Community Structure Of Denitrifying Bacteria In Static Aerobic Composting Of Dairy Manure

Composting is the mainstream technology of resource disposal,reduction and innocuous treatment of dairy manure and straw.In order to overcome the shortcomings of traditional composting,such as long compost cycle and unstable product quality,a novel static aerobic high temperature composting technology was evaluated in this study.Denitrifying bacteria is the driving force of denitrification process,which lead to nitrogen loss and pollutes the environment,it also plays an important role in composting process.Therefore,the research on denitrifying bacteria community was helpful to further understand the theory of nitrogen cycling in composting process and improve composting technology.The physicochemical and biological indexes of composting process（temperature,pH,water content,carbon to nitrogen ratio,ammonium nitrogen,nitrate nitrogen,ammonium nitrogen to nitrate nitrogen ratio and seed germination index）were determined in this study.The community structure and diversity of denitrifying bacteria in dairy manure and straw co-composting were analyzed by high-throughput sequencing technique.The correlation between dominant denitrifying bacteria and environmental factors in compost was analyzed by Spearman correlation heatmap.The main results were as follows:（1）The composting lasted 17 days,entered the thermophilic stage on the 4th day of composting,and maintained a high temperature for 14 days.Through comprehensive evaluation of pH,water content,total organic carbon（TOC）,Kjeldahl nitrogen（TKN）,C/N,ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）,NH₄⁺-N/NO₃^--N and seed germination index（GI）,results showed that the compost products treated with the novel compost technology reached maturity.（2）High throughput sequencing technique was used to analyze the dynamic changes of nirK bacterial community in the co-composting of dairy manure and straw process.The results revealed that denitrifying bacteria community was the most abundant at the initial stage of the composting.Alpha diversity analysis showed that the diversity index of denitrifying bacteria was different within the composting periods,and the diversity of denitrifying bacteria community was the highest at the initial stage of the composting.The results of beta diversity analysis showed that the bacterial community structure significantly changed during composting.（3）The community structure of nirK denitrifying bacteria in the co-composting of dairy manure and straw was analyzed at the phylum classification level,the results showed that denitrifying bacteria were mainly Proteobacteria.The large proportion of denitrifying bacteria in unclassified indicated that there were a large number of undiscovered denitrifying bacteria in compost.At the initial stage of the composting,the main denitrifying bacteria wereβ-proteobacteria.With the composting process,the relative abundance ofβ-proteobacteria decreased gradually,andα-proteobacteria gradually occupied the dominant position.Analysis of order classification level showed,the dominant genera belong to Rhizobium,Erythrobacter,and Burkholderiales.Among them,Rhizobium species were the most,but the relative abundance of Burkholderiales was the highest.The relative abundance of Rhizobiales increased gradually during composting and reached the highest at the maturity stage.The relative abundance of Burkholderiales was large at the initial and thermophilic stages,and it was the dominant population of these two periods.With the increase of temperature,the relative abundance of Burkholderiales decreased gradually,and then increases slightly at the maturity stage.Rhodobacterales mainly existed at the initial stage,and decreased gradually with the increase of the temperature,reached 0.19%at the thermophilic stage and 0.01%at the maturity stage.Analysis of genus classification level showed that most dominant bacteria were unclassified denitrifying bacteria in compost.（4）Spearman correlation analysis showed that the 10 nirK denitrifying bacteria in addition to unclassified_Alcaligenaceae and Ochrobactrum were related to environmental factors.Unclassified_knorank_dBacteria and unclassified_oRhizobiales were negatively correlated with TOC,C/N,Water content,and pH,and positively correlated with TKN,NO₃^--N and GI.The other dominant bacteria were positively correlated with TOC,C/N,Water content and pH,and negatively correlated with TKN,NO₃^--N and GI.There was a significant negative correlation between unclassified_oRhizobiales and NH₄⁺-N（p<0.001）.Unclassified_pProteobacteria and Pusillimonas were significantly positively correlated with NH₄⁺-N,respectively（p<0.001）.There was a positive correlation between Paracoccus and NH₄⁺-N（p<0.05）.All dominant genera were not related to temperature.