Insights into compositional changes of dissolved organic matter during a full-scale vermicomposting of cow dung by combined spectroscopic and electrochemical techniques

Biogas slurry-derived dissolved organic matter inhibited oxytetracycline adsorption by tropical agricultural soils

The increasing presence of oxytetracycline (OTC) in agricultural soils has raised global environmental concerns. We investigated the environmental behavior and fate of OTC in two types of tropical agricultural soils, focusing on the impact of dissolved organic matter (DOM) from biogas slurry. Techniques such as three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEM), Fourier Transform Infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and Ultraviolet-visible spectrophotometer (UV-vis) were used to explore the adsorption mechanisms. Our findings revealed that biogas slurry-derived DOM decreased the OTC adsorption on soils and extended the time to reach adsorption equilibrium. Specifically, the equilibrium adsorption of OTC by the two soils decreased by 19.41 and 15.32 %, respectively. These adsorption processes were effectively modelled by Elovich, intraparticle diffusion, linear, and Freundlich thermodynamic models. Thermodynamic parameters suggested that OTC adsorption onto soils was spontaneous and endothermic, with competitive interactions between biogas slurry-derived DOM and OTC molecules intensifying at higher DOM concentrations. The adsorption mechanisms were governed by both physical and chemical processes. Furthermore, the presence of Ca2+ and Na+ ions significantly inhibited OTC adsorption. These insights advanced our understanding of the fate and risk of OTC in soil environments influenced by DOM, contributing to more informed agricultural and environmental management practices.

Reuse of composted food waste from rural China as vermicomposting substrate: effects on earthworms, associated microorganisms, and economic benefits

ABSTRACTAerobic composting of food waste (FW) from rural China using a composting device results in a substantial financial burden on the government. This study aimed to assess the feasibility of mitigating this cost using vermicomposting of composted FW. The specific aims were to elucidate the effects of composted FW on earthworm growth and reproduction, reveal the changes in the physical and chemical properties of earthworm casts during vermicomposting, identify the microbial community structure associated with vermicomposting, and perform a financial analysis based on the yield of earthworms and earthworm casts. Mixing composted FW and mature cow dung in an equal ratio achieved the highest earthworm reproduction rate, where 100 adult earthworms produced 567 juvenile earthworms and 252 cocoons in 40 d. Earthworms reduce salt content of vermicomposting substrates by assimilating Na+ and promoting humification by transforming humin into humic and fulvic acid, thus producing earthworm casts with a high generation index > 80%. When composted FW was added to a vermicomposting substrate, a distinctive microbial community structure with alkaliphilic, halophilic, and lignocellulolytic microorganisms dominated the microflora. The dominant bacterial species was Saccharopolyspora rectivirgula, and the dominant fungal species changed from Kernia nitida to Coprinopsis scobicola. Furthermore, microbial genes for refractory organic matter and fat degradation were observed in Vibrio cholerae, Kernia nitida, and Coprinopsis scobicola. Financial analysis showed that vermicomposting has the potential to reduce the cost associated with FW disposal from $ 57 to $ 18/t.

Phage lysate can regulate the humification process of composting

Phages play a crucial role in orchestrating top-down control within microbial communities, influencing the dynamics of the composting process. Despite this, the impact of phage-induced thermophilic bacterial lysis on humification remains ambiguous. This study investigates the effects of phage lysate, derived explicitly from Geobacillus subterraneus, on simulated composting, employing ultrahigh-resolution mass spectrometry and 16S rRNA sequencing techniques. The results show the significant role of phage lysate in expediting humus formation over 40 days. Notably, the rapid transformation of protein-like precursors released from phage-induced lysis of the host bacterium resulted in a 14.8 % increase in the proportion of lignins/CRAM-like molecules. Furthermore, the phage lysate orchestrated a succession in bacterial communities, leading to the enrichment of core microbes, exemplified by the prevalence of Geobacillus. Through network analysis, it was revealed that these enriched microbes exhibit a capacity to convert protein and lignin into essential building blocks such as amino acids and phenols. Subsequently, these components were polymerized into humus, aligning with the phenol-protein theory. These findings enhance our understanding of the intricate microbial interactions during composting and provide a scientific foundation for developing engineering-ready composting humification regulation technologies.

Accelerating the humification of mushroom waste by regulating nitrogen sources composition: Deciphering mechanism from bioavailability and molecular perspective

Regulating nitrogen source composition is efficient approach to accelerate the spent mushroom substrate (SMS) composting process. However, currently, most traditional composting study only focuses on total C/N ratio of initial composting material. Rarely research concerns the effect of carbon or nitrogen components at different degradable level and their corresponding decomposed-substances on humification process. This study deciphers and compares the mechanism of mixed manure-N sources on SMS humification from bioavailability and molecular perspective. Two different biodegradable manure-N sources, cattle manure (CM) and Hainan chicken manure (CH), were added into the SMS composting with the different CM:CH ratio of 1:0, 3:1, 1:1, 1:3, and 0:1, respectively. The physicochemical properties and humic substances were determined to evaluate the compost quality. Coupling analysis of spectroscopy, fluorescence, and humic intermediate precursors were conducted to characterizing molecular formation process of humic acid (HA). The results indicated that regulating the carbon-nitrogen nutrient biodegradability of composting material by adding mixed nitrogen sources is an effective strategy to accelerate the SMS humification process. The C1H3 (CM:CH ratio of 1:3) and CH treatments obtained great physicochemical properties and the highest growth rate of HA (31.96% and 27.02%, respectively). The rapid reaction of polysaccharide, ketone, quinone, and amide in DOM (LCP1) might be the key for the fast humification in C1H3 and CH. The polyphenol, reducing sugar and amino acid originated from the labile-carbon-proportion I (LCP1) and recalcitrant-carbon-proportion (RCP), labile-carbon-proportion II (LCP2) and RCP, and labile-nitrogen-proportion I (LNP1), respectively, were the main driving intermediate precursors for the formation of HA. This study deciphers the SMS humification mechanism at molecular level and provides a strategy in accelerating-regulating the composting process. which will be beneficial for enhancing the disposing efficiency of SMS, producing high-quality organic fertilizer, and even popularizing to the similar types of organic waste in practical field.

