Study On The Mechanism Of Organic Waste Compost Degrading Soil PAHs

Polycyclic aromatic hydrocarbons?PAHs?are a class of persistent organic pollutants with two or more aromatic rings structure.PAHs are susceptible to retention in the soil and may cause adverse effects on human health and ecosystems through the transmission of food chains and step-up amplification.In this study,Comamonas Testosteroni was used as an external bacterial agent,and four treatment groups were set up:cow manure and pastoral soil compost treatment?CK?,cow manure and PAHs-contaminated soil compost treatment?PAH?,and external bacterial agent-cow manure and the pastoral soil compost treatment?CK+?and the external bacterial agent-cow manure and PAHs-contaminated soil compost treatment?PAH+?,setting up a cow manure and PAHs-contaminated soil simulation composting system.Changes in the content of naphthalene?Nap?,phenanthrene?Phe?,benzo[a]pyrene?BaP?,physicochemical parameters,and enzyme activity during the composting humification process were determined by qPCR and high-throughput sequencing was used to analyze the dynamic changes of the microbial community structure during this process,and the degradation of PAHs in the PAHs-contaminated aged soil was investigated by compost-produced fertilizer.The main results of the study are as follows:?1?In the course of 65 days of combined composting experiments with cattle manure and PAHs contaminated soil at a dry weight ratio of 6:4,the organic matter content in the system decreased from 51.00%to 59.02%to 27.57%to 31.06%,and the degradation rate slowed down with time.The presence of PAHs can significantly improve the rate of organic matter degradation during composting,but the dual role of PAHs and external bacteria is the opposite.After the high temperature period,neither of them affects the conversion of organic matter during the composting process.The content of soluble sugars and soluble amino acids in PAHs-contaminated soil in the later period of composting was significantly lower than that in non-PAHs-contaminated soil.The main reason for this change was the stress of PAHs on microorganisms,which caused microorganisms to increase their metabolism to resist adverse environments.The ratio of humic acid to fulvic acid in the four treatment groups increased gradually,and was greater than 1.9 on the 65th day,which met the compost maturity standard,and the content of humic substances and humic substances in the PAH+group was significantly higher than that of the other treatment groups.?2?The dynamic changes of PAHs in the cattle manure-soil combined composting system were measured by GC-MS.At the end of the composting,the degradation rates of Nap,Phe and BaP were 81.97%?82.16%,79.44%?82.89%,59.63%?At 61.55%,all three PAHs were significantly degraded;the external fungus agent quickly took effect in the early stage of composting,which significantly improved the degradation efficiency of Phe and BaP within the initial 7 days.During the first 12 days of composting,PAHs and external bacteria maked the activity of dehydrogenases?DHA?in the sample significantly higher than that of CK.Therefore,the presence of PAHs and the introduction of exogenous bacteria can strengthen the overall metabolic activity of microorganisms from different angles.Both polyphenol oxidase?PPO?and laccase?Lac?in the PAH+group showed high activity,and could play an important role in the bioremediation of PAHs.?3?The copy number changes of 16S rRNA,3?-HSD/CR,and PAH-RHD?GNF during composting process were analysed by qPCR.It was found that the external bacteria can increase the biomass of bacteria in the system,and at the same time,the PAH-RHD?GNF gene copy number increased,while PAHs had a stress effect on the bacteria in the system,and at the same time,the 3?-HSD/CR gene copy number decreased slightly.High-throughput sequencing analysis of bacterial 16S rRNA showed that both PAHs and exogenous bacteria had an effect on species richness and diversity during composting.The bacteria in the compost mainly comes from 21 phylums.The dominant phylums are Proteobacteria,Bacteroidetes,Firmicutes,Actinobacteria,Chloroflexi,and Gemmatim-onadetes.Statistical analysis of species abundance at the genus level,and Pearson correlation analysis,redundancy analysis,and network analysis with physical and chemical indicators,enzyme activity,functional genes,and PAHs content found that external bacterial agents can significantly improve indigenous microorganisms Phe and BaP.The degradation ability of microbial community structure also improved the adaptability of microbial community structure to PAHs and the response relationship between microbial groups and PAHs.?4?20%of the weight PAHs-contaminated aged soil was added with compost-produced fertilizer,and the PAHs content was measured during the 30 days culture period.It was found that the application of the compost-fertilizer can significantly improve the degradation efficiency of PAHs in the aged soil within 30 days and the degradation rates of LMW-PAHs and HMW-PAHs are 58%?79.93%and 31.15%?36.58%respectively.Except for individual treatment groups,the degradation rate of?_{13 PAHs} can reach more than 45%,and the degradation effect is good.At the same time,the application of decomposed fertilizer can significantly increase the activity of PPO,DHA and urease,indicating that the decomposed fertilizer produced by cow manure-soil combined composting can effectively improve the microbial biomass and metabolic activity in aging soil,which is beneficial to the degradation of PAHs.In summary,this paper builds a combined composting system of organic waste and PAHs contaminated soil,exploring the degradation of PAHs in compost and inoculated compost,and the dynamic changes of microbial community structure during the composting process.The above experiments show that compost and inoculate composting can improve the degradation efficiency of PAHs in PAHs contaminated soil to a certain extent and improve the physical and chemical indicators and enzyme activities that affect the composting system;at the same time,the composting process can significantly increase the copy number of PAHs degradation function genes;although it does not change the composition of the microbial community structure,However,it has an influence on the relative abundance and inter-genus correlation of bacteria within the phylum level and genus level of the composting system,and the correlation between the genus and various indicators.The decomposed fertilizer produced by the cow manure-soil combined composting process was co-cultivated with PAHs-contaminated soil.Experiments confirmed that adding decomposed fertilizer to PAHs-contaminated aging soil can be used as a soil amendment to bioremediation the aging soil.Therefore,while exploring the mechanism of using the composting process to degrade PAHs in the soil,this article provides a theoretical basis and application basis for the treatment of organic waste and the"in-situ compost restoration"of soil PAHs.