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The present study aimed to accelerate the humification and to investigate how MnO2 modification of biochar (MBC) drives the humus formation during composting with chicken manure. In this study, compared with the control group (CK), the addition of MBC caused an increase in the concentration of both humus and humic acid (HA), with a respective enhancement of 29.1% and 37.2%. In addition, MBC also improved the stability of compost products. Hetero two-dimensional correlation spectra further exhibited that the MBC could alter the formation mechanism of humus fractions during composting. Random forest analysis showed that Microbacterium, Bacteroides, Kroppenstedtia, Gracilibacillus, and Lentibacillus were significantly related to humus formation (P < 0.05). MBC enhanced the absolute abundance of these five genera during composting. The structural equation model further confirmed that these five genera could be indirectly involved in humus formation, through the production of aromatic compounds via secondary metabolism. Additionally, these five genera could directly transform organic components into macromolecular humus structures. Therefore, the increase in these five genera might be a direct response to the acceleration of the humification during MBC composting. These findings demonstrate the potential value of MBC in harmless disposal of hazardous biowastes through composting. Highlights MnO2 modification of biochar changed the formation mechanism of humus fractions. Key genera involved in humus formation were identified. Among of MnO2 modification of biochar, key genera and humus formation were revealed. [ABSTRACT FROM AUTHOR]

Heilongjiang is a "browning" river that receives substantial terrestrial organic matter, where reactivity of dissolved organic matter (DOM) may have important effect on ecosystem function and carbon biogeochemical cycle. However, little is known about microbial transformations of different DOM components, which could provide valuable insight into biogeochemical reactivity of DOM. In this study, bioavailability experiments were conducted for 55 days to determine changes of different DOM components by microbial transformations. Labile matter (C1) was detected only in initial DOM, and tryptophan-like substances (C4) were observed from day 5 onwards. Thus, three individual components were identified at each sampling time of the bioavailability experiment. The increase of Fmax in DOM components revealed that microbial humic-like substances (C2), terrestrial humic-like substances (C3), and C4 were produced by microbial transformation, especially in the spring samples. Further, two-dimensional correlation spectroscopy (2D-COS) indicated that shorter wavelength tryptophan-like and microbial humic-like substances can be degraded by microbes or transformed into longer wavelength complex substances. Relatively simple microbial humic-like substances were preferentially produced compared to complex terrestrial humic-like substances. The results make sense to understand the biogeochemical cycling and environmental effects of DOM in the Heilongjiang River. [ABSTRACT FROM AUTHOR]

A study was conducted to determine the fluorescence characteristics of various molecular weight (MW) fractions of dissolved organic matter (DOM) derived from different composts based on fluorescence regional integration (FRI) and parallel factor (PARAFAC) modeling of excitation-emission matrix (EEM) fluorescence spectra. The significant differences in the locations of EEM peaks indicated that there were differences in the molecular structures of the MW >5 kDa fraction and the other fractions. FRI indicated that the degree of humification increased with increasing MW for all of the MW fractions, except for the MW >5 kDa fraction, suggesting that FRI was not suitable for analyzing MW fractions with differences in their molecular structures. PARAFAC modeling resulted in three components for the MW >5 kDa fraction and two components for the other MW fractions. Red-shifting occurred in components 1 (Microbial Humic-like) and 2 (Terrestrial Humic-like) as the MW increased. And component 3 was categorized as a protein-like substance. PARAFAC modeling of the MW fractions can isolate more components and resolve more fluorophores than can DOM analysis. [ABSTRACT FROM AUTHOR]

Composting is a biodegradation and transformation process that converts lignocellulosic biomass into value-added products, such as humic substances (HSs). However, the recalcitrant nature of lignocellulose hinders the utilization of cellulose and hemicellulose, decreasing the bioconversion efficiency of lignocellulose. Pretreatment is an essential step to disrupt the structure of lignocellulosic biomass. Many pretreatment methods for composting may cause microbial inactivation and death. Thus, the pretreatment methods suitable for composting can promote the degradation and transformation of lignocellulosic biomass. Therefore, this review summarizes the pretreatment methods suitable for composting. Microbial consortium pretreatment, Fenton pretreatment and surfactant-assisted pretreatment for composting may improve the bioconversion process. Microbial consortium pretreatment is a cost-effective pretreatment method to enhance HSs yields during composting. On the other hand, the efficiency of enzyme production during composting is very important for the degradation of lignocellulose, whose action mechanism is unknown. Therefore, this review describes the mechanism of action of lignocellulase, the predominant microbes producing lignocellulase and their related genes. Finally, optimizing pretreatment conditions and increasing enzymatic hydrolysis to improve the quality of composts by controlling suitable microenvironmental factors and core target microbial activities as a research focus in the bioconversion of lignocellulose during composting in the future. • The pretreatment destroys the structure of refractory lignocellulose. • Combined pretreatment and composting have a synergistic effect on HSs formation. • The key roles of lignocellulase during composting have been confirmed. • Directional control HSs formation has been proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nitrogen source supply affects humifaction process of rice straw composting. • HS and HA concentrations at 50 d of composting is higher in R_CM than that in R_UR. • Functional bacteria have specificity under driven by different nitrogen source. • Protein-like nitrogen source alleviate competition within bacterial community. • Chicken manure supply stimulates collaborative division of bacterial community. This study investigated effects of high-nitrogen source (urea) (R_UR) and protein-like nitrogen source (chicken manure) (R_CM) on humification process during lignocellulose biomass composting. It demonstrated that decreasing ratio of crude fiber (CF), polysaccharide (PS) and amino acids (AAs) in R_CM (29.75%, 53.93% and 73.73%, respectively) was higher than that in R_UR (14.73%, 28.74% and 51.92%, respectively). Humic substance (HS) concentration increased by 7.51% and 73.05% during R_UR and R_CM composting, respectively. The lower total links, more independent modularization and higher proportion of positive correlations between functional bacteria and organic components was observed with R_CM network than R_UR, indicating that protein-like nitrogen source supply may alleviate competition within bacterial community. Moreover, chicken manure supply favorably selects greater special functional bacterial taxa (Pusillimonas, Pedomicrobium, Romboustia and other 24 genus) related to AAs and stimulates the collaborative division of bacterial community. This is significance for strengthening effective transformation of organic components. [ABSTRACT FROM AUTHOR]

