, et al. Journal of Hazardous Materials, 2009, 165(1-3): 1120.
