Exploring the impact of biochar on antibiotics and antibiotics resistance genes in pig manure aerobic composting through untargeted metabolomics and metagenomics

Mechanism of reducing antibiotic resistance genes by nano-selenium during composting: insight into host microorganisms and a two-component system

Nano-selenium (nano-Se) makes up for the low biological activity and high toxicity of inorganic selenium, and is widely used as a novel Se fertilizer in agriculture. This study investigated the effects of different concentrations of nano-Se on Antibiotic resistance genes (ARGs) and host microorganisms during aerobic composting of cow manure. The results showed that high-concentration nano-Se had excellent effects on ARG removal, effectively inhibiting the dominant ARG subtypes such as aada and tetA(48). Transposase was the mobile genetic element (MGE) that deserves the most attention, which maintained a relatively high abundance in each sample. Antibiotic efflux-transposase had the highest abundance of all ARG resistance mechanism-MGE types. At the end of composting, antibiotic efflux-transposase abundance in group Se_H decreased by 7.14 %, 21.96 %, and 54.92 %, respectively, compared with groups CK, Se_L, and Se_M. High-concentration nano-Se inhibited the activity of transposase to reduce the risk of horizontal transmission of antibiotic efflux ARGs. Proteobacteria and Actinobacteria were the main ARG host microorganisms, and Actinobacteria had a lower tolerance threshold for nano-Se. Network analysis and Mantel testing demonstrated that high-concentration nano-Se could weaken the association between host microorganisms and the ARG subtypes. Structural equation models showed that high-concentration nano-Se regulated the transfer of ARGs primarily by stimulating signal transduction in host microorganisms. Thus, the addition of proper concentrations of nano-Se during aerobic composting of cow manure was effective in reducing ARG exposure risk by regulating transposase and host microorganisms.

Degradation of typical tetracycline antibiotics in landfill leachate by three-dimensional aerated electrocatalytic reactor (3D-AER): electrode properties, influencing factors and degradation mechanism

Anode Ti/RuO2/IrO2-SnO2 was prepared, and coconut shell biochar (CBC) was applied as the particle electrode to construct a three-dimensional aerated electrocatalytic reactor (3D-AER) for enhanced treatment of landfill leachate, especially for tetracycline antibiotics (TCs), including tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC) in it. The characterization results of the anode indicated that the dense RuO2-IrO2-SnO2 solid solution on the surface of the electrode coating had excellent electrocatalytic activity, high oxygen evolution potential, and low chlorine evolution potential. The investigation of influencing factors showed that under the conditions of pH 10, current density of 30 mA/cm2, KCl concentration of 100 mg/L, CBC dosage of 7 g/L, CBC particle size of 1.7-2.36 mm, and electrode spacing of 3 cm, excellent treatment effects were achieved for nitrogen and carbon removal from leachate. The removal rates of COD, NH3-N, TC, OTC, and CTC reached 82.76 %, 66.02 %, 86.58 %, 87.46 %, and 81.88 %, respectively, and the degradation process of TCs followed pseudo-first-order kinetics. The presence of CBC enhanced mass transfer and electrolysis efficiency, promoting the generation of ·OH. The degradation modes of macromolecular organic compounds and TCs in leachate included anodic direct oxidation and indirect oxidation (free radical pathway(·OH) and non-free radical pathway (1O2 and active chlorine)), among which 1O2 dominated. In the denitrification process, NH3-N accounted for 79.69 % of TN, most of which was converted into N2 and NO3-, and Cl· and active chlorine played an indispensable role. Therefore, this study shares a theoretical basis for further understanding the electrocatalytic process, degradation mechanism of TCs in chlorine containing leachate.

The new strategies for high efficiency removal of antibiotics and antibiotic resistance genes by direct bio-drying of biogas slurry: Microbiological mechanisms