Deciphering the bioavailability of dissolved organic matter in thermophilic compost and vermicompost at the molecular level

Studies on compost dissolved organic matter (DOM) previously focus on its composition and humification, without considering DOM bioavailability to understand compost fertility. To decipher the fertility basis of compost, DOM bioavailability in thermophilic compost (TC) and vermicompost (VC) was investigated and linked with its molecular composition. Results showed that DOM bioavailability of VC (36 % BDOC) was generally higher than that of TC (22 % BDOC) due to containing more tannin-like substances. Inversely, only lipid-/carbohydrate-/protein-like substances contributed to DOM bioavailability in TC. Moreover, these differences of bioavailability expanded with C/N decreased in composting materials. Specifically, the %BDOC of VC with N-rich materials (C/N < 25) was 2.1-3.0 times higher than that in TC, while it was only 1.2-1.4 times for C-rich materials (C/N < 25), because N-surplus facilitated the formation of O-/N-containing aromatics (e.g., CHON and tannin) in VC, but inhibited the decomposition of organic materials into small bioactive molecules in TC.

Electric field applications enhance the electron transfer capacity of dissolved organic matter in sludge compost

Dissolved organic matter (DOM) plays an important role in heavy metal passivation and organic pollutant degradation owing to its redox ability. The structure and composition of DOM are determinants of redox ability changes during composting. Electric field-assisted aerobic composting (EAAC) has been shown to promote the degradation and humification of organic matter in compost. However, how EAAC affects the redox ability of DOM remains unclear. Hence, electron transfer capacity (ETC) of DOM extracted from EAAC was studied using the electrochemical method. Various spectral methods, such as excitation-emission matrix and ultraviolet and visible spectrophotometry were used to study the relationship of ETC with the compositional and structural changes of DOM. Results indicated that EAAC enhanced ETC of DOM at the later stage of composting, and ETC of DOM extracted from the final EAAC product was 10.4% higher than that of the control group. Spectral and correlation analyses showed that EAAC resulted in structural and compositional changes of DOM, and humification degree, aromatic compounds, molecular weight, and fulvic- and humic-like substance contents were improved in EAAC. This conversion increased ETC of DOM. Results of this study will contribute to the understanding of the redox of DOM and in expanding the application of EAAC products.

Study on the mechanism of co-pyrolysed biochar on soil DOM evolution in short-term cabbage waste decomposition

Cabbage waste returned soil was studied to assess the short-term influences of the application of cabbage waste biochar (CB), pine wood biochar (PB), and co-pyrolysed biochar (PCB) on soil dissolved organic matter (DOM) evolution. The decrease in DOM and soil organic matter (SOM) content was greater in the biochar-added soils during 35 days of decomposition. The DOM and SOM content in PCB added group decreased by 26.96 mg L-1 and 4.48 g kg-1, respectively. The increase in relative abundance of humic acid-like substances in DOMs was higher in the biochar-added soils during decomposition, which increased by 4.29% in PCB added group. PCB addition also resulted in a high SOM content (initial content of 78.82 g kg-1), and mineral elements were introduced into the soil, thus increasing soil pH (7.81) and electrical conductivity (574.67 μs cm-1). Moreover, the addition of biochars attenuated the decrease in average relative abundance of Bacillaceae and promoted bacterial proliferation during decomposition. The application of biochars regulated the soil bacterial community and promoted organic matter conversion and soil DOM evolution.

Effects of oyster shells on maturity and calcium activation in organic solid waste compost

With the rapid development of aquaculture, the production of oyster shells has surged, posing a potential threat to the environment. While oyster shell powder is widely recognized for its inherent alkalinity and rich calcium carbonate content, making it a superior soil conditioner, its role in organic solid waste composting remains underexplored. To investigate the effects of varying concentrations of oyster shell powder on compost maturation and calcium activation, this study employed thermophilic co-composting with acidic sugar residue and bean pulp, incorporating 0% (control), 10% (T1), 20% (T2), 30% (T3), and 40% (T4) oyster shell powder. Findings revealed that appropriate proportions of oyster shell powder significantly enhance temperature stability during composting and elevate maturation levels, notably reducing ammonia emissions between 62.5% and 76.7%. Intriguingly, the calcium in the oyster shell powder was significantly activated during composting, with the 40% addition group achieving the highest calcium activation rate of 48.5%. In summation, the inclusion of oyster shell powder not only optimizes the composting process but also efficiently activates the calcium, resulting in an alkaline organic-inorganic composite soil conditioner with high exchangeable calcium content. This research holds significant implications for promoting the high-value utilization of oyster shells.

