Study On Agricultural Waste Composting Performance And Related Functional Microorganisms Based On Nanomaterials Regulation

China is a large agricultural country,which generates more and more agricultural waste every year.It is of great practical significance to promote the treatment and resource utilization of agricultural waste.The traditional composting method has a long composting cycle,it is of great significance to promote the degradation of organic matter?OM?during the composting process to improve the composting efficiency.In addition,some nitrogen elements will be lost as the form of NH₃,N₂O,NO_x and N₂ during the composting process,thus causing environmental pollution and reducing the fertility of compost products.Therefore,it is also very important to effectively reduce the nitrogen loss during composting process to improve the fertility of composting products.In order to solve the above problems,different concentrations of silver nanomaterials?AgNPs?and different types of iron-oxide nanomaterials?FeONPs?were added into the composting system of agricultural waste to systematically study their effects on OM degradation and nitrogen biotransformation.In addition,different physico-chemical parameters during composting have different effects on microbial abundance and community structure as well as composition,and different microbial populations have different cell biochemical and metabolic characteristics,so they respond differently to the same physico-chemical parameters.Therefore,this study also used the method of multivariate statistical analysis to study the relationships between different microbial population abundances and environmental factors to determine the significant factors driving variations of microbial population abundances,and calculated the contribution of single siginificant factor by eliminating the influences of other significant factors using partical redundancy analysis?partial RDA?.These will provide good theoretical guidances for further regulation and optimization of composting process parameters and improving composting efficiency.The specific research content and results of this paper include the following aspects:?1?Low concentration of AgNPs?2 mg/kg?was added into the agricultural waste composting system to study its effect on OM degradation and nitrogen transformation during composting.Two groups of treatments were set up,in which blank control was labeled as Pile 1,and the AgNPs treatment group was labeled as Pile 2.The results showed that at the end of composting,the OM loss in Pile 2?57.96%?was less than that in Pile 1?61.66%?,and 27.22%and 30.1%of the initial TOC was decomposed in pile 1 and pile 2,respectively.The final concentation of WSC in Pile 1 and Pile 2 was25642.75 and 23559.27 mg/kg dry compost,respectively.Therefore,it can be concluded that in the presence of AgNPs at 2 mg/kg compost,OM degradation during composting process of agricultural waste was inhibited,while the mineralization of TOC and WSC was promoted.In addition,the presence of AgNPs at 2 mg/kg compost reduced the loss of TN,but it could increase the loss of mineral N,this is mainly due to the microbial fixation of mineral N,that is,the transformation of mineral N into organic N in the AgNPs treatment group is stronger than that in the control group.?2?Further studied the nitrogen transformation pathway and mechanism during composting under the regulation of AgNPs with different addition amounts,and studied the changes of nitrogen transformation functional enzyme activity,functional gene abundance and community diversity.Five groups of treatments were set up in the experiment,namely Pile 1⁵,in which the addition amounts of AgNPs were 0,2,10,20,and 30 mg/kg compost.As the temperature in pile 5 did not meet the composting requirement,other parameters in it were not analyzed.The results showed that the mineral N in Pile 3 which was added with AgNPs at a concentration of 10 mg/kg compost?6.58 g/kg dry compost?was higher than that in other treatment groups,and its TN loss?47.07%?was the lowest among all treatments.The enzyme activity analysis showed that AgNPs had different degree of inhibition.Quantitative PCR?qPCR?results of functional genes showed that in Pile 3,the addition of AgNPs at a concentration of 10 mg/kg compost had the most significant inhibitory effect on amoA,nxrA and nirK which encode the ammonia monooxygenase,nitrite oxidoreductase,and copper-containing nitrite reductase,respectively,while when compared with the control group,it showed promoting effect on narG,nirS and nosZ which encode the membrane-bound nitrate reductase,cd₁-containing nitrite reductase,and nitrous oxide reductase,respectively.Pearson correlation analysis results showed that in Pile 3,the relationship between TN loss and bacterial amoA gene abundance was the most significant,with negative correlation between them.Therefore,high-throughput sequencing analysis of the bacterial amoA gene was also conducted in this study.The results showed that during the composting process of the AgNPs treatment gr oup with the addition amount of 10 mg/kg compost,the dominant family of AOB was Nitrosomonadaceae,and the number of Operational Taxonomic Units?OTUs?decreased after the composting compared with that of day 1.To sum up,the addition of AgNPs at 10 mg/kg compost could significantly reduce the