High levels of antibiotics and antibiotic resistance genes (ARGs) still exist in biogas slurry after anaerobic digestion of cow manure. In this study, direct bio-drying strategies of cow manure biogas slurry without solid-liquid separation for the removal of antibiotics and ARGs were explored. The results showed that, after direct bio-drying of biogas slurry, the moisture contents decreased to 25.2 %-31.5 %. The maximum temperatures of the piles reached 76.1-77.4 °C, which is close to ultra-high temperatures (>80 °C). Direct biogas slurry bio-drying (CK treatment) achieved efficient removal of antibiotics, ARGs, and mobile genetic elements (MGEs) (95.4 %, 98.6 % and 86.7 % removal, respectively). Compared to the CK treatment, molecular membrane covering (MMC) alone was the most effective in further significantly decreasing the antibiotic concentration and the abundance of ARGs and MGEs in the final bio-dried samples, followed by food waste hydrochar (FHC) addition alone. Methanogenic archaea were identified as potential hosts for ARGs based on Network analysis. FHC addition-MMC increased the abundance of potential hosts for ARGs and promoted the expression of microbial methane metabolism function relative to the CK treatment during the later stages of bio-drying, thereby decreasing the removal efficiency of ARGs. The results of structural equation model and redundancy analysis showed that MGEs had the most significant direct effect on ARGs and moisture content had the highest relative contribution to changes in ARGs. In summary, direct bio-drying strategies were able to efficiently remove antibiotics and ARGs from cow manure biogas slurry and also achieve biological dewatering of the biogas slurry.

Biochar for ameliorating soil fertility and microbial diversity: From production to action of the black gold

This article evaluated different production strategies, characteristics, and applications of biochar for ameliorating soil fertility and microbial diversity. The biochar production techniques are evolving, indicating that newer methods (including hydrothermal and retort carbonization) operate with minimum temperatures, yet resulting in high yields with significant improvements in different properties, including heating value, oxygen functionality, and carbon content, compared to the traditional methods. It has been found that the temperature, feedstock type, and moisture content play critical roles in the fabrication process. The alkaline nature of biochar is attributed to surface functional groups and addresses soil acidity issues. The porous structure and oxygen-containing functional groups contribute to soil microbial adhesion, affecting soil health and nutrient availability, improving plant root morphology, photosynthetic pigments, enzyme activities, and growth even under salinity stress conditions. The review underscores the potential of biochar to address diverse agricultural challenges, emphasizing the need for further research and application-specific considerations.

Metagenomic insights on promoting the removal of resistome in aerobic composting pig manure by lightly burned modified magnesite

The antibiotic resistance genes (ARGs) have become a serious issue facing public health. In this study, light-burned magnesite with a high specific surface area at 650 °C (MS650) was used for aerobic composting, evaluating its effect on the resistome during pig manure composting. Different concentrations of MS650 reduced the abundance of the resistome, including seven high-risk ARGs, class two metal and biocide resistance genes (MBRGs), and human pathogenic bacteria (HPBs). The addition of 2.5 % MS650 (L1) in the composting had the best reduction effect on ARGs, MBRGs and HPBs. ARG and microbial community assembly are deterministic processes. Proteobacteria and Actinobacteria was the main factor associated with the decrease in ARGs, followed by virulence factor genes (VFGs, 44.2 %). The reduction in MBRGs by MS650 mainly suppressed HGT by reducing the Isfinder abundance. To summarize, MS650 is an effective method to improve emission reduction of ARGs and MBRGs. This study provided a theoretical basis for improving the engineering application potential of MS650.

Optimizing the management of aerobic composting for antibiotic resistance genes elimination: A review of future strategy for livestock manure resource utilization

Aerobic composting technology is an efficient, safe and practical method to reduce the residues of antibiotics and antibiotic resistance genes (ARGs) due to unreasonable disposal of livestock manure. Nowadays, it remains unclear how aerobic composting works to minimize the level of remaining antibiotics and ARGs in manure. Moreover, aerobic composting techniques even have the potential to enhance ARGs level. Therefore, this study conducted a literature review on ARGs variation during the composting process to assess the fate, migration, and risk features of antibiotics and ARGs in different livestock manure and compost. The relationship between ARGs reduction and crucial factors (temperature, heavy metal, and microbial community structures) in the composting process was discussed. The merits and limitations of different technologies used in compost was summarized. The effects on ARGs reduction in the aerobic composting process with various strategies was examined. We attempt to provide a fresh and novel viewpoint on the advancement of global aerobic composting technology.