The microbial oxidation of pharmaceuticals in an anaerobic aqueous environment: Effect of dissolved organic matter fractions from different sources

Previous studies have demonstrated the importance of dissolved organic matter (DOM) on the biodegradation of trace organic contaminants occurred in the hyporheic zone. However, the role of diverse DOM fractions with distinct physicochemical properties on the biodegradation of pharmaceuticals under reducing conditions is scarcely known. To address this knowledge gap, DOMs derived from road-deposited sediment, soil, and active sludge (namely allochthonous DOM) and algae (namely autochthonous DOM) were collected and isolated into different fractions. Thereafter, the effect of DOM fractions on the anaerobic microbial oxidation of two typical pharmaceuticals, i.e., ritonavir (RTV) and tetracycline (TC) was explored by using simulated anaerobic microcosms. Mechanistic insights into how DOM fractions from different sources influence pharmaceutical biodegradation processes were provided by optical and electrochemical analyses. Results showed that humic acid and fulvic acid fractions from allochthonous DOM could enhance the biodegradation of TC (12.2 % per mgC/L) and RTV (14.5 % per mgC/L), while no significant impact was observed for that of hydrophilic fractions. However, autochthonous DOM promoted the biodegradation of TC (4.17 % per mgC/L) and inhibited that of RTV. Mechanistic analysis showed that the higher of humification and aromatization level of DOM components, the stronger their promotive effect on the biodegradation of TC and RTV. Further, the promotive mechanism could be attributed to the response of quinone moieties in DOM as extracellular electron acceptors that yields more energy to support microbial metabolism. These results provide a more comprehensive understanding of diverse DOM fractions mediating microbial anaerobic oxidation of trace organic pollutants, and extend our insights into contamination control and remediation technologies.

Compost-derived humic and fulvic acid coupling with Shewanella oneidensis MR-1 for the bioreduction of Cr(Ⅵ)

The compost-derived humic acids (HA) and fulvic acids (FA) contain abundant active functional groups with strong redox capacity, which can function as an electron shuttles for promoting the reduction of heavy metals, thus changing the form of the pollutants in the environment and reducing their toxicity. Therefore, in this study, UV-Vis, FTIR, 3D-EEM, electrochemical analysis were applied to study the spectral characteristics and electron transfer capacity (ETC) of HA and FA. Upon analysis, the results showed an increasing trend of ETC and humification degree (SUVA254) for both HA and FA during composting. However, the aromatic degree (SUVA280) of HA was higher than FA. After 7 days of culture, 37.95% of Cr (Ⅵ) was reduced by Shewanella oneidensis MR-1 (MR-1) alone. Whereas, only if HA or FA existed, the diminution of Cr (Ⅵ) reached 37.43% and 40.55%, respectively. However, the removal rate of Cr (Ⅵ) by HA/MR-1 and FA/MR-1 increased to 95.82% and 93.84% respectively. It indicated that HA and FA acted as electron shuttles, mediating the transfer of electrons between MR-1 and the final electron acceptor, effectively facilitating the bioreduction of Cr (Ⅵ) to Cr (Ⅲ) and also determined via correlation analysis. This study suggested compost-derived HA and FA coupling with MR-1 exhibited excellent performance for the bioreduction of Cr (Ⅵ) to Cr (Ⅲ).

Ammonia assimilation is key for the preservation of nitrogen during industrial-scale composting of chicken manure

Nitrogen loss from compost is a serious concern, causing severe environmental pollution. The NH4+-N content reflects the release of NH3. However, the nitrogen conversion pathway that has the greatest impact on NH4+-N content is still unclear. This study attempted to explore the key pathways, core functional microorganisms, and mechanisms involved in the transformation of ammonia nitrogen during composting. KEGG (Kyoto Encyclopedia of Genes and Genomes) metabolic pathways revealed that ammonia assimilation was dominated by the glutamate dehydrogenase (GDH) pathway (53.4%), which is crucial for nitrogen preservation. The combined analysis of KEGG, NR species annotation, and co-occurrence network identified 20 easy-to-regulate obligate core nitrogen-transforming functional microorganisms, including 18 ammonia-assimilating bacteria. Furthermore, the effects of environmental parameters on the obligate core functional microorganisms were investigated. The present study results provided a theoretical basis for the utilization of ten ammonia-assimilating bacteria, such as Paenibacillus, Erysipelatoclostridium, and Defluviimonas to improve the quality of compost.

Molecular insight into the transformation of dissolved organic matter during sewage sludge composting: An investigation of a full-scale composting plant

The aim of the study was to explore the molecular dynamics and transformation pathways of dissolved organic matter (DOM) in sewage sludge (SS) during composting, and the DOM of raw material, material experiencing thermophilic phase and material collected from humification phase were characterized using electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry. The results indicated that there were approximately 85% of aliphatic/proteins and 75% of carbohydrate preferentially decomposed in the thermophilic phase. Moreover, lignins/carboxylic-rich alicyclic molecules (CRAM) were the main N-containing substances evolved in the decomposition, which leading to a reduction of N/C ratio from 0.073 to 0.041. Whereas aliphatic acids and tryptophan in lignins/CRAM with high oxidizing capacities are preferentially decomposed in the thermophilic phase. As for maturity phase, the carbon of the newly generated compounds (belonging to lignins/CRAM and tannins), possessed an oxidation state that similar to sulfonates and sulfonamides, and these DOM are beneficial for the humic substances formation. Moreover, it was found that the newly formed N2Ox and N3Ox compounds had a more significant contribution to the double bond equivalent (DBE) of the compost, corresponding to 1.0 and 1.7 DBE, respectively. The results would help explore the understanding of DOM transformation and humification during SS composting in the microscopic molecular level.

