Humus Accumulation And Its Driving Mechanisms In Bauxite Residue Disposal Areas

Bauxite residue is a strong alkaline solid waste generated by the alumina industry,with a low comprehensive utilization rate.Currently,it can only be mainly stacked,but it has a high environmental risk.Ecological restoration of bauxite residue disposal areas is an effective disposal method to reduce the risk of bauxite residue disposal areas.However,bauxite residue has strong saline-alkalinity characteristic,low organic matter content,limited humus accumulation content,and it is difficult for plants to grow.The problem of humus deficiency was solved by anthropogenic improvement methods in bauxite residue disposal areas,based on the reduction of saline-alkalinity.In this paper,the accumulation characteristics of humus was investigated during the natural weathering process of bauxite residue disposal areas.Then,the effects of organic modifier on accumulation of humus in bauxite residue disposal areas was explored.Subsequently,the strengthening role of Penicillium oxalicum（acid producing microorganism screened by bauxite residue disposal areas）on the accumulation of humus was clarified in bauxite residue disposal areas.Further,the specific mechanism of P.oxalicum to enhance bauxite residue humification was elucidated and the ecological restoration method of bauxite residue disposal areas to promote the accumulation of humus was established by the nitrogen coupling P.oxalicum and organic modifier.All of those abovementioned were based on 3D fluorescence spectrum,2D infrared correlation spectrum,high-throughput sequencing technology and other means through the combination of field investigation,simulation experiment and field test of bauxite residue disposal areas:（1）It was found that plant grew well in the bauxite residue disposal areas with high humus content and humification level,according to the nutrient and spatial differentiation investigation.Overall,the nutrient content of the bauxite residue disposal areas was at a low level,but the nutrient content showed a significant spatial differentiation with plant colonization.When plant grew,the overall quality index（SQI）of bauxite residue was improved,with humus having the highest percentage among the nutrient factors.Compared with the bare area,the humus content in the plant growth area increased from 0.98 g/kg to 1.52 g/kg,among which the humic acid content increased from 0.53 g/kg to 1.04 g/kg,and the H/F（humic acid content/fulvic acid content）increased from 1.3 to 2.2.Environmental factors such as pH and microbial indicators such as enzyme activity significantly influenced the humus accumulation in the bauxite residue disposal areas.（2）The addition of organic modifier could promote humus accumulation in bauxite residue on the basis of stabilizing its pH.The addition of corn straw（BGS）,chicken manure（BGM）and nitro humic acid（BGH）all reduced the alkalinity of bauxite residue and could inhibit the pH rebound,and the final pH was stabilized at about 8.At 12th months,the humus contents were 4.47,3.17 and 8.26 g/kg,respectively.Further,the humic acid contents were 3.21,1.16 and 7.97 g/kg,respectively.In addition,the order of H/F and PQ values was BGM<BGS<BGH.The organic improver increased the intensity of humic acid peak of bauxite residue type and enhanced the aromatic functional groups in humic substances.Phenol-OH stretching and precursors for humus formation were important process in humus formation,where BGS treatment had the highest polyphenol content,β-glucoside,xylanase,B-D-cellulose glycosidase and polyphenol oxidase,with higher potential for humus formation.The humification process in bauxite residue was mainly:organic materials are decomposed into precursor substances such as polyphenols by microbial action and further precursor substances were polymerized into humus by the action of polyphenol oxidase.（3）Generally,on the basis of organic amendment,the addition of P.oxalicum enhanced bauxite residue humus accumulation and increased the content and structural stability of bauxite residue humic acid and fulvic acid.The bauxite residue content was increased by 11%and 10%for primary and segmental addition,respectively,and humic acid content was increased by 9%and 6%,respectively.It promoted the decomposition of difficult-to-use organic matter and the utilization of easily decomposed small molecules,in which the polyphenol and amino acid content decreased.P.oxalicum improved the aromaticity and structural stability of humic acid,enhanced the fluorescence intensity of fulvic acid,and promoted bauxite residue humification,but the enhanced effect of segmental addition of P.oxalicum on bauxite residue humification was not significant.The addition of P.oxalicum increased the bacterial and fungal community richness and species diversity of bauxite residue,and enhanced enzyme activity.Structural equations demonstrated that microorganisms promoted humification by secretingβ-glucosidase,xylanase,polyphenol oxidase,acetylaminoglucosidase and cellulose disaccharidase influencing the production of humification precursors.P.oxalicum has a bifunctional mechanism（i.e.,catabolism and polymerization）during humification,and precursor substances such as polyphenols and amino acids are intermediate products of humification.（4）The synergistic effect of Al₂O₃ and Fe₂O₃（main components in bauxite residue）and P.oxalicum could strengthen the reaction condensation of precursor substances into humic substances,indicating that the nature of bauxite residue was favorable to the formation of humic substances.The oxidative decarboxylation reaction occurred after the oxidative cyclic cleavage of the humic precursors gallic acid,the production of aliphatic substances and the decomposition of glycine,which contributed to the conversion of small molecules of organic matter of the system to humus.Al₂O₃,Fe₂O₃ and P.oxalicum could effectively promote the utilization of phenolic and protein amino groups and the formation of humus.Additionaly,the synergistic effect of metal oxides and P.oxalicum had better results.Al₂O₃,Fe₂O₃ treatment humification process is preferentially phenol into quinone,promoting the small molecules into the polymerization of the formation of large molecules aromatic ring group.Oxalic acid Penicillium synergistic Al₂O₃,Fe₂O₃treatment in the hydroxyl group was oxidized under the action of microorganisms.It is the main way to produce quinones.The synergistic effect of P.oxalicum coupling Al₂O₃,Fe₂O₃ increased the reaction rate of the humification process and promoted the polymerization of aromatic ring substances of the system,in which nitrogenous substances enter the structure of humic acid,which was an important mechanism for the formation and stabilization of humic substances.（5）Nitrogen-coupled P.oxalicum synergistically promoted humus accumulation in the bauxite residue disposal areas with biomass.Further the addition of nitrogen under the reinforcing effect of P.oxalicum,the humus and humic acid contents were increased by 20%and 31%,respectively.Compared with the addition of P.oxalicum,H/F reached 2.6and increased the degree of bauxite residue humification.The overall nutrient content of bauxite residue was increased and the overall quality（SQI）of the bauxite residue disposal areas was improved.Nitrogen addition promoted the formation of bauxite residue microaggregates of 5-20μm particle size,which further increased the elemental C,N and O content.The enhanced biomass decomposition by P.oxalicum could promote the accumulation of loose-knotted humus,and on this basis,the addition of N increased the content of associative humus.More small-molecule organic matter aggregated into the bauxite residue humus after N addition,and at the same time promoted the conversion of humic acid-like to more stable humic acid-like,which further enhanced the structural stability of humus.The input of exogenous nitrogen promoted the growth of bacteria with cellulose decomposition and nitrogen fixation functions.Furthermore,P.oxalicum could survive in the bauxite residue disposal areas,further promoting the bauxite residue humification process.The synergistic effect of nitrogen-coupled P.oxalicum and biomass can achieve humus accumulation in bauxite residue disposal areas,providing theoretical basis and practical experience for the ecological restoration of disposal areas.