• Malonic acid inhibits CO 2 release by affecting the tricarboxylic acid cycle. • MnO 2 acts through biological and non-biological pathways. • Malonic acid, MnO 2, malonic acid + MnO 2 can promote the formation of HA. • Malonic acid and MnO 2 can change bacterial community structure during composting. The aim of this study was to explore the effects of malonic acid (MA), manganese dioxide (MnO 2), malonic acid combined with manganese dioxide (MA + MnO 2) addition on reducing CO 2 emission and promoting humic substance (HS) formation during composting. The result showed that the addition of MA and MnO 2 were an efficient way to reduce CO 2 emission. Meanwhile, the CO 2 emissions in the MA + MnO 2 treatment was 36.8% less than that of the CK, and the amount of humic acid (HA) produced in the MnO 2 treatment was 38.7% higher than that of the CK. Structural equation models demonstrated that the addition of exogenous substance promoted the conversion of amino acids and reducing sugars to HA. The addition of exogenous substances was the main reason for influencing the concentration of HA. In general, this research provided theoretical supports for the addition of exogenous substances to promote HA formation during composting. [ABSTRACT FROM AUTHOR]

• Identified effects of ammonia-oxidizing bacteria (AOB) on humus formation. • AOB can increase humus production by releasing NH 3 emission. • AOB has also promoted nitrogen transformation. • AOB has dual functions to reduce nitrogen loss and improve compost quality. The aim of this study was to identify the effects of ammonia-oxidizing bacteria (AOB) inoculation on humus formation. Both nitrogen conversion and humus formation were considered as the main processes, because NH 4 +-N-like compounds not only substrates of nitrification, but also precursors of humus. During composting, the inoculation of AOB indeed increased humus concentration by fixing NH 3 emission as NH 4 +-N, but it has also promoted nitrogen transformation. While the main reason was the changed bacteria community structure caused by inoculating AOB. Moreover, the relationship between bacteria and nitrogen transformation and humus formation has become closer. And bacteria were more likely to synthesize humus. Therefore, it is conjectured that AOB inoculation could not only provide NH 4 +-N for humus formation, but also enhance the anabolism of microorganisms. This suppose has been confirmed by structural equation model in this study. Therefore, AOB inoculation has a driving effect on promoting humus formation. [ABSTRACT FROM AUTHOR]

• Fenton pretreatment promoted the organic fractions degradation. • Fenton pretreatment promoted the humic substances (HS) formation. • Bacterial communities structure and diversity were changed by Fenton pretreatment. • Key environmental factors and microbes were identified. The goal of this work was to explore the effect of Fenton pretreatment combined with bacteria inoculation on the formation of humic substances (HS) during rice straw composting. In this study, the compound bacterial agents were inoculated after Fenton pretreatment during rice straw composting. The results suggested that the coupling effects of Fenton pretreatment and bacteria inoculation promoted the humification process, which might be the reason of organic fractions degradation and transformation. In addition, the bacterial communities structure and diversity were changed by Fenton pretreatment and inoculation. Key microbial genera linking to the transformation of organic fractions were determined by network analysis. Redundancy analysis and structural equation model analysis indicated that Fenton pretreatment, inoculation, amino acid, soluble sugar and beta-diversity as the key factors affecting organic fractions transformation during composting. Therefore, the combined application Fenton pretreatment with bacteria inoculation provided a new method to promote the HS amount. [ABSTRACT FROM AUTHOR]

[Display omitted] • Catechol addition reduced organic matter loss during chicken manure composting. • The addition of catechol improved the formation of humus. • Degree of aromatization of humic acid was increased by adding catechol. • Key bacteria involved in humus transformation were identified. • Catechol enhanced contribution of biotic and abiotic factors for humus formation. In this present study, catechol was added to promote humus formation in chicken manure composting using 12.5-L working volume lab-scale reactors. Chicken manure composting were carried out on two treatments, control (CK) and 0.5% (w/w) catechol treated group. Results showed that catechol addition reduced organic matter loss and improved the content of recalcitrant components. Humus and humic acid (HA) contents were, respectively, 69.2% and 82.3% higher in catechol treated-compost than in the control. With the addition of catechol, the bacterial community composition and richness was changed, e.g., the relative percentage of Bacillus and Sinibacillus were increased, which might contribute to humus and HA formation. Additionally, the activities of polyphenol oxidase and laccase (related to humus formation) were increased by the addition of catechol. Importantly, the catechol addition reduced the polyphenol content by 39.6% compared to control. Overall, the addition of catechol enhanced the biotic and abiotic factors to promote the humification process during composting, which might be a promising approach for accelerating the humification process and reducing contamination of phenolic compounds. [ABSTRACT FROM AUTHOR]