Black soldier fly pretreatment promotes humification and phosphorus activation during food waste composting

Black soldier fly (BSF) and thermophilic composting (TC) treatments are commonly adopted to manage food waste. In this study, 30 days of TC of food waste following seven days BSF pretreatment (BC) was compared to 37 days of TC of food waste (TC, the control). Fluorescence spectrum and 16S rRNA high-throughput sequencing analysis were used to compare the BC and TC treatments. Results showed that BC could decrease protein-like substances and increase humus substances more quickly, and that the humification index of compost products was 106.8% higher than that of TC, suggesting that the humification process was accelerated by BSF pretreatment resulting in a 21.6% shorter maturity time. Meanwhile, the concentrations of total and available phosphorus rose from 7.2 and 3.3 g kg-1 to 44.2 and 5.5 g kg-1, respectively, which were 90.5% and 118.8% higher in compost products from BC as compared to those in TC. Furthermore, BC had higher richness and diversity of humus synthesis and phosphate-solubilizing bacteria (PSB), with Nocardiopsis (53.8%) and Pseudomonas (47.0%) being the dominant PSB. Correlation analysis demonstrated that the introduction of BSF gut bacteria contributed to the effectiveness of related functional bacteria, resulting in a rapid humification process and phosphorus activation. Our findings advance understanding of the humification process and provide novel perspectives on food waste management.

Long-term microbial community succession and mechanisms of regulation of dissolved organic matter derivation in livestock manure fermentation system

Industrial-scale aerobic fermentation was conducted with livestock manures. Microbial inoculation promoted the growth of Bacillaceae and consolidated its position as the dominant microorganism. Microbial inoculation substantially influenced dissolved organic matter (DOM) derivation and variations of related components in the fermentation system. The relative abundance of humic acid-like substances of DOM increased from 52.19% to 78.27% in microbial inoculation system, resulting in a high humification level. Moreover, lignocellulose degradation and microbial utilization were the important factors influencing DOM content in fermentation systems. The fermentation system was regulated by microbial inoculation, thus achieving a high level of fermentation maturity.

Isolation of dissolved organic matter from aqueous solution by precipitation with FeCl3: mechanisms and significance in environmental perspectives

Ferric ions can bind strongly with dissolved organic matter (DOM), including humic acids (HA), fulvic acids (FA), and protein-like substances, whereas isolation of Fe-DOM precipitates (Fe-DOM_P) and their biochemical characteristics remain unclear. In this work FeCl₃ was used to isolate DOM components from various sources, including river, lake, soil, cow dung, and standard tryptophan and tyrosine, through precipitation at pH 7.5-8.5. The Fe-DOM_P contribute to total DOM by approximately 38.6-93.8% of FA, 76.2% of HA and 25.0-30.4% of tryptophan and tyrosine, whilst fluorescence spectra allowed to monitor/discriminate the various DOM fractions in the samples. The relative intensity of the main infrared peaks such as 3406‒3383 cm^-1 (aromatic OH), 1689‒1635 cm^-1 (‒COOH), 1523-1504 cm^-1 (amide) and 1176-1033 cm^-1 (‒S=O) show either to decline or disappear in Fe‒DOM_P. These results suggest the occurrence of Fe bonds with various functional groups of DOM, indicating the formation of π-d electron bonding systems of different strengths in Fe‒DOM_P. The novel method used for isolation of Fe-DOM_P shows promising in opening a new frontier both at laboratory and industrial purposes. Furthermore, results obtained may provide a better understanding of metal-organic complexes involved in the regulation of the long-term stabilization/sequestration of DOM in soils and waters.

Accelerating the humification mechanism of dissolved organic matter using biochar during vermicomposting of dewatered sludge

The use of biochar can accelerate the vermicomposting process of dewatered sludge. However, the underlying mechanism of vermicompost maturity with biochar is still unclear. This study investigated the effect of biochar on sludge maturation during vermicomposting by analyzing the spectroscopic characteristics of dissolved organic matter. For this, dewatered sludge mixed with and without 5 % biochar were separately vermicomposted. The results showed that the biochar could significantly increase the biomass of earthworms as well as the activity and abundances of bacteria and eukaryotes (P < 0.05) during vermicomposting of sludge. The addition of biochar resulted in a 23.35 % increase in the ratio of absorbances at wavelengths of 250 nm and 365 nm (E₂₅₀/E₃₆₅), and a 20.50 % decrease in aromatic proteins of sludge during vermicomposting, compared to the control. The ratio of fulvic acid to humic acid contents rapidly increased from 10 to 15 days in biochar added treatment, which was earlier than that in the control. Compared to control, the biochar addition enhanced the contents of fulvic acid and humic acid in sludge vermicompost by 79.97 % and 91.54 %, respectively. During vermicomposting, the DOM maturated parameter displayed stronger correlation (P < 0.05) between each other in the treatment with biochar, rather than the control. This study suggests that the biochar addition significantly modifies the degradation pathway of dissolved organic matter, thus promoting sludge maturation during vermicomposting.

Insights into organic matter evolution during food waste stabilization induced by 14-hour high-temperature fermentation

Large amounts of secondary pollutants are released during traditional composting, and rapid fermentation is desirable for the stabilization of food wastes. Food wastes were mixed with rice husk, placed in a bioreactor, and stirred continuously to achieve high-temperature fermentation for 14 h. The transformations of the mixtures were analyzed using elemental and spectral analysis combined with kinetic equations and two-dimensional correlation spectroscopy. The carbohydrates, proteins, and aliphatic compounds of food waste were degraded after 4 h of fermentation. Transformations of dissolved organic and sulfur- and nitrogen-containing substances followed first-order kinetic equations with reaction rate constants of 0.142 h^-1, 0.098 h^-1, and 0.016 h^-1, respectively. Organic matter conversion was in the following order: aliphatic → protein → carbohydrate and followed the order, acrylamide C → O-alkyl C → anomeric C at the molecular level. The fermentation process was characterized by the increase in protein- and fulvic-like compounds. Fulvic acid substances gradually accumulated during the late fermentation period. Thus, dissolved organic matter components were gradually transformed into humic substances with increasing fermentation time. The sequence of transformation during the fermentation process was, tyrosine-like → tryptophan-like → fulvic-like substances. Humification mainly occurred in the mature stage of composting; therefore, it was verified that the food waste was stabilized by a 14-h fermentation.