Metabolomics analysis of advancing humification mechanism in secondary fermentation of composting by fungal bioaugmentation

The aim of this study was to investigate the differential metabolites and core metabolic pathways caused by fungal bioaugmentation (pH regulation and Phanerochaete chrysosporium inoculation) in secondary fermentation of composting, as well as their roles in advancing humification mechanism. Metabolomics analyses showed that inoculation strengthened the expression of carbohydrate, amino acid, and aromatic metabolites, and pH regulation resulted in the up-regulation of the phosphotransferase system and its downstream carbohydrate metabolic pathways, inhibiting Toluene degradation and driving biosynthesis of aromatic amino acids via the Shikimate pathway. Partial least squares path model suggested that lignocellulose degradation, precursors especially amino acids and their metabolism process enhanced by the regulation of pH and Phanerochaete were the main direct factors for humic acid formation in composting. This finding helps to understand the regulating mechanism of fungal bioaugmentation to improve the maturity of agricultural waste composting.

Deciphering and predicting changes in antibiotic resistance genes during pig manure aerobic composting via machine learning model

Livestock manure is one of the most important pools of antibiotic resistance genes (ARGs) in the environment. Aerobic composting can effectively reduce the spread of antibiotic resistance risk in livestock manure. Understanding the effect of aerobic composting process parameters on manure-sourced ARGs is important to control their spreading risk. In this study, the effects of process parameters on ARGs during aerobic composting of pig manure were explored through data mining based on 191 valid data collected from literature. Machine learning (ML) models (XGBoost and Random Forest) were utilized to predict the rate of ARGs changes during pig manure composting. The model evaluation index of the XGBoost model (R2 = 0.651) was higher than that of the Random Forest (R2 = 0.490), indicating that XGBoost had better prediction performance. Feature importance was further calculated for the XGBoost model, and the XGBoost black box model was interpreted by Shapley additive explanations analysis. Results indicated that the influencing factors on the ARGs variation in pig manure were sequentially divided into thermophilic period, total composting period, composting real time, and thermophilic stage average temperature. The findings gave an insight into the application of ML models to predict and decipher the ARG changes during manure composting and provided suggestions for better composting manipulation and optimization of process parameters.

Occurrence and dissemination of antibiotic resistance genes in the Yellow River basin: focused on family farms

As an emerging contaminant, antibiotic resistance genes (ARGs) have attracted growing attention, owing to their widespread dissemination and potential risk in the farming environment. However, ARG pollution from family livestock farms in the Yellow River basin, one of the main irrigation water sources in the North China Plain, remains unclear. Herein, we targeted 21 typical family farms to assess the occurrence patterns of ARGs in livestock waste and its influence on ARGs in receiving environment by real-time quantitative PCR (qPCR). Results showed that common ARGs were highly prevalent in family livestock waste, and tet-ARGs and sul-ARGs were the most abundant in these family farms. Most ARG levels in fresh feces of different animals varied, as the trend of chicken farms (broilers > laying hens) > swine farms (piglets > fattening pigs > boars and sows) > cattle farms (dairy cattle > beef cattle). The effect of natural composting on removing ARGs for chicken manure was better than that for cattle manure, while lagoon storage was not effective in removing ARGs from family livestock wastewater. More troublesomely, considerable amounts of ARGs were discharged with manure application, further leading to the ARG increase in farmland soil (up to 58-119 times), which would exert adverse impacts on human health and ecological safety.

Relationships between arsenic biotransformation genes, antibiotic resistance genes, and microbial function under different arsenic stresses during composting

Although the arsenic contamination and antibiotic resistance genes (ARGs) during composting have been studied separately, there is limited information on their interactions, particularly, the relationship between arsenic biotransformation genes (ABGs) and ARGs. Therefore, the present study used different forms of arsenic stress (organic and inorganic arsenic at 10 and 50 mg/kg) in pig manure and straw co-composting, to evaluate the effects of arsenic stress on microbial community structures, metabolic function, ABGs, and ARGs. The results showed that arsenic stress had different effects on different parameters and promoted the microbial formation of humic acid and the biodegradation of fulvic acid. Inorganic arsenic showed more rapid effects on microbial community structure, visible within about 20 days, while the effects of organic arsenic were later (about 45 days) due to the necessity of transformation. Moreover, the addition of organic roxarsone and inorganic arsenic resulted in higher expression of ABGs and ARGs, respectively. Arsenic addition also caused increased expression of genes associated with replication and repair. A significant relationship was observed between ABG and ARG expression, for instance, genes involved in arsenic reduction and oxidation were influenced by genes involved in aminoglycoside and chloramphenicol resistance genes (p < 0.05). These complex interactions among microorganisms, functional genes, and external parameters contribute to the understanding of the mechanisms underlying cross-contamination.

