Study On The Mechanism Of Bio-synergistic Degradation In Vermicomposting System Of Municipal Sludge

Vermicomposting system is an artificially enhanced ecosystem based on the detritus food web.In this system,organic detritus,microbes,microfauna and earthworms make up a vermicomposting food web,in which various biological groups affect the derogation and transformation of organic matter by synergistic effect and restriction mechanism among organisms.Therefore,study on the synergistic effect and restriction mechanism among organisms is an important aspect to illustrate the internal cause of organic degradation in vermicomposting system of municipal sludge.In this study,mechanism of organic degradation and sludge vermistabilization were revealed from the perspective of food web through exploring the regulatory mechanism of top-down effects and the bottom-up effects on vermicompost food web by designing the two-factor control experiment.The main results were as follows:（1）Earthworms are the top trophic level organism in the vermicomposting food web.The nutrient supply in matrix is the main factor affecting population dynamics of earthworms.（2）In this study,30 genera of microfauna,belonging to 3 phyla,8 class,13 orders and23 families,were detected in all treatments.According to their characteristics during the whole sludge degradation process,microfauna was divided into three groups,that was perishable microfauna,testate amoebae and nematodes.In detail,the number of perishable microfauna decreased rapidly in all treatments due to environmental changes of municipal sludge caused by mechanical dewatering.Testate amoebae were the dominant group,accounting for about 85%of the total amount of microfauna.Nutrient supply and earthworm predation pressure in the substrate significantly affected the number of testate amoebae and nematodes.Bacterivorous nematodes were influenced by bottom-up effects caused by nutrient supply in the substrate in the initial stage and the top-down effects caused by earthworm predation pressure in the mid-later period,respectively.Fungivorous nematodes were closely related to the nutrient supply and fungal abundance,however,the quantity of omnivore-predators was mainly regulated by the top-down effects caused by earthworm predation pressure.（3）Suitable earthworm biomass can promote the proliferation of microbial population.Bacterial community was affected by both bottom-up effects and top-down effects,otherwise,the fungal community was mainly influenced by the bottom-up effects caused by the nutrient supply in the substrate.In addition,Amines and phenolic compounds were the main carbon sources for microbial utilization.The amount of carbon source in substrate was the main factor affecting metabolic intensity of microorganisms.（4）The soluble organic matter of the municipal sludge used in this study was mainly composed of aromatic protein substances,soluble microbial by-products and a small amount of humus.With the degradation of sludge,humus content displayed a trend of continuous decrease.Fulvic acid was the main component of humus,accounting for 65.8%.In the range of substrate environment carrying capacity,increasing earthworm biomass can promote the degradation of aromatic protein substances and soluble microbial by-products,improve the humification degree of fulvic acid and humic acid,and accelerate the mineralization rate of organic nitrogen and phosphorus,thus improve the quality of vermicomposting products.（5）The vermicomposting process of municipal sludge can be divided into three stages:（1）In the early stage of nutrient enrichment,earthworms,microfauna（Sessile、Rotifera、Euglypha、Eggs、Arcella）,microbes（Bacteroidetes、Latescibacteria、Fusobacteria、Acidobacteria、Ascomycota、Firmicutes、Nitrospirae）had the positive relationship with OM,TN and TOC.The above organisms realized the rapid degradation of easily utilized organic matter through biological assimilation and efficient transformation of organic matter in bacterial energy channels.（2）In the middle period when nutrients were relatively scarce,the number and correlation of fungivorous nematodes with Cryptomycota、Saccharibacteria and Actinobacteria increased.Fungal energy channels play a major role,which accelerate the decomposition and transformation of refractory organic compounds.（3）In the late stage of sludge stabilization and humification,the effect of bacterial energy channels was enhanced.Microorganisms,testate amoebae,bacterivorous nematodes and earthworms formed a relatively stable food web,and made the vermicomposting products tend to be stable.（6）In the process of vermicomposting,part of the organic matter in sludge was converted into inorganic substances,such as NO₃^--N and AP,meanwhile the CO₂and H₂O discharged from system.Organic matter accounting for 15.99%～21.96%of total organics in vermicomposting products transformed into humus with more stable properties and higher humification degree.Meanwhile,some organics formed the shells of testate amoebae and eggs of earthworms,which were difficult to decompose in a short period.Moreover,some organic compounds entered the microfood web and participated in the formation of stable microbial communities.In addition,other organics assimilated by earthworm biomass,which retained or left the substrate with the change of earthworm population.Thereinto,content of inorganic substances,humification degree and content of humus,stability of microbial community directly affected the characteristic and availability of vermicomposting products.