Insight into humification of mushroom residues under addition of Rich-N sources: Comparing key molecular evolution processes using EEM-PARAFAC and 2D-FTIR-COS analysis

Accelerating the humification of organic solid waste is one of the most important issues in composting. This present study aims to study and compare the humification process of different rich-N sources (chicken manure, cattle manure, and urea) addition during the composting of mushroom residues, from macro physicochemical properties to micro humic molecular structure evolution process. The physicochemical elements and humic components were determined for evaluating the compost quality and humification degree as composting proceed. The coupled analysis of excitation-emission matrix with parallel factor analysis (EEM-PARAFAC) and two-dimensional correlation with Fourier transform infrared spectrum (2D-FTIR-COS) were used to characterize the functional molecular structure evolution of dissolved organic matter during humification process. The results indicated that the rank order for humification level were the treatments of chicken manure (HM), urea (UM), cattle manure (CM), and single mushroom residue treatment (CK), with their humification index of 22.18%, 22.05%, 18.47%, and 16.52%, respectively. Humic substance, humic acid, and fulvic acid were obtained the highest in HM treatment with contents of 35.41 ± 0.86%, 23.32 ± 1.57%, and 10.97 ± 0.52%, respectively. The rich-N source addition enhanced the degradation of protein-like and polysaccharides-like substances in dissolved organic matter, thus accelerating the humification process of mushroom residues. The key structure evolution of dissolved organic matter in the HM treatment, in which the CO and CC stretching of quinone, amide, or ketone, and the C-O stretching of polysaccharides may be responsible for the faster formation of humus compared to the other nitrogen treatments. In this study, redundancy analysis indicated that the total nitrogen (TN) and nitrate nitrogen (NO₃^--N) may be the potential indicators for determining the humification level as composting proceed. The result provides significant insight into the humification mechanism of mushroom residue under different types of nitrogen sources at the molecular level, and will be reference for improving the composting technique in practical field.

Characterization of vermicompost of coconut husk mixed with cattle dung: physicochemical properties, SEM, and FT-IR analysis

The present work evaluated the potential of vermicomposting in management of different ratios of coconut husk waste (CH) and cattle dung (CD) viz (waste: CD) 0:100 (CH₀), 25:75 (CH₂₅), 50:50 (CH₅₀), 75:25 (CH₇₅), and 100:0 (CH₁₀₀) using Eisenia fetida for 120 days. The physicochemical properties were analyzed in vermicompost samples taken on the 0 and 120th day. Co-composting with cattle manure improved their acceptability for E. fetida as well as their physicochemical properties. In a 50:50 (CH₅₀) ratio, the lowest mortality and maximum growth in terms of number and biomass of earthworms were observed. The results revealed that during pre-vermicompost to post-vermicompost, nutrients such as nitrogen, phosphorus, and sodium increased, whereas in all vermicomposting end products organic carbon and the C:N ratio decreased significantly. Except zinc, all heavy metals decreased significantly (p < 0.05) over initial in all the feed mixtures. Seed germination tests indicated that the mature and non-phytotoxic vermicompost has been formed at the end of the experiment. The Fourier transmission infrared spectroscopy (FT-IR) and scanning electron microscopic (SEM) images of vermicompost demonstrated the excellent maturity of the compost.

Deciphering the structural characteristics and molecular transformation of dissolved organic matter during the electrolytic oxygen aerobic composting process

Electrolytic oxygen aerobic composting (EOAC) effectively treats organic solid waste by using in-situ electrolytic oxygen for aeration. However, the fundamental mechanism of compost maturity is still unclear. Therefore, we comprehensively characterized dissolved organic matter (DOM) transformation closely related to compost maturity during EOAC. Excitation-emission matrix-parallel factor (EEM-PARAFAC) and Fourier transform infrared (FTIR) analysis confirmed that EOAC quickly decreased organic matter and increased humus substances, accelerating the compost humification process compared with conventional aerobic composting. Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis reveals that the double bound equivalent and aromaticity index during EOAC are higher than in conventional aerobic composting (CAC), suggesting more aromatic compounds in EOAC. DOM's detailed transformation investigation suggested that low O/C and high H/C compounds were preferentially decomposed during EOAC. Our investigation firstly extends the in-depth molecular mechanisms of humification during EOAC, and reveals its practical engineering applications.

The migration and microbiological degradation of dissolved organic matter in riparian soils

Riparian zones are important natural means of water purification, by decreasing the aqueous concentration of terrestrial organic matter (OM) through adsorption and microbial degradation of the organic matter within the aquatic ecosystem. Limited studies have been reported so far concerning the migration of dissolved organic matter (DOM) in the horizontal and vertical planes of riparian zones. In this study, the migration of DOM in riparian zones, from forest soil to wetland soil, and with soil depth, were explored, based on a case study reservoir. Results showed that riparian wetlands can absorb the OM from the forest soils and adjacent reservoir, and act as a major OM sink through microbial action. Methylomirabilota and GAL15 bacteria increased with soil depth for the two soil systems, and the wetland soil system also contained microbial sulfates, nitrates and carbonates. These microorganisms successfully utilize the Fe³⁺, SO₄^-, and CO₃^- as electron acceptors in the wetland system, resulting in enhanced OM removal. Although the variation of soil DOM in the vertical direction was the same for both forest and wetland soils, the Chemical structure of the DOM was found to be significantly different. Furthermore, the soil was found to be the main source of DOM in the forest ecosystem, with lignin as the main ingredient. The lignin structure was gradually oxidized and decomposed, with an increase in carboxyl groups, as the lignin diffused down into the soil and the adjacent reservoir. PLS-PM analysis showed that the soil physicochemical properties were the main factors affecting DOM transformation. However, microbial metabolism was still the goes deeper affecting factor. This study will contribute to the analysis that migration and transform of soil organic matter in riparian zone.

