| Aerobic composting is an important technology to treat and reuse organic solid waste such as sludge.The emergence of new types of pollutants such as antibiotic resistance genes?ARGs?puts forward higher requirements for the resource utilization of sludge.Whether from the perspective of recycling of organic solid waste,or from the problem of reducing environmental pollution by solid waste,the maturity effect of composting sludge with agricultural waste additives,and the transmission of ARGs issues in the composting process and its application are all worthy of attention.Starting from the actual problem of poor maturity of traditional agricultural waste and sludge compost,this project intends to carry out evaluation of the effect of composting with straw substrate and sludge,and reveal the key factors of ARGs reduction in the aerobic composting process.Besides,the driving mechanism of the spread of ARGs in the soil-plant systems following the land use of the compost will be explored.The main research conclusions are as follows:1.The straw substrate was used to mix straws for aerobic composting.The temperature of the straw fermentation back-mixed compost rose rapidly in the initial stage,which entered the thermophilic stage??45??on the 3rd day and maintained for5 days.The content of loosely combined humic acid and stably combined humic acid increased significantly at the maturity stage?P<0.05?,from 7.72±0.31 g/kg to18.64±0.05 g/kg and 0.72±0.15 g/kg to 14.68±0.29 g/kg,respectively.Firmicutes,Proteobacteria,Bacteroides and Actinobacteria were the dominant phyla in the aerobic composting process.With the progress of aerobic composting,the relative abundance of Firmicutes decreased while the numbers of Proteobacteria,Bacteroides and Actinobacteria increased significantly,respectively.Network analysis showed that total phosphorus had a greater impact on humus than C/N ratio and available phosphorus.In the metabolic function of the microbial community,the abundance of Carbohydrate metabolism and Amino acid metabolism sequence was dominant.Besides,the sequences related to Carbohydrate metabolism increased significantly with composting.It indicated that Carbohydrate metabolism plays a vital role in the degradation of hemicellulose and cellulose.The content clarified the main microbial community structure succession features of using the back-mixed fermentation substrate to prepare the straw subatrate and revealed the characteristics of the metabolic function,providing an important theoretical basis for the metabonomics research of the composting system.Moreover,it provided the material for the subsequent sludge compost.2.The sludge was composted aerobically combining the straw substrate,rice straw,wheat straw and sawdust,respectively.It was found that during the aerobic composting process of the straw subatrate and the sludge,the temperature rose rapidly to 75°C and maintained at?45°C for 12 days,which were significantly more advantageous than other treatment groups.Furthermore,the relative root length and germination index?GI?of the compost product were both greater than 80%.The microbial community structure of the straw subatrate combined with the sludge treatment group was significantly different from the other four groups.The results suggested that the differences in the physical and chemical properties of sludge compost caused by the added materials will affect the diversity of the compost microbial community.Correlation analysis showed that the physical and chemical properties of the straw subatrate and sludge synergistic aerobic composting process significantly affected the microbial community diversity and biological indicators?seed germination,relative root growth and GI?.The comprehensive evaluation of principal component analysis showed that the straw subatrate had the highest comprehensive score for the product of sludge aerobic composting,and it was the best composting material.The research results confirmed that the straw subatrate as a sludge composting conditioning material is beneficial to promote the maturity of the sludge compost and reduce the toxicity of the compost product,providing a new idea for aerobic composting of organic solid waste.3.Although the straw substrate helps to promote the maturity of the sludge compost,the sludge contains a lot of ARGs,which need to be controlled during the composting process.The C/N ratio plays an important role in the composting process,but the influence of different C/N ratios on the ARGs is still unknown.The research is aimed at the composting with straw subatrate and sludge aerobic,where the C/N ratio was optimized.Meanwhile,the fate and influence mechanism of ARGs in the aerobic composting of straw subatrate and sludge were explored.The results showed that the initial C/N ratio could significantly affect the fate of ARGs in the aerobic composting of straw substrate and sludge.The removal rate of tetracycline resistance genes?tet M and tet Q?,?-lactam resistance genes(bla TEM and bla OXA)and the encoding gene of the multidrug excretion pump?mex F?were 94.69%?95.78%,90.24%?96.20%and 96.66%,respectively with a C/N ratio of 30:1.By optimizing the C/N ratio?30:1?during the composting process,the thermophilic period could be prolonged and the bioavailable heavy metals could be reduced.Correlation analysis showed that the reduction of the bioavailable heavy metals was also conducive to reducing the abundance of ARGs.Different initial C/N ratio treatments had significant changes in the abundance of compost microbial community composition.Host microorganisms were the main factor driving the spread of ARGs in the process of aerobic composting of straw subatrate and sludge.Optimizing the initial C/N ratio could reduce the host bacteria of ARGs.Network analysis showed that the changes of sul1,sul2,and aad A1 were jointly affected by multiple metabolic pathways such as Genetic information processing,Environmental information processing,and Metabolism.The research results confirmed that the effective reduction of ARGs could be achieved by adjusting the C/N ratio,which can provide technical support for controlling the spread of ARGs in sludge composting.4.The resource-based products of sludge compost are usually used for soil improvement and other purposes,but the impact of ARGs spreading in the soil-plant systems due to the accumulation of heavy metals?Cd?is rarely reported.This work studies the spread of ARGs in the soil-plant systems during the process of improving poor soil by applying the compost of straw subatrate and sludge.The results showed that the relative abundance of target ARGs in bulk soil,rhizosphere soil,and endophytes increased under cadmium?Cd?stress in the application of compost products.The relative abundance of target ARGs in the rhizosphere soil was lower than that in the bulk soil.With the increase of the Cd stress concentration,the abundance of the target ARGs in the above-parts of the shallot?stems and leaves?increased.The composition of fungal community was the main driving factor for changes in soil ARGs.However,endophytic bacteria were the main driving factors that promote the spread of ARGs in plants.In addition,the endophytic bacteria Sphingobacterium and Alcaligenes were potential hosts for ARGs.The long-term application of aerobic composting products of sludge?under Cd stress?will increase 3.23 times of the abundance of ARGs in the shallot,compared with none Cd stress.The risk of ARGs spreading to the human through the intake of raw vegetables was increased.These findings indicated that the application of sludge compost products?with Cd stress?can promote the spread of ARGs in crops,which is of great significance for the rational assessment of the risk of sludge organic fertilizer application.Furthermore,the results will provide an important theoretical basis for controlling the AGRs from soil to plants in agricultural production activities.In summary,this study revealed the succession of the microbial community structure during the back-mixing aerobic composting process of the straw subatrate and confirmed the superiority of the straw subatrate as composting material mixed with sludge.By optimizing the initial C/N of the straw subatrate and sludge composting process,the target ARGs were reduced,and the driving force of ARGs spreading in the soil-plant system during the land application process of compost products was clarified.This conclusion helps to further clarify the spreading mechanism of ARGs in the aerobic composting process of organic solid waste and the spreading risk of ARGs in the soil-plant systems caused by land application. |
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