Biochar induces different responses of intracellular and extracellular antibiotic resistance genes and suppresses horizontal transfer during lincomycin fermentation dregs composting

This study aims to indicate the influence of biochar on extracellular and intracellular ARGs (e/iARGs) variation and proliferation during lincomycin fermentation dregs (LFDs) compost. Biochar addition made iARGs keep reducing but eARGs increase to the maximum at the middle thermophilic phase and reduce at the end of the compost. Compared to control 3.15-log and 5.42-log reduction of iARGs and eARGs were observed, respectively. Biochar addition, bacterial community, and MGEs were the major contributors to iARGs and eARGs removal, with the contribution percentages of 38.4%, 31.0%, 23.7%, and 27.2%, 29.1%, and 34.9%, respectively. Moreover, biochar significantly inhibited eARGs transformation and RP4 plasmid conjugative transfer among E. coli DH5α and Pseudomonas aeruginosa HLS-6. The underlying mechanism involved in broken cell membranes of bacteria, and altered expression of oxidative stress genes and save our souls (SOS) response-related genes. The results indicated that biochar addition in composting could limit the dissemination of ARGs.

Fungus reduces tetracycline-resistant genes in manure treatment by predation of bacteria

New strategies to remove antibiotic resistance genes (ARGs), one of the most pressing threats to public health, are urgently needed. This study showed that the fungus Phanerochaete chrysosporium seeded to a composting reactor (CR) could remarkably reduce tetracycline-resistant genes (TRGs). The reduction efficiencies for the five main TRGs (i.e., tetW, tetO, tetM, tetPA, and tet(32)) increased by 8 to 100 folds compared with the control without P. chrysosporium, and this could be attributed to the decrease in the quantity of bacteria. Enumeration based on green fluorescence protein labeling further showed that P. chrysosporium became dominant in the CR. Meanwhile, the bacteria in the CR invaded the fungal cells via the cell wall defect of chlamydospore or active invasion. Most of the invasive bacteria trapped inside the fungus could not survive, resulting in bacterial death and the degradation of their TRGs by the fungal nucleases. As such, the predation of tetracycline-resistant bacteria by P. chrysosporium was mainly responsible for the enhanced removal of TRGs in the swine manure treatment. This study offers new insights into the microbial control of ARGs.

Effect of municipal sludge-based biochar produced at different pyrolysis temperatures on humification and oxytetracycline degradation of pig manure composting

This study explored the influence of pyrolysis temperatures on the properties of municipal sludge-based biochar (MSB) and evaluated the impact of MSB on humification and oxytetracycline (OTC, a broad-spectrum antibiotic) degradation in pig manure composting. Three types of MSB were produced from sewage sludge pyrolyzed at 300 °C, 500 °C, and 700 °C, respectively. Results indicated that pyrolysis temperature adjusted the formation sequence of the functional groups in MSB, and higher pyrolysis temperatures enriched the aromaticity of the biochar and augmented the concentrations of humic precursor compounds. The MSB addition to pig manure composting enhanced the peak temperature and prolonged the thermophilic phase. Moreover, the MSB addition significantly increased the HI (humic acid/fulvic acid) values (1.6-2.57) compared with the control (1.28), with a more pronounced effect observed at higher biochar pyrolysis temperatures. Furthermore, the MSB reduced the half-life of OTC degradation (1.47-2.44 d) during composting, accelerating its degradation compared with the control (2.66 d). The study demonstrated that the MSB provided a substantial amount of humic precursor materials into the composting process while also expediting the degradation of organic matter, thereby enhancing the humification process. Moreover, the extended duration of the thermophilic phase accelerated the degradation of OTC and shortened its half-life. Notably, the MSB at 700 °C had the best performance compared with other MSBs.

Risk characteristics of resistome coalescence in irrigated soils and effect of natural storage of irrigation materials on risk mitigation