[Display omitted] • FeZ+ increased precursor content and conversion rates. • FeZ+ significantly enhanced HS content from 120.0 mg/g to 180.9 mg/g. • Microbial abundance and diversity increased and community structure changed. • FeZ+ promoted HS content and stability through biological pathways. Lignocellulose is difficult to directly degrade and mineralize during composting, which further leads to low humic substance content. Therefore, we investigated the effect of the combined Fenton-like pretreatment of the straw materials with clay minerals addition on humic substance during composting. We created six different treatments as CK, Z, FeZ, CK+, Z+, and FeZ+. The results showed that FeZ treatment increased the humic substance precursors content (31.3 %). The addition of clay minerals promoted the humic substance precursors formation, and the order of their content was as follows: FeZ+ > FeZ>Z+ > Z>CK+ > CK. The Fenton-like pretreatment with clay minerals addition increased humic substance (120.0 mg/g in CK, 141.6 mg/g in CK+, 135.8 mg/g in Z, 160.4 mg/g in Z+, 165.2 mg/g in FeZ and 180.9 mg/g in FeZ+ treatment). Upon coupling treatment increased the diversity of microbial communities, changed the composition of dominant genera, thus resulting in complex microbial structure. The structural equation model confirmed that the combined Fenton-like pretreatment with clay minerals addition could affect the content of humic substance by promoting their precursor formation. In addition, it could change the structure of microbial community structure to promote humic substance production and fixation. Our study provides new insights and theoretical support for the stable increase of humic substance content and humic substance fixation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Phenolic substances self-polymerize and polymerize faster than phenolic amines. • The phenol-amine reaction preferentially produces fulvic acid. • Humus units are an intermediate product of humus derived from phenol-amine reaction. • Humus units polymerize small molecule substances to form humic acid. Humus is the stable form of carbon storage in straw compost. The phenol-amine reaction is a pathway for humus formation in straw compost. In this study, two reaction systems, GP group (pyrogallol and glycine) and GCP group (catechol, pyrogallol, and glycine), were constructed in a simulated composting environment and revealed the molecular binding mechanism of the phenol-amine reaction through spectroscopy and mass spectrometry. The results showed that phenolic self-polymerization was faster than phenol-amine reaction. Therefore, the aromatization degree of GP was 27.14 % higher than that of GCP. The phenol-amine reaction first produced fulvic acid, and then formed humus units rich in active functional group structures (i.e., phenolic hydroxyl and carboxyl groups). These units further captured small molecule compounds to form humic acid eventually. This study would provide theoretical support for exploring the humus formation process and the promotion of straw humification by adding phenol or amino acids to compost. [ABSTRACT FROM AUTHOR]

Nitrobenzene (NB) mineralization problem exists during co-composting of NB-polluted soil and cow manure (CM). Regulating the conversion of NB degradation into humus (HS) synthesis offers a new way to circumvent the aforementioned problem. To explore the influence of the presence of NB on humification process during co-composting, co-composting of NB-polluted soil and cow manure was conducted with composting of clean soil and CM, serving as the control. Results showed that a 54.66% of NB-degradation rate was achieved after 47 d of composting. Simultaneously, compared to CK, not only HS content increased by 18.75%, but also the proportion of fluorescent component with the highest complexity of humic and fulvic acid increased by 21.92% and 44.44%, respectively. NB intermediate metabolites substantially contributed to HS formation. Meanwhile, a markedly strong interaction between bacteria and environmental factors may be crucial for enhancing the humification process in the NB group. It was verified that the addition of NB increased the content of polyphenols (λ = 0.912, p ＜0.001) and further improved the synthesis of humic acid (λ = 0.420, p ＜0.05) by strengthening the Maillard reaction. Simultaneously, the inhibitory effects of NB and polyphenols on bacterial growth prevented excessive depletion of ammonia nitrogen, favoring humic acid synthesis. These results illustrated that NB intermediate metabolites promoted HS synthesis, which was the prerequisite for subsequent establishment of targeted regulation technology of NB degradation-HS synthesis during co-composting. [Display omitted] • A 54.66% of nitrobenzene degradation rate was attained by co-composting technology. • Humus content increased by 18.75% when adding nitrobenzene. • Intensified interaction of bacteria and basic indexes contributed to humus formation. • The inhibitory effect of nitrobenzene and polyphenols favored humic acid synthesis. [ABSTRACT FROM AUTHOR]

This paper researched the effect of functional genes in the Fenton-like reactions in the hydrolysis of lignocellulosic biomass during composting. Four treatment groups were set up: CK (control), Fe (addition of Fe(II)), Z1 (inoculation of Aspergillus fumigatus), Fe+Z1 (inoculation of A. fumigatus and addition of Fe(II)). The addition of Fe(II) and inoculation of A. fumigatus was treated as a Fenton-like reaction group. Results showed that the organic matter degradation ratio of Fenton-like reactions increased by 21.18%. In addition, Fenton-like reactions increased the numbers of potential host activities of lignocellulose-degrading genes (except for the degradation of mannan) and shikimic acid pathway-related genes, and decreased potential host activities of glycolysis metabolism-related genes. Furthermore, Fenton-like reactions increased number of feature fungal species, and the diffusion of fungal species became stronger. Additionally, Fenton-like reactions enhanced the correlation between functional genes, potential hosts, and humic substance. Bacterial hosts of functional genes were affected by cellulose, hemicellulose, amino acid, reducing sugar, lignin, organic matter and C/N, while fungal hosts of functional genes were affected by pH and total nitrogen. This study confirmed that functional genes were key factors in promoting the hydrolysis of lignocellulose based on Fenton-like reactions, which provided a theoretical guidance for the degradation of agricultural wastes. • Organic matter degradation ratio of Fenton-like reactions increased by 21.18%. • Fenton-like reactions increased the potential hosts of functional genes. • Fenton-like reactions increased number of feature fungal species. • Fenton-like reactions raised correlation between genes, potential hosts, and humic substance. • Limiting factors of functional genes in different microbial communities were determined. [ABSTRACT FROM AUTHOR]