loss of TN and retain more mineral N,thus improving the nitrogen utilization efficiency of composting products.?3?Multivariate relationships between microbial community abundance and environmental variables during composting of agricultural waste under the regulation of AgNPs with different addition amounts.The results of manual forward selection showed that the variation in 16S rDNA was significantly explained by NO ₃^--N,while nitrifying genes were significantly related with pH,and denitrifying genes were driven by NO₃^--N and TN.Partial RDA further revealed that NO₃^--N solely explained28.8%?P=0.012?of the variation in 16S rDNA abundance,while pH accounted for61.8%?P=0.002?of the variation in nitrifying genes.NO₃^--N and TN accounted for34.2%?P=0.002?and 9.2%?P=0.026?of denitrifying genes,respectively.In fact,this did not mean that other environmental variables were not influential,as some of the total variations in microbial abundances were caused by the synergistic effect of the significant parameters and other indistinctive parameters.The RDA triplots showed that different genes shared different relationships with environmental parameters.?4?Changes of OM degradation and enzyme activity during composting of agricultural waste under FeONPs regulation.In this part,Fe₂O₃ NPs and Fe₃O₄ NPs were added to the composting system respectively,with an amount of 10 mg/kg compost.Results showed that composting amended with FeONPs,especially Fe₂O₃NPs,was effective to facilitate OM degradation and enzymatic activities.The results of RDA and Pearson correlation analysis indicated that the relationships between enzyme activities and environmental variables were changed by different treatments,but temperature was the most influential?P<0.05?to variations in both DHA and UA in all composting treatments.Compared with DHA,UA was more closely related to the environmental variables.Seed germination test indicated that amendment of FeONPs,especially Fe₂O₃ NPs,could promote seed germination and seedling growth.Therefore,composting amended with FeONPs was a feasible and promising method to promote and accelerate the OM degradation,and improve the enzyme activities during composting process as well as quality of the final compost product.?5?The effects of FeONPs regulation on nitrogen transformation and microbial community succession during composting were further studied.The results showed that the addition of FeONPs especially Fe₂O₃ NPs,could significantly reduce the TN loss and improve the retention of mineral N during the composting process.Moreover,by inhibiting the oxidation of NH₄⁺-N,more NH₄⁺-N could be retained in the FeONPs treatment groups,thereby improving the nitrogen utilization efficiency of composting products.qPCR results showed that the abundances of ammonia-oxidizing bacteria?AOB?and archaea?AOA?in FeONPs treatment groups were lower than that in the control group,and it was the lowest in Fe₂O₃ NPs treatment group.Pearson correlation analysis results showed that AOB played a dominant role in the nitrification process,and the reduction of AOB contributed more to the weakening of NH₄⁺-N oxidation,the retention of more NH₄⁺-N and mineral N,and the reduction of TN loss than that of AOA.In addition,the addition of Fe₂O₃ NPs significantly increased the average abundance of 16S rRNA gene?P<0.05?,while the effect of Fe₃O₄ NPs was not significant.The composition of the bacteri al community at the phylum level of all samples was divided into three different groups according to different composting stages,namely 1d,5d and 17-60d,rather than clustering according to different treatments,indicating that the composition of the bac terial community at the phylum level did not shifted significantly with the addition of FeONPs.Firmicutes,Actinobacteria,and Proteobacteria were the three most dominant phyla in all treatments.In general,this study provided a method to effectively reduce TN loss and improve mineral N reservation during composting process,and gave a deep insight into the role of AOB and AOA in nitrogen transformation.?6?The changes of different microbial population abundances?bacteria,fungi and actinomycetes?during composting under FeONPs regulation and the multivariate relationships with compost matrix parameters were investigated.Three treatments were set up in the experiment:T-control was a blank control group without FeONPs were added;T-nanohematite and T-nanomagnetite were amended with Fe₂O₃ NPs and Fe₃O₄ NPs at a dosage of 10 mg/kg compost,respectively.The results showed that the addition of Fe₂O₃ NPs was more beneficial to increase the abundances of bacteria and fungi,while Fe₃O₄ NPs was more effective to increase actinomycetes abundance.RDA analysis showed that temperature was the most significant factor affecting the microbial abundances in all treatments.PLS-PM results further indicated that in T-control and T-nanomagnetite,the total effect of temperature was increased as both the direct and indirect effects were positive.In T-nanohematite,the total effect was reduced because the direct effect was positive and the indirect effect was negative.Partial RDA indicated that bacteria,fungi and actinomycetes shared different relationships with composting parameters.In general,actinomycetes was likely more sensitive to the changes of composting parameters than bacteria and fungi.