Irrigation and fertilization are the routinely agricultural practices but also cause resistome coalescence, by which the entire microbiomes from irrigation materials invade soil microbial community, to transfer antibiotic resistance genes (ARGs) in the coalesced soils. Although studies have reported the effect of irrigation or fertilization on the prevalence and spread of ARGs in soils, risk characteristics of resistome coalescence in irrigation system remain to be demonstrated and few has shown whether natural storage of irrigation materials will reduce resistance risks. To fill the gaps, two microscopic experiments were conducted for deeply exploring resistance risks in the soils irrigated with wastewater and manure fertilizer from a perspective of community coalescence by metagenomic analysis, and to reveal the effect of natural storage of irrigation materials on the reduction of resistance risks in the coalesced soils. Results showed irrigation and coalescence significantly increased the abundance and diversity of ARGs in the soils, and introduced some emerging resistance genes into the coalesced community, including mcr-type, tetX, qacB, and an array of genes conferring resistance to carbapenem. Procrustes analysis demonstrated microbial community was significantly correlated with the ARGs in coalesced soils, and variance partitioning analysis quantified its dominant role on shaping resistome profile in the environment. Besides ARGs, abundant and diverse mobile genetic elements (MGEs) were also identified in the coalesced soils and co-existed on the ARG-carrying contigs, implying potential transfer risk of ARGs in the irrigation system. Further, the analysis of metagenome-assembled genomes (MAGs) confirmed the risk by recovering 358 ARGs-carrying MAGs and identifying the resistant bacteria that co-carried multiple ARGs and MGEs. As expected, the natural storage of irrigation water and manure fertilizer reduced about 27%-54% of ARGs, MGEs and virulence factors in the coalesced soils, thus caused the soils to move towards lower resistance risks to a certain extent.

Biochar applications in microbial fermentation processes for producing non-methane products: Current status and future prospects

The objective of this review is to encourage the technical development of biochar-assisted microbial fermentation. To this end, recent advances in biochar applications for microbial fermentation processes (i.e., non-methane products of hydrogen, acids, alcohols, and biofertilizer) have been critically reviewed, including process performance, enhanced mechanisms, and current research gaps. Key findings of enhanced mechanisms by biochar applications in biochemical conversion platforms are summarized, including supportive microbial habitats due to the immobilization effect, pH buffering due to alkalinity, nutrition supply due to being rich in nutrient elements, promoting electron transfer by acting as electron carriers, and detoxification of inhibitors due to high adsorption capacity. The current technical limitations and biochar's industrial applications in microbial fermentation processes are also discussed. Finally, suggestions like exploring functionalized biochar materials, biochar's automatic addition and pilot-scale demonstration are proposed. This review would further promote biochar applications in microbial fermentation processes for the production of non-methane products.

Effects of magnesium-modified biochar on antibiotic resistance genes and microbial communities in chicken manure composting

Abatement of antibiotic resistance genes (ARGs) in livestock manure by composting has attracted attention. This study investigated the effect of adding magnesium-modified biochar (MBC) on ARGs and microbial communities in chicken manure composting. Twelve genes for tetracyclines, sulfonamides, and macrolides, and mobile genetic elements were measured in the compost pile. The results showed that after 45 days of the composting, the treatment groups of MBC had longer high temperature periods, significantly higher germination indices (GI) and lower phytotoxicity. There were four major dominant phyla (Firmicutes, Actinobacteriota, Proteobacteria, and Bacteroidota) in the compost. The abundance of Firmicutes decreased significantly during the compost cooling period; tetracycline resistance genes demonstrated an extremely significant positive correlation with Firmicutes, showing a trend of the same increase and decrease with composting time; tetT, tetO, tetM, tetW, ermB, and intI2 were reduced in the MBC group; the total abundance of resistance genes in the 2% MBC addition group was 0.67 times that of the control; Proteobacteria and Chloroflexi were also significantly lower than the other treatment groups. Most ARGs were significantly associated with mobile genetic elements (MGEs); MBC can reduce the spread and diffusion of ARGs by reducing the abundance of MGEs and inhibiting horizontal gene transfer (HGT).

Effect of hydrochar on antibiotic-resistance genes and relevant mechanisms during chicken manure composting

The reduction of enhanced antibiotic resistance genes (ARGs) in compost is important to mitigate the risk of ARG transmission in agricultural production. Hydrochar is used in many applications as a functional carbon material with adsorption and catalytic properties. This study investigated the effects of hydrochar addition on bacterial communities, mobile genetic elements (MGEs), and ARGs in chicken manure composting. The addition of 2%, 5%, and 10% hydrochar (dry weight) reduced the total numbers of target ARGs and MGEs in the compost products by 40.13-55.33% and 23.63-37.23%, respectively. Hydrochar changed the succession of the bacterial population during composting, lowering the abundance of potential pathogens and promoting microbial activity in amino acid and carbohydrate metabolism. A significant possible microbial host for ARGs was found to be Firmicutes. Hydrochar was found to affect the host microorganisms and MGEs directly by altering environmental factors that indirectly impacted the ARG profiles, as shown by partial least squares pathway modeling analysis. In conclusion, the addition of hydrochar to compost is a simple and effective method to promote the removal of ARGs.