Effect of ventilation quantity on electron transfer capacity and spectral characteristics of humic substances during sludge composting

Humic substances (HSs) can ameliorate soil pollution by mediating electron transfer between microorganisms and contaminants. This capability depends on the redox-active functional structure and electron transfer capacity (ETC) of HS. This study mainly aimed to analyze the effects of different ventilation quantities on the ETC and spectral characteristics of HS (including humic acids (HAs) and fulvic acids (FAs)) during sludge composting. HS was extracted from compost with different ventilation quantities (0.1, 0.2, and 0.3 L kg^-1 dry matter min^-1, denoted as VQ1, VQ2, and VQ3, respectively). The ETC of HS was measured by electrochemical method. Excitation-emission matrix (EEM) spectroscopy, ultraviolet and visible (UV-Vis) spectrophotometry, and Fourier transform infrared (FT-IR) spectroscopy were conducted to understand the evolution of HS composition during composting. Results indicated that the ETC of HA and FA increased during composting, and VQ2 had stronger ETC and electron recycling rate than VQ1 and VQ3 at the end of composting. UV-Vis analysis revealed that the humification degree, aromatization degree, and molecular weight of HA and FA increased during composting, while the content of lignin decreased. EEM-PARAFAC results suggested that VQ2 accelerated the degradation of protein-like substances. FT-IR revealed a decrease trend in polysaccharide and aliphatic, and the carboxyl content increased in VQ2 and VQ3 while decreased in VQ1. Correlation analysis was used to study the relationship between HS components and ETC. The results advance our further understanding of the pollution remediation mechanism of HS.

Alternating electric field enables hyperthermophilic composting of organic solid wastes

Hyperthermophilic composting (HTC) achieves compost temperatures above 80 °C, usually depending on the inoculated hyperthermophilic bacteria, which has been well used in full-scale plants. However, the scarcity of hyperthermophilic bacteria and the high cultivation cost hinder the development of HTC. Recently, a direct-current electric field applied on conventional aerobic composting raised compost temperature to 70-75 °C, but gradient moisture distribution under the action of the direct-current electric field affected microbial metabolic heat and limited the temperature rise. Herein the effects of alternating electric field (AEF) promoting a uniform water distribution and further raising the temperature to achieve HTC were investigated. Our results demonstrated that AEF raised the compost temperature to 90 °C, and the period with temperatures above 80 °C lasted 4 days. The physicochemical properties and maturity index showed that the AEF improved the biodegradation and humification of organic matter due to the generation of metabolic heat. The AEF enriched thermophilic bacteria (Ureibacillus: by 52.36% on day 3; Navibacillus: by 46.54% on day 41). A techno-economic analysis indicated that the proposed approach with the AEF had a cost advantage over HTC with the inoculation of hyperthermophilic bacteria. Therefore, the AEF composting system represents a novel and applicable strategy for HTC.

In-situ electrolytic oxygen is a feasible replacement for conventional aeration during aerobic composting

Aerobic composting is an effective recycling method for the disposal and resource utilization of organic solid waste. However, the inappropriate aeration mode used during conventional aerobic composting (CAC) often results in low oxygen utilization efficiency and loss of temperature, which further leads to a long maturation period and large odorous gas (NH₃) pollution. Herein, a novel electrolytic oxygen aerobic composting (EOAC) process was invented first using in-situ oxygen generation for aeration by the electrolysis of water in compost. Our results demonstrated that the germination index (GI) significantly increased during EOAC, and the maturation time of compost was shortened by nearly 50% during EOAC compared to CAC, indicating higher oxygen utilization efficiency during EOAC. Meanwhile, NH₃ emissions, N₂O emissions, and nitrogen loss during the EOAC process decreased by 61%, 46%, and 21%, respectively, compared to CAC. The total relative abundance of thermophilic and electroactive bacteria during EOAC increased remarkably. EOAC inhibited ammoniation, nitrification, and denitrification, and weakened N-associated functional genes. A techno-economic analysis indicated that EOAC had greater technical superiority and cost advantages compared to CAC. This study represents proof-of-principle for EOAC and suggests that in-situ electrolytic oxygen is a feasible replacement for conventional aeration during aerobic composting.

Moisture migration driven by the electric field causes the directional differentiation of compost maturity

Electric field-assisted aerobic composting (EAC) has been recently believed as a novel and effective process for the resource utilization of organic solid waste. However, the effect of electric field in composting process needs to be further clarified. Herein, moisture migration and compost maturity along electric-field-direction (from anode to cathode) in EAC was first to be explored. It was found that moisture content and compost maturity changed regularly from anode to cathode. At the end of composting, the moisture content of S3 (cathodic zone) was 30% and 62% higher than that of S2 (middle zone) and S1 (anodic zone), respectively. The germination index (a key parameter for compost maturity) in S3 (138.92%) was significantly higher than that of S2 (104.98%) and S1 (84.45%). However, temperatures in S3 were lower than that of S1 and S2, indicating the moisture content played a more important role than temperature for compost maturity in EAC. Furthermore, the microbial activities in S3 were also higher than that of S1 and S2, supporting the trend of compost maturity. This pioneering study demonstrates the electric field can drive moisture gradient migration to control the directional differentiation of compost maturity, showing a great application potential in aerobic composting.