The pollution and harm of food waste (FW) are increasingly concerned, which has the dual attributes of pollutants and resources. This study aimed to improve the synthesis efficiency of FW humic substances (HS), and investigating the effect of catechol on the formation mechanism and structure of humic acid (HA) and fulvic acid (FA). Results indicated that catechol incorporation could enable to exhibit higher HS yield and more complex structure, especially the maximum particle size of FA reached 4800 nm. This was due to the combination of catechol with multiple nitrogenous compounds, which accelerated molecular condensation. Spectroscopic scans analysis revealed that Maillard reaction occurs first. Subsequently, Maillard reaction products and amino acids were combined with different sites of catechol, which leads to the difference of molecular structure of HS. The structure of FA is characterized by an abundance of carboxyl and hydroxyl groups, whereas HA is rich in benzene and heterocyclic structures. The structural difference was responsible for the disparity in the functional properties of FA and HA. Specifically, the presence of amino, hydroxyl, pyridine, and carboxyl groups in FA contributes significantly to its chelating activity. This research provides an efficient and sustainable unique solution for the high-value of FW conversion, and provides evidence for understanding the structural evolution of HA and FA. [Display omitted] • Catechol as carbon skeleton increased the humification rate of food waste. • The synergistic effect of catechol and MnO 2 increased the molecular size of FA. • Different substrate binding forms form different humic acid molecules. • Maillard reaction plays a key role in promoting humification. • FA is rich in carboxyl and hydroxyl, while HA is rich in unsaturated carbon structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • The degradation of organic matter is accelerated by the addition of MnO 2. • Humification degree of fulvic acid is increased by adding MnO 2. • The MnO 2 enhances the biological activities of humic acid in co-composting. • The role of microorganism on the formation of humic-like acid are identified. Improving the structure and quantity of humus is important to reduce agriculture organic waste by composting. The present study was aimed to assess the role of δ-MnO 2 on humus fractions formation during co-composting of chicken manure and rice straw. Two tests (control group (CK), the addition of δ-MnO 2 (M)) were carried out. The results showed that organic matter content decreased by 34% and 29% at M and CK, suggesting the process of organic waste disposal was accelerated by adding δ-MnO 2. The structures and quantity of fulvic acid (FA) and humic acid (HA) (as the main fractions of humus) were investigated. The δ-MnO 2 had no significant effect on improving the concentration of FA and HA (p > 0.05). However, the addition of δ-MnO 2 caused different effects on the FA and HA structure. The humification degree of FA improved, while bioavailability of HA increased through adding δ-MnO 2. The addition of δ-MnO 2 rephased the bacterial community structure, slowing down the succession rate of the bacterial community in M composting. After adding δ-MnO 2, the structural equation modeling results showed that environmental factors could directly drive changes in FA and HA by modulating the bacterial community. Furthermore, the role of FA and HA in the soil amendment was also demonstrated. Therefore, the addition of MnO 2 might be promising for agriculture organic waste treatment and environmental repair during composting. [ABSTRACT FROM AUTHOR]

The incorporation of malonic acid (MA) into compost as a regulator of the tricarboxylic acid (TCA) cycle has the potential to increase carbon sequestration. However, the influence of MA on the transformation of the microbial community during the composting process remains unclear. In this investigation, MA was introduced at different stages of chicken manure (CM) composting to characterize the bacterial community within the compost using high-throughput sequencing. We assess the extent of increased carbon sequestration by comparing the concentration of total organic carbon (TOC). At the same time, this study examines whether increased carbon sequestration contributes to humus formation, which was elucidated by evaluating the content and composition of humus. Our results show that the addition of MA significantly improved carbon sequestration within the compost, reducing the carbon loss rate (C loss (%)) from 64.70% to 52.94%, while increasing HS content and stability. High throughput sequencing and Random Forest (RF) analysis show that the introduction of MA leads to a reduction in the diversity of the bacterial communities, but enhanced the ability of bacterial communities to synthesize humus. Furthermore, the addition of MA favors the proliferation of Firmicutes. Also, the hub of operational taxonomic units (OTUs) within the community co-occurrence network shifts from Proteobacteria to Firmicutes. Remarkably, our study finds a significant decrease in negative correlations between bacteria, potentially mitigating substrate consumption due to negative interactions such as competition. This phenomenon contributes to the improved retention of TOC in the compost. This research provides new insights into the mechanisms by which MA regulates bacterial communities in compost, and provides a valuable theoretical basis for the adoption of this innovative composting strategy. [Display omitted] • Malonic acid (MA) directionally shapes bacterial communities in compost. • MA facilitates the growth of Firmicutes in chicken manure compost. • MA addition simplifies bacterial communities to reduce carbon loss. • MA increases the importance of Firmicutes in bacterial interaction networks. • MA increases the efficiency of bacterial synthesis of humic substances. [ABSTRACT FROM AUTHOR]