External static magnetic field potentiates the reduction of antibiotic resistance genes during swine manure composting

Livestock and poultry manure are repositories of antibiotic resistance genes (ARGs). Accumulating evidence suggests that composting is an important way to effectively attenuate ARGs, but how to reinforce the reduction in ARGs during composting needs to be further investigated. This study explored the influence of an external static magnetic field on ARG mitigation enhancement during swine manure composting. The results showed that a total of 12 high-risk ARGs were identified. A relatively high magnetic field intensity (14.81 mT) was more effective in reducing the abundance of high-risk ARGs, and the removal rate was 20.66-100 %. It also reduced the abundance of 27.14 % of integrons, 79.44 % of insertion sequences, and 8.78 % of plasmids. Partial least squares path modeling showed that a relatively high magnetic field intensity treatment promoted the reduction in ermB by reducing the abundance of Phascolarctobacterium, Streptococcus, and insertion sequences. It also mitigated sul1 expression by reducing the abundance of Acinetobacter and integrons, and it mitigated tetM expression by decreasing Lactobacillus, Streptococcus, insertion sequences, and plasmids. These findings demonstrate that an external static magnetic field is an effective method for intensifying the reduction in ARGs, providing a feasible reference for controlling the potential ARG risk of organic waste composting.

Synergistic effects of rice husk biochar and aerobic composting for heavy oil-contaminated soil remediation and microbial community succession evaluation

Soil petroleum pollution is an urgent problem in modern society, which seriously threatens the ecological balance and environmental safety. Aerobic composting technology is considered economically acceptable and technologically feasible for the soil remediation. In this study, the combined experiment of aerobic composting with the addition of biochar materials was conducted for the remediation of heavy oil-contaminated soil, and treatments with 0, 5, 10 and 15 wt% biochar dosages were labeled as CK, C5, C10 and C15, respectively. Conventional parameters (temperature, pH, NH₄⁺-N and NO₃^--N) and enzyme activities (urease, cellulase, dehydrogenase and polyphenol oxidase) during the composting process were systematically investigated. Remediation performance and functional microbial community abundance were also characterized. According to experimental consequences, removal efficiencies of CK, C5, C10 and C15 were 48.0%, 68.1%, 72.0% and 73.9%, respectively. The comparison with abiotic treatments corroborated that biostimulation rather than adsorption effect was the main removal mechanism during the biochar-assisted composting process. Noteworthy, the biochar addition regulated the succession process of microbial community and increased the abundance of microorganisms related to petroleum degradation at the genus level. This work demonstrated that aerobic composting with biochar amendment would be a fascinating technology for petroleum-contaminated soil remediation.

Industrial-scale aerobic composting of livestock manures with the addition of biochar: Variation of bacterial community and antibiotic resistance genes caused by various composting stages

The presence of large amounts of antibiotic resistance genes (ARGs) in livestock manures poses an impending, tough safety risk to ecosystems. To investigate more comprehensively the mechanisms of ARGs removal from industrial-scale composting of livestock manure based on biochar addition, we tracked the dynamics of bacterial community and ARGs at various stages of aerobic composting of livestock manures with 10% biochar. There were no significant effects of biochar on the bacterial community and the profiles of ARGs. During aerobic composting, the relative abundance of ARGs and mobile genetic elements (MGEs) showed overall trends of decreasing and then increasing. The key factor driving the dynamics of ARGs was bacterial community composition, and the potential hosts of ARGs were Caldicoprobacter, Tepidimicrobium, Ignatzschineria, Pseudogracilibacillus, Actinomadura, Flavobacterium and Planifilum. The retention of the thermophilic bacteria and the repopulation of the initial bacteria were the dominant reasons for the increase in ARGs at maturation stage. Additionally, among the MGEs, the relative abundance of transposon gene was substantially removed, while the integron genes remained at high relative abundance. Our results highlighted that the suitability of biochar addition to industrial-scale aerobic composting needs to be further explored and that effective measures are needed to prevent the increase of ARGs content on maturation stage.