Molecular characterization of the composition and transformation of dissolved organic matter during the semi-permeable membrane covered hyperthermophilic composting

Current knowledge of dissolved organic matter (DOM) in semi-permeable membrane-covered thermophilic compost (smHTC) is limited. Therefore, this study provided a comprehensive characterization of composition and transformation of DOM in smHTC using multiple spectroscopic methods and ultrahigh resolution mass spectrometry. The results showed that the values of SUVA₂₈₀, SUVA₂₅₄, A_240-400 (0.042, 0.048, 34.193) in smHTC were higher than those of conventional thermophilic composting (cTC) (0.030, 0.037, 18.348), and the increment of P_V,n in smHTC were 2.4 times higher than that of cTC. These results suggested that smHTC accelerated the humification process by promoting the degradation of labile DOM and the production of humus-like substances. Mass spectrometry further confirmed that the DOM of smHTC possessed higher degree of aromatization and humification, based on the lower H/C (1.14), higher aromaticity index (0.34) and double bond equivalence (10.36). Additionally, smHTC increased the proportion of carboxyl-rich, unsaturated and aromatic compounds, and simultaneously improved the degradation of aliphatic/proteins, lipids, carbohydrates, along with even some refractory substances such as CHO subcategory (24.1%), especially lignin-like structures (14.8%). This investigation provided molecular insights into the composition and transformations of DOM in smHTC, and extended the current molecular mechanisms of humification in composting.

Microbial Activity during Composting and Plant Growth Impact: A Review

Replacing harmful chemical pesticides with compost extracts is steadily gaining attention, offering an effective way for plant growth enhancement and disease management. Food waste has been a major issue globally due to its negative effects on the environment and human health. The methane and other harmful organisms released from the untreated waste have been identified as causes of this issue. Soil bacteria impart a very important role in biogeochemical cycles. The interactions between plants and bacteria in the rhizosphere are some of the factors that determine the health and fertility of the soil. Free-living soil bacteria are known to promote plant growth through colonizing the plant root. PGPR (Plant Growth Promoting Rhizobacteria) inoculants in compost are being commercialized as they help in the improvement of crop growth yield and provide safeguard and resistance to crops from disease. Our focus is to understand the mechanism of this natural, wet waste recycling process and implementation of a sustainable operative adaptation with microbial association to ameliorate the waste recycling system.

Vermicomposting of cow manure: Effect of time on earthworm biomass and chemical, physical, and biological properties of vermicompost

Vermicomposting is a biological process for efficient cattle manure treatment, but the vermicomposting time determines the quality of the vermicompost. The objective of this study was to evaluate the effect of cattle manure vermicomposting time on earthworm biomass and the changes in physical, chemical, and biological in properties of the vermicompost. The cattle manure was inoculated with Eisenia andrei earthworms and conducted vermicomposting for 0, 15, 30, 45, 60, and 120 days. The analysis of 44 chemical, physical, and biological properties allowed the vermicomposting process to be divided into initial (<45 days) and final (45-120 days) phases. The initial phase was characterized by high microbial activity and the final by high physical-chemical transformation of the vermicompost and an increase in earthworm density. The organic matter aromaticity increased until the 45th day, subsequently decreasing. Although 30 d of vermicompost are sufficient to obtain a high-quality organic fertilizer, 120 d are necessary for producing matrices.

Accelerated humification and alteration of microbial communities by distillers' grains addition during rice straw composting

This research explored the influence of distillers' grains amendment on the humification performance and microbial communities during rice straw composting. The composition of dissolved organic matter and maturity index were analyzed by the fluorescence excitation emission matrix spectroscopy and parallel factor analysis. High-throughput sequencing and redundancy analysis were employed for revealing the structure dynamics for microbial community and their shaping factors, respectively. Results indicated that addition of distillers' grains effectively increased the microbial activity, which was beneficial to the organic matter degradation and nitrogen conservation. Microbial community structures were significantly changed with different amendment strategies. Nitrate, water soluble carbon, organic matter, ammonium were the key parameters influencing the variation of bacterial community in different treatments. Water soluble carbon significantly affected the dominant fungal community dynamics. These results showed that addition of distillers' grains effectively improved the nutritional status and changed the microbial communities during rice straw composting.

Linking microbial community structure with molecular composition of dissolved organic matter during an industrial-scale composting

This study explored the interactions between dissolved organic matter (DOM) composition and microbial community structure during an industrial-scale composting by Fourier transform ion cyclotron resonance mass spectrometry and 16S rRNA sequencing analysis. The results revealed that DOM from matured compost contained primarily lignins/carboxylic-rich alicyclic molecules (73.6%), the higher double bond equivalent (5.97) and aromaticity index (0.18), indicating that the molecular composition of DOM had changed substantially. Drastic changes in microbial community structure were also observed along with the DOM transformation process of composting. Network analysis further indicated that Caldicoprobacter, Bacillus, and Dechloromonas were associated with the most DOM subcategories. Caldicoprobacter could degrade carbohydrates, Bacillus accelerated the humification by transforming N-containing compounds, and Dechloromonas could degrade polycyclic aromatic hydrocarbons distributed in low O/C. These findings are helpful for understanding the molecular mechanisms of DOM transformation and humification of sludge composting.