This study explored the microbial mechanism of humic substance (HS) formation in the lignocellulosic biomass composting by inoculation with different functional microbes. Typical types of functional bacterial and fungal strains with higher lignocellulose degradation characteristics were inoculated into compost. Three treatment groups were set up: uninoculated microbes (CK treatment), inoculation of Bacillus subtilis (B4 treatment) and Aspergillus fumigatus (Z1 treatment). Results showed that the HS concentration for inoculation B. subtilis and A. fumigatus increased by 15.42% and 56.31%, respectively. B. subtilis could produce phenolic metabolites, cellobiose and maltose, and then enter the tricarboxylic acid cycle, thereby releasing CO2, while A. fumigatus could promote HS formation by producing phenolic metabolites, 2,4-Di-tert-butylphenol, cellobiose, maltose, glucose, galactose, xylose, arabinose, mannose, ribose and deoxyribose. In addition, inoculating A. fumigatus enhanced the correlation among lignocellulose-degrading metabolites and HS, and increased the relative abundance and numbers of microbial hosts of lignocellulose-degrading genes, promoting the HS formation. In addition, cellulose-degrading genes and lignin-degrading genes differences were the main drivers of HS formation in the inoculated treatment groups. Soluble sugar, organic matter, lignin and pH were key drivers in influencing the lignocellulose-degrading genes. This study was important for analyzing microbial biological mechanisms of HS formation during composting. [Display omitted] • HS concentration in B4 and Z1 treatment increased by 15.42% and 56.31%, respectively. • B. subtilis could produce phenolic metabolites, cellobiose and maltose, and then enter the tricarboxylic acid cycle. • A. fumigatus could promote HS formation by producing phenolic metabolites and holocellulose-degrading metabolites. • Cellulose and lignin-degrading genes differences were the main drivers of HS formation in the inoculated treatment groups. • Soluble sugar, organic matter, lignin and pH were key drivers in influencing the lignocellulose-degrading genes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microbial pretreatment produces more reducing sugar than chemical pretreatment. • Bacillus oryzaecorticis has more influence on OH and COOH of FA structure. • Bacillus licheniformis contributed more to the aliphatic structure of HA. The study aimed to investigate the pretreatment characteristics of food waste (FW) by Bacillus licheniformis and Bacillus oryzaecorticis , and to determine the contribution of microbial hydrolysis in the structure of fulvic acid (FA) and humic acid (HA). FW was pretreated with Bacillus oryzaecorticis (FO) and Bacillus licheniformis (FL), and the resulting solution was heated to synthesize humus. The results showed that the acidic substances produced by microbial treatments led to a decrease in pH. In addition, Bacillus oryzaecorticis degraded starch and released a large amount of reducing sugar, providing OH and COOH to FA molecules. Bacillus licheniformis showed a positive effect on the HA structure, which had higher OH, CH 3 and aliphatics. FO is more beneficial to retain OH and COOH, while FL is more beneficial to retain amino and aliphatics. This study provided evidence for the application of Bacillus licheniformis and Bacillus oryzaecorticis in waste management. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • MnO 2 has been added in composting materials to promote humus formation. • Humus production increased by 38.7% in treatment group of corn straw composting. • Presence of MnO 2 emphasizes the importance of reducing sugar in humus formation. • The roles of MnO 2 in affecting humus formation are substrate dependent. Abstract The aim of this study is to reveal the roles of MnO 2 in Maillard reaction of biotic composting, and to identify its effectiveness in promoting humus formation. Corn straw (CS) and chicken manure (CM) have been chosen to be composted. During CS composting, addition of MnO 2 rapidly reduced reducing sugars concentration (decreased by 84.0%) in 5 days and significantly improved humus production by 38.7% compared with treatment without MnO 2. Whereas in CM composting, the promoting effect of MnO 2 on humus formation was relatively weak by comparing with the treatment group of CS. Additionally, the presence of MnO 2 has reshaped bacteria community, which might be the reason of MnO 2 stimulated bacteria to utilize organic matter during CM composting. Therefore, the structural equation modeling has confirmed that MnO 2 mainly performed as chemical catalyst to promote humus formation during CS composting. Besides catalyst, MnO 2 also played as a bioactive activator in CM composting. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Jointly adding BA and BD promoted HS formation, especially HA formation. • Adding precursors strengthened the relationship among them. • Significant difference of the bacterial community existed in the different treatments. • Changes in the composting environment weaken the utilization of bacteria for precursors. • An effective approach for promoting HS formation by adding precursors was proposed. Abstract The aim of this work was to explore the effect of the addition of exogenous precursors on humic substance (HS) formation during composting. HS formation is a complex biochemical process that occurs during composting. In addition, HS precursors and bacterial communities were recognized as the key factors that affect HS formation. The addition of exogenous precursors can promote the humification process during composting, but few studies have explored the potential relationships between the proportion of additional exogenous precursors, the bacterial community and HS formation. Jointly adding benzoic acid (BA) and soybean residue after extracted oil (SR) treatment can promote HS formation, especially humic acid formation. In addition, the increase in the proportion of exogenous precursors added could strengthen the relationship among different precursors, thereby changing the bacterial community composition and further promoting the humification process during composting. In addition, a structural equation model (SEM) showed that precursors were the key factors to regulate HS formation and certain bacteria as the direct drivers to affect HS formation. This model provides more possibilities to regulate HS formation during composting and enhances its potential applicability under real conditions. [ABSTRACT FROM AUTHOR]

The aim of this work was to identify the factors which can affect humic substance (HS) formation. Composting periods, HS precursors, bacteria communities and environment factors were recognized as the key factors and few studies explored the potential relationships among them. During composting, HS precursors were mainly formed in the heating and thermophilic phases, but HS were polymerized in the cooling and mature phases. Moreover, bacterial species showed similar classification of community structure in the same composting period of different materials. Furthermore, structural equation model showed that NH 4 − -N and NO 3 − -N were the indirect environmental factors for regulating HS formation by the bacteria and precursors as the indirect and direct driver, respectively. Therefore, both environmental factors and HS precursors can be the regulating factors to promote HS formation. Given that, a new staging regulating method had been proposed to improve the amount of HS during different materials composting. [ABSTRACT FROM AUTHOR]

The inoculum containing four cellulolytic thermophilic actinomycetes was screened from compost samples, and was inoculated into co-composting during different inoculation phases. The effect of different inoculation phases on cellulose degradation, humic substances formation and the relationship between inoculation and physical–chemical parameters was determined. The results revealed that inoculation at different phases of composting improved cellulase activities, accelerated the degradation of cellulose, increased the content of humic substances and influenced the structure of actinomycetic community, but there were significant differences between different inoculation phases. Redundancy analysis showed that the different inoculation phases had different impacts on the relationship between exogenous actinobacteria and physical–chemical parameters. Therefore, based on the promoting effort of inoculation in thermophilic phase of composting for the formation of humic substances, we suggested an optimized inoculation strategy to increase the content of humic substances, alleviate CO 2 emission during composting. [ABSTRACT FROM AUTHOR]

The aim of this work was to put forward a method to improve HS amount by studying the formation regularity of HS. Five precursors have been detected and few researches combined them with bacteria to study HS formation. During composting, the polyphenols, carboxyl and amino acids concentration decreased by 75.8%, 63.2% and 68.3% on average, respectively. However, the polysaccharides, reducing sugars and HS concentration increased by 61.2%, 47.1% and 37.33% on average. Relationships between precursors and HS concentration showed that the HS formation were significantly affected ( p < 0.05). The key bacteria community and physical–chemical parameters which affected HS formation have also been identified by redundancy analysis. Twelve key bacteria communities have been selected, which were significantly affected by physical–chemical parameters ( p < 0.05). Accordingly, we proposed an adjusting method to promote HS amount during composting based on the relationship between the key bacteria communities and the precursors as well as physical–chemical parameters. [ABSTRACT FROM AUTHOR]