Exploring the impact of biochar supplement on the dynamics of antibiotic resistant fungi during pig manure composting

The purpose of this study was to investigate antibiotic resistant fungal (ARF) communities in pig manure (PM) composting employing two different biochar (coconut shell-CSB and bamboo biochar-BB) as amendment. Three treatments (Control, 10% CSB and 10% BB) were designed and indicated with T1 to T3. Experimental results declared that the fungal abundance significantly reduced among the both biochar applied treatments but three dominant phyla Ascomycota, Basidiomycota and Mucoromycota were still relatively greater abundance present. There were significant differences (p < 0.05) in the relative abundance and diversity of fungi among all three treatments. Interestingly, biochar addition regulated the overall fungal community in final compost. Compared with the control group, the abundance of fungi was positively mobilized, and especially CSB showed a better effect. Conclusively, biochar has potential to inhibit and reduce the ARGs population and mobility in compost. Thus, these findings offer new insight to understand the succession of ARFs during PM composting.

Inoculating indoleacetic acid bacteria promotes the enrichment of halotolerant bacteria during secondary fermentation of composting

The secondary fermentation stage is critical for stabilizing composting products and producing various secondary metabolites. However, the low metabolic rate of mesophilic bacteria is regarded as the rate-limiting stage in composting process. In present study, two indoleacetic acid (IAA)-producing bacteria (Bacillus safensis 33C and Corynebacterium stationis subsp. safensis 29B) were inoculated to strengthen the secondary fermentation stage to improve the plant-growth promoting potential of composting products. The results showed that the addition of IAA-producing bacteria promoted the assimilation of soluble salt, the condensation and aromatization of humus, and the accumulation of dissolved organic nitrogen (DON) and dissolved organic carbon (DOC). The bioaugmentation strategy also enabled faster microbial community succession during the medium-late phase of secondary fermentation. However, the colonization of Bacillus and Corynebacterium could not explain the disproportionate increase of IAA yield, which reached up to 5.6 times compared to the control group. Deeper analysis combined with physicochemical properties and microbial community structure suggested that IAA-producing bacteria might induce the increase of salinity, which enriched halotolerant bacteria capable of producing IAA, such as Halomonas, Brachybacterium and Flavobacterium. In addition, the results also proved that it was necessary to shorten secondary fermentation time to avoid IAA degradation without affecting composting maturity. In summary, enhancing secondary fermentation of composting via adding proper IAA-producing bacteria is an efficient strategy for upgrading the quality of organic fertilizer.

Three-dimensional printed bulking agents reduce antibiotic resistance genes in swine manure aerobic composting by regulating oxygen concentration to alter host microorganisms and mobile genetic elements

Antibiotic resistance genes (ARGs) in manure aerobic composting are a potential environmental pollutant. Therefore, reducing the abundance of ARGs is crucial. The effects of adding three-dimensional printed bulking agents (3DBAs) on ARGs in aerobic composting of swine manure were investigated in this study. Compared with the control group, 3DBAs with different addition dosages can greatest reduce the total ARGs by 5.98%, tetracycline resistance genes by 14.02%, macrolide resistance genes by 9.65%, and sulfonamide resistance genes by 20.59%. By further combining physicochemical parameters, host microorganisms, and mobile genetic elements (MGEs) for analysis, it was found that oxygen concentration was vital for ARGs reduction, and 3DBAs with regular porosity and uniform size indirectly regulate the activity of host microorganisms and MGEs abundance by changing the oxygen consumption, finally reducing vertical or horizontal ARGs transfer risks. Overall, 3DBAs addition is an effective strategy to reduce the abundance of ARGs in aerobic composting.

The effects of biochar on antibiotic resistance genes (ARGs) removal during different environmental governance processes: A review

Antibiotic resistance genes (ARGs) pollution has been considered as one of the most significant emerging environmental and health challenges in the 21st century, many efforts have been paid to control the proliferation and dissemination of ARGs in the environment. Among them, the biochar performs a positive effect in reducing the abundance of ARGs during different environmental governance processes and has shown great application prospects in controlling the ARGs. Although there are increasing studies on employing biochar to control ARGs, there is still a lack of review paper on this hotspot. In this review, firstly, the applications of biochar to control ARGs in different environmental governance processes were summarized. Secondly, the processes and mechanisms of ARGs removal promoted by biochar were proposed and discussed. Then, the effects of biochar properties on ARGs removal were highlighted. Finally, the future prospects and challenges of using biochar to control ARGs were proposed. It is hoped that this review could provide some new guidance for the further research of this field.