Linking the electron transfer capacity with the compositional characteristics of dissolved organic matter during hyperthermophilic composting

Redox-active functional groups in dissolved organic matter (DOM) can mediate reductions in organic pollutants and the passivation of heavy metals, which are related to the humification process of composting. Hyperthermophilic composting (HTC) has been shown to promote changes in the composition and structure of DOM and accelerate humification. However, how HTC affects the redox properties of DOM remains unclear. Here, we fractionated DOM into humic acid (HA), fulvic acid (FA) and hydrophilic (HyI) fraction to study their electron transfer capacities (ETC) and the relationship between ETC and compositional characteristics using electrochemical method and excitation-emission matrix-parallel factor analysis. HTC accelerated the formation of component 3 containing quinone-like moieties, which mainly existed in the HA, improving the electron accepting capacity (EAC) of DOM. The rapid degradation of component 4 containing tryptophan-like substances of HA, FA and HyI strengthened the electron donating capacity of DOM in HTC. Partial least squares path model also showed that compositional changes and the stronger ETC of DOM in HTC had a positive effect on the maturity degree, revealing that the EAC of HA could be used as a maturity index for compost. This study advances our understanding of the humification process and the contamination control mechanism of HTC.

Nutrient recovery from pineapple waste through controlled batch and continuous vermicomposting systems

The largest portion of pineapple peels and pulp generated from production points is disposed of haphazardly contributing to a number of environmental and health challenges. However, these wastes contain valuable plant nutrients that could be recovered to boost soil fertility, and increase agricultural production. This study evaluated the variation in physico-chemical parameters in batch and continuous vermicomposting systems as potential pathways for nutrient recovery from pineapple waste. The study compared the efficiency of waste reduction and nutrient recovery for batch (B), and continuous (C) vermicomposting systems during a 60-day period. The substrates were pineapple peels (PW), and cattle manure (CM) fed in a ratio of 4:1 (w/w). Control reactors were fed with 100% CM in both the feeding modes. Results indicated that waste degradation was 60%, and 54% while earthworm biomass increased by 57% and 129% for BPW, and CPW, respectively. pH significantly decreased with time in both systems. Total phosphorous increased with vermicomposting time with that of B being significantly higher than C systems. Nitrogen, potassium, and sodium significantly increased in the control experiments while the three elements significantly reduced for BPW, and CPW owing to high leachate production in the latter. The N, P, K, and C retention in vermicompost was 24.2%, 90.4%, 67.5%, 41.1%, and 32.6%, 91.2%, 79.3%, 46.1%, for BPW and CPW, respectively. Continuous systems produced higher earthworm biomass and retained more nutrients in vermicompost than batch systems, and can therefore, be recommended as better systems for pineapple waste vermicomposting.

Alkali lignin and sodium lignosulfonate additives promote the formation of humic substances during paper mill sludge composting

Alkali lignin (AL) and sodium lignosulfonate (SLS) are by-products of the papermaking industry and could influence composting processes due to their rich aromatic structures. In this study, the roles of AL and SLS additives in the formation of humic substances (HS) during paper mill sludge composting were investigated. Results showed that HS content and degree of polymerization of the final products in AL (44.42 mg·g^-1 and 0.70, respectively) and SLS (45.87 mg·g^-1 and 1.14, respectively) treatments were appreciably higher than those of the control sample (34.36 mg·g^-1 and 0.67). Excitation-emission matrix-parallel factor coupled with two-dimensional FT-IR correlation spectroscopy analysis suggested that AL and SLS additives could speed the transformation of quinone-like substances by increasing the amounts of low molecular weight lignin depolymerized products, which led to higher HS concentrations. This work provided a way of promoting HS formation and the comprehensive utilization of papermaking wastes.

Nutrient recovery and vermicompost production from livestock solid wastes with epigeic earthworms

The aim of this work was to study nutrient recovery and vermicompost production from livestock solid wastes of indigenous and exotic cow breeds with epigeic earthworms. Fourteen days pre-decomposed dung of Vechur native (CD1) and exotic Jersey (CD2) breeds were vermicomposted with Perionyx excavatus and Eudrilus eugeniae for 45 days (Cycle I) and 90 days (Cycle II) including respective controls without earthworms. Vermicomposts from CD1 and CD2 substrates during Cycle I and II showed increase in NPK, Ca and micro-nutrients (P < 0.05); whereas, pH, total organic carbon, C/N and C/P ratios disclosed decrease (P < 0.05) over initial levels for both the earthworms. E. eugeniae was found efficient in vermicomposting cattle solid wastes in 45 days and CD1 yielded nutrient rich vermicompost. The study concludes that Cycle I is suitable for nutrient recovery and vermicompost production, in addition to mass multiplication of earthworms in Cycle II.

Valorization of Orange Peel Waste Using Precomposting and Vermicomposting Processes

The industrialization process of oranges generates waste, which is inadequately disposed of; this produces adverse effects on the environment. Among the alternatives for valorization is the vermicomposting process, which consists of the degradation of organic waste through the action of earthworms and microorganisms. Therefore, this research aimed to study this process using orange peel (OP) waste at the laboratory level. For this purpose, it was necessary to determine the degradation conditions through the monitoring of physicochemical parameters (temperature, pH, humidity, organic matter (OM), total organic carbon (TOC), total nitrogen (TN) and the carbon/nitrogen (C/N) ratio). To balance the substrate’s nutrients, load material (LM) that included vegetable waste and eggshells was added to three different mixtures: M1 (50% OP + 50% LM), M2 (40% OP + 60% LM) and M3 (60% OP + 40% LM). To condition the substrate for earthworm (Eisenia fetida) activity, a previous precomposting process was performed. The results showed that all the mixtures fulfilled the requirements for a quality and mature vermicompost; however, the highest concentrations for TN were in the mixtures M1 and M2. The total time required for degradation of the OP waste was 13 weeks.