[Display omitted] • Adding rhamnolipids after Fenton pretreatment and fungal inoculation into compost. • Adding rhamnolipids promote the humic substances formation. • The main fungi in composting is Ascomycota. • RS, pH, and TN were the key environmental factors to form HA. Composting is a green and sustainable way to dispose and reuse agricultural wastes, but the low degradation rate during composting hinders its application. This study was conducted to explore the effect of added surfactant rhamnolipids after Fenton pretreatment and inoculation of fungi (Aspergillus fumigatus) into the compost on the formation of humic substances (HS) during rice straw composting, and explored the effect of this method. The results showed that rhamnolipids speeded up the degradation of organic matter and HS formation during composting. Rhamnolipids promoted the generation of lignocellulose-degrading products after Fenton pretreatment and fungal inoculation. The differential products benzoic acid, ferulic acid, 2, 4-Di- tert -butylphenol and syringic acid were obtained. Additionally, key fungal species and modules were identified using multivariate statistical analysis. Reducing sugars, pH, and total nitrogen were the key environmental factors that affected HS formation. This study provides a theoretical basis for the high-quality transformation of agricultural wastes. [ABSTRACT FROM AUTHOR]

The purposes of this study were to explore the effect of Fenton-like reactions on the hydrolysis efficiency of lignocellulose and the formation mechanism of humic substance (HS) during rice straw composting based on genomics and metabolomics sequencing. The inoculation of Aspergillus fumigatus and adding Fe (II) into compost were used to establish Fenton-like reactions. Fenton-like reactions have been successfully established in vitro and in the composting process. In addition, Fenton-like reactions increased HS and humic acid concentration by 96.38% and 255.80%, respectively, further promoting the humification process. The key differential lignocellulose-degrading products in the Fenton-like reactions, especially for maltose, glucose, xylose, galactose, phenols, butyrophenone, vanillin, syringe acid, vanillic acid hydroxycinnamic acid and benzoic acid, were identified. Additionally, network analysis was divided into four modular microbes in the fungal network. The functional fungal modules' functions and their related influencing factors showed essential roles in the hydrolysis efficiency of lignocellulose and HS formation during composting. This research provides new insight into the hydrolysis efficiency of lignocellulose and then promotes the formation of HS in the Fenton-like reactions during agricultural waste composting. [Display omitted] • Fenton-like reactions have been successfully established during composting. • Fenton-like reactions increased humic substance (HS) concentration by 96.38%. • Fenton-like reactions increased the production of lignocellulosic metabolites. • Amino acid, reducing sugar and total nitrogen as key factors to promote HS formation. • Key functions and related influencing factors of fungal modules were determined. [ABSTRACT FROM AUTHOR]

[Display omitted] • Evaluated humic substance formation differences in different materials composting. • To confirm the effect of shikimic acid pathway (SAP) to humic substance formation. • Identified the core bacteria related to converting SAP products to humic substance. • Identified the key impact factors of humic substance formation. • A new idea for promoting humic substance formation in composting was proposed. This study aimed to evaluate differences in humic substance (HS) formation based on the shikimic acid pathway (SAP) during five different materials composting. The results showed that compared with other three materials, gallic acid, protocatechuic acid and shikimic acid of the SAP products in lawn waste (LW) and garden waste (GW) compost decreased significantly. Furthermore, as important indicators for evaluating humification, humic acid and degree of polymerization increased by 39.4%, 79.5% and 21.8%, 87.9% in LW and GW, respectively. Correlation analysis showed that SAP products were strongly correlated with HS fractions in LW and GW. Meanwhile, network analysis indicated that more core bacteria associated with both SAP products and HS were identified in LW and GW. Finally, the structural equation model proved that SAP had more significant contribution to humification improvement in LW and GW. These findings provided theoretical foundation and feasible actions to improve compost quality by the SAP. [ABSTRACT FROM AUTHOR]

[Display omitted] • The key factors affecting shikimic acid (SA) pathway were identified. • The key influencing factors of each metabolite were obviously different. • Only the SA content in lawn waste could be affected by physicochemical factors. • The more complex environment, the more factors that restrict protocatechuic acid. • A new direction for improving SA metabolic intensity during composting. Metabolites of shikimic acid (SA) pathway can be used as humic substance (HS) precursors. Due to the complexity of SA anabolism, there were few studies on SA pathway during composting. The aim of this study was to identify the key drivers of SA pathway during different materials composting. During composting, the SA, protocatechuic acid (PA) and gallic acid (GA) decreased by 57.09%, 72.27% and 54.04% on average, respectively. The structural equation model showed that SA had key driving factors (organic matter and pH) during lawn waste composting. In addition, the complexity of material structure was the main factor affecting PA driving factors. The factors and degree of influence on GA varied with different materials. Accordingly, this study provided theoretical support for the improvement of SA metabolic intensity by single material and mixed material composting, and further provided a new direction for future HS research. [ABSTRACT FROM AUTHOR]

Promoting humus production by composting is meaningful for decreasing organic wastes pollution, and repairing contaminated environment. However, the interaction of multiple humus formation pathways has limited to understand how to improve humus formation by composting. Here, humus formation has been divided into biotic and abiotic pathways to quantify their synergy to humus formation. Monomolecular reducing sugars, amino acids and protocatechuic acid acted as precursors to simulate humus formation during composting in 20 days. PARAFAC analysis of humic acids structure presented that the humus formed by abiotic process was more complex and quicker. While two-dimensional correlation spectra illustrated that compounds derived from biotic process were steadier than the abiotic one. The core microorganisms which transform simple components into complex ones could explain it, which further illustrated that the microbial metabolism can effectively promote the complexity of humus either. Additionally, to analyze the contribution of biotic and abiotic pathways to humus formation, the synergy of the two pathways has been quantified. Abiotic synthesis, biotic degradation, and biotic synthesis were accounted for 30.68%, 43.58% and 25.74%. Therefore, promoting humus production by abiotic process may be an efficient way to reduce organic waste pollution with composting, especially for the protein-like and cellulose wastes. [Display omitted] • Humus formation has been approximately quantitative analyzed. • Abiotic synthesis contributed much to humus formation during stimulate composting. • Humus structure formed by abiotic pathway was faster. • Biotic pathway promoted HA structural complexity with higher efficiency. [ABSTRACT FROM AUTHOR]

[Display omitted] • The degree of humification of HS decreases with decreasing lake depth. • Nutrient and physicochemical factors influenced HS in deep-lake sediments. • Physicochemical factors influenced HS in shallow-lake sediments. • The complex components of HS were converted into simple components in sediments. As an important part of lake ecosystems, sediments play a key role in the carbon cycle in lakes. Due to the complex sources of organic matter in sediments, the link between sequestration and transformation of humic substance is largely unknown. Therefore, it is essential to understand the characteristics of humic substance in lake sediments. In this study, the relationship between the characteristics of humic substance and environmental conditions was found in sediment samples from six typical lakes of northeast China. The structure of humic substance (HS), fulvic acid (FA) and humic acid (HA) tended to be simplified with decreasing lake depth. Pearson analysis indicated that characteristics of humic substances in deep lake sediments were dominantly determined by nutrient factors (e.g., NO 3 −-N, NO 2 −-N, NH 4 +-N) and physicochemical factors (e.g., pH, water temperature, dissolved oxygen), while the changes of HS in shallow lake sediments were dominantly determined by physicochemical factors (e.g., pH, water temperature, dissolved oxygen). Variation partitioning results also indicated that nutrient factors and physicochemical factors explained 64% and 24.2% of the variations of HS content in boreal deep lakes. In comparison, physicochemical factors explained most of the variation (35.6%) of HS composition, while nutrient factors only explained 3.4% of the variation in shallow lakes. Structural equation models further confirmed that complex components were transformed into simple components by nutrient and physicochemical factors with decreasing lake depth. This study suggests that changes in environmental conditions with lake depth may result in changes in the quality of HS in lake sediments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microbes of humic substances precursors cleavage enriched after nitrate addition. • Most of genes involved in aromatic precursors cleavage pathways were increased. • Aroma-degradation microbes exhibited negative correlation with humic substances. • Nitrate shifted environmental factors and microbial communities. • Microenvironment drove the link of functional microbes and humic substances. The aim of this study was to clarify the aromatic cleavage pathways and microbes involved in the adverse effect of nitrate on aromatic compounds humic substances during sludge composting. Results showed that the functional microbes involved in aromatic compounds humic substances precursors (catechol, tyrosine, tryptophan and phenylalanine) cleavage pathways significantly enriched after nitrate addition. Linear regression analysis showed that aromatic-ring cleavage functional microbes exhibited significant negative correlation with aromatic humic substances (p < 0.05). Furthermore, network analysis indicated that most of microbial communities prefer cooperative with aromatic-ring cleavage functional microbes. Structural equation model further revealed that composting microenvironment drove aromatic-ring cleavage functional microbes activities, resulting in the biodegradation of complex aromatic compounds. This study parsed the effect of a negative factor on aromatic compounds humic substances from an opposing perspective. Properly controlling nitrate concentration and aromatic-ring cleavage functional microbes involved in precursors cleavage was suggested to the practice of composting. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lower moisture (LM) or biochar addition (+C) regulated denitrification. • Denitrification regulations promoted aromatic humic substance formation. • LM or + C decreased denitrifying-aroma degradation microbial activities. • LM or + C inhibited denitrifying-genes and aromatic degradation pathways. • Simultaneously reduced pollution risk and delivered end-produce benefits. Denitrification during composting is a hidden danger that causes environmental pollution risk and aromatic humic substance damage, which needs to be better regulate urgently. In this study, two denitrification regulation methods, moisture and biochar amendment, were conducted during chicken manure composting. Denitrification performance data showed two regulation methods obviously reduced NO 3 −-N, NO 2 −-N and N 2 contents. Humic substance increased by 25.3 % and 29.1 % under two regulations. Microbiological analysis indicated that two regulation methods could decreasing denitrifying functional microbes with aroma degradation capability. Subsequently, denitrification gene narG , nirS , nosZ were significantly inhibited (p < 0.05) and the aromatic degradation metabolism pathways were down-regulated. Correlation analysis further revealed the important influence of interspecific interactions and non-biological characteristics on functional microbes. These results provided important scientific basis to denitrification regulation in the practice of composting, which achieved the purpose of simultaneously controlling environmental pollution risk and conducing end-product formation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Exogenous amino acids promoted lignocellulose degradation and humus synthesis. • Amino acids promoted humus synthesis by acting as precursors. • Amino acids promoted humus synthesis by affecting bacterial communities. • Key bacteria involved in humus synthesis were identified. • The metabolic function of bacterial community was analyzed. This study was aimed at exploring the mechanism of promoting humus formation by the addition of exogenous amino acids. Amino acids not only participated in the synthesis of humus directly as precursors, but also changed the functions of bacterial communities. The composition and diversity of bacterial community changed with the addition of amino acids. The ability of bacterial community to degrade lignocellulose was enhanced, which provided precursors for humus synthesis. The key bacteria for humus formation and organic matter transformation were identified using random forests. These bacteria showed growth advantage with the addition of amino acids. The results showed that exogenous amino acids tended to transform organic matter and synthesize humus. Variance partitioning analysis confirmed that the bacterial community was the driving force of humus synthesis. These results were further verified by the structural equation model. These findings provided new ideas and understanding for straw waste composting. [ABSTRACT FROM AUTHOR]

Composting, as a controlled decomposition, is the aerobic biodegradative process of organic residues. Denitrification is a facultative respiratory biochemistry pathway mediated by denitrifying microbes, which convert nitrate to nitrogen gas or nitrous oxide, and constitutes one of the main branches of nitrogen cycle during composting. Still, lots of details about denitrification during composting have not been systematically provided. This review summarizes the current findings of denitrification, which deepen understanding and emphasize the importance of denitrification during composting. Denitrification during composting is undoubtedly confirmed, mainly due to the agglomerates of composting material easily lead to local anaerobic. The loss of nitrogen and the degradation of crucial products (aromatic humic substances) caused by denitrification are the significance of denitrification studies during composting. Furthermore, this review outlines future research direction in order to increase nitrogen use efficiency and improve composting quality. Optimizing the composting process, governing the end product of nitrate respiration, and exploring biochemical processes competitive with denitrification is essential for controlling denitrification. • The occurrence of denitrification in composting has been undoubtedly confirmed. • Denitrification cause nitrogen loss during composting. • Denitrification adversely affects the formation of composting core products. • The proposals of directional control denitrification have been proposed. [ABSTRACT FROM AUTHOR]

• Functional materials (FM) disturb the effect of precursors on humus formation. • Adding FM in early stage of composting can promote organic waste reduction. • Adding FM in later stage of composting is beneficial to raise humus production. • FM promote humus formation by abiotic ways. The aim of this study is to detect the action properties of functional materials (FM) in transforming waste into resource products with high humus content. FM (MnO 2 and reducing sugar) were added in different periods of chicken manure composting. During composting, concentration of humic acids (HA) as aromatic fraction of humus, was increased by FM. The promotive effects of adding FM in later period was the most obvious. While adding FM in the beginning period could accelerate organic matter degradation, but it did not promote HA formation. Meanwhile, the microbial diversity was higher in groups by adding FM in the beginning and thermophilic periods. Therefore, it was speculated that FM might improve HA formation by promoting the abiotic polymerization of precursors. Eventually, structural equation model showed that FM was beneficial to abiotic pathway of HA formation. But the formation efficiency was reduced by interfering with biotic pathway. [ABSTRACT FROM AUTHOR]

• Enhanced denitrification reduced HS formation during sludge aerobic fermentation. • Increased denitrifying genes abundance caused enhanced denitrification. • Denitrification aromatic-degrading microbes are the host of denitrifying gene. • Denitrification aromatic-degrading microbes were vital for reducing HS formation. Humic substance (HS), as an aromatic compound, is the core product of aerobic fermentation. Denitrification-dependent degradation of aromatic compounds have been repeatedly observed in environment. However, few studies have elucidated the relationship between denitrification and aromatic HS during sludge aerobic fermentation. This study was conducted to investigate the effect of enhanced denitrification on aromatic HS formation. On the 24th day of sludge aerobic fermentation, five tests (CK, Run1, Run2, Run3 and Run4) were executed, and nitrate concentrations were adjusted to 480 ± 20, 500 ± 20, 1000 ± 20, 1500 ± 20 and 2000 ± 20 mg/kg with potassium nitrate, respectively. Analytical results demonstrated that nitrate addition increased denitrifying genes abundance and enhanced denitrification, which further reduced aromatic HS formation (p < 0.05). Especially in Run3, the concentrations of HS and humic acid on the 52nd day dramatically decreased by 12.9 % and 34.2 % in comparison with those on the 31st day. High-throughput sequencing revealed that enhanced denitrification effectively stimulated the metabolism of denitrifying microorganisms with aromatic-degrading capability. Co-occurring network analysis indicated that some keystone taxa of denitrification aromatic-degrading microorganisms involved in the conversion of nitrate to nitrite were the most crucial for enhancing denitrification and reducing aromatic HS formation. [ABSTRACT FROM AUTHOR]

• Each neutral sugar is converted by a specific group of bacteria in RS composting. • A group of bacteria can convert multiple neutral sugars during MD and L composting. • The diversity of bacterial community lead to the difference in the conversion. • The GM/AX is found to characterize the neutral sugars conversion during composting. The aim of this study was to explore the conversion characteristics of neutral sugars during different lignocellulose wastes composting from rice straw (RS), leaf (L) and mushroom dreg (MD). The results showed that the changes of neutral sugars were different during different wastes composting, but the changes of various hexose or pentose were similar during composting of the same material. The diversity of bacterial community led to different conversion characteristics of neutral sugars. During RS composting, each neutral sugar was transformed by a specific group of bacteria. However, a group of bacteria could transform multiple neutral sugars during MD and L composting. Furthermore, GM/AX value was first applied to composting, which could be used to characterize the conversion of neutral sugars during composting. This will help to provide more efficient recommendations for lignocellulose wastes treatment and accelerating humic substances synthesis during composting. [ABSTRACT FROM AUTHOR]

• The addition of inhibitors has different effects on labile and refractory materials. • Both ATP and MA can reduce CO 2 emission during CM composting. • Only MA can reduce CO 2 emission during LW and GW composting. • Both ATP and MA can promote the formation of HS, especially for ATP. • Both ATP and MA can change the bacterial community structure during composting. The aims of this study are to reveal the roles of tricarboxylic acid (TCA) cycle regulators in reducing CO 2 emission and promoting humic substance (HS) formation during composting with different materials. The results showed that the addition of adenosine tri-phosphate (ATP) or malonic acid (MA) reduced CO 2 emission during chicken manure composting. However, only the addition of MA reduced CO 2 emission during lawn waste and garden waste composting. In addition, both of the two inhibitors promoted HS formation, especially for ATP. Structural equation models further confirmed that ATP and MA reduced CO 2 emission by inhibiting the decomposition of amino acid by microorganisms. Meanwhile, ATP promoted the conversion of amino acid and soluble sugars to HS, while MA only promoted the conversion of soluble sugars to HS. In summary, this study provides a theoretical basis for the application of inhibitors to reduce CO 2 emission and promote HS formation during composting. [ABSTRACT FROM AUTHOR]