Effect Of Biochar On The Diversity And Function Of Diazotrophs In Cattle Manure Composting

Composting is an effective method degraded agricultural waste through microbial metabolism.Biochar as a pivotal factor in regulating physicochemical factors promoted nitrogen transformation and reduced ammonia loss during composting by their abundant pore structure and complex functional groups.Diazotrophs are special functional group of microorganisms responsible for nitrogen fixation,which could increase nitrogen content in the environment and affected other nitrogen transformation processes.However,to our knowledge,few researches have been conducted on diazotrophic community in composting especially under the influence of biochar,and its role in nitrogen transformation are rarely explored.The lack of information on diazotrophs in composting may limit the understanding of the whole microbial communities and nitrogen transformation.In this study,dairy manure and corn stalk were used as raw materials,rice straw biochar was used as additive?10%?.Diversity and community structure of diazotrophs were analyzed by high-throughput sequencing.The effect of biochar on dynamics of diazotrophs,correlation between diazotrophs and physicochemical factors,and key diazotrophic bacteria affecting nitrogen transformation in compost were studied.The aims of this study are to provide theoretical basis for further understanding of composting nitrogen cycle and improving composting technology.The main results are listed as below:?1?Biochar promoted transformation of nitrogen and changed the physicochemical properties of compost.Compared with the control check?CK?,the concentration of ammonium nitrogen?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?and total nitrogen in biochar composting?BC?decreased by22.94%,39.54%and 17.37%respectively.Biochar increased the p H in the heating and cooling phase significantly,and increased the humic acid content and seed germination index at the end of compost.?2?During composting process,the activity of diazotroph community inhibited by high temperature significantly,the quantities of diazotrophs decreased first and then increased.Biochar could promote the growth and reproduction of diazotroph bacteria.At the end of composting,the quantities of diazotrophs in BC increased 41.03%compared with CK.?3?The diversity of diazotroph community in compost changed significantly.Biochar increased the diversity of diazotrophs on mesophilic and maturation phase.At phylum level,Proteobacteria,Actinobacteria and Firmicutes are the dominant phyla in both treatments.Biochar increased the relative abundance of Actinobacteria on thermophilic phase,and increased the relative abundance of Firmicutes at the end of composting.At the genus level,Psychrobacter and Streptomyces are dominant diazotrophic bacteria on initial phase,and Psychrobacter,Streptomyces,Bradyrhizobium and Azospira are the dominant diazotrophs in mesophilic phase.On thermophilic phase,Streptomyces and Azoarcus are the dominant diazotrophs.During late composting process,Azotobacter and Azoarcus are dominant diazotroph community in CK,and Bradyrhizobium,Azoarcus,Clostridium,Pseudomonas and Pseudoxanthomonas are the dominant diazotrophic bacteria in BC.Biochar altered the community structure of diazotrophic bacteria significantly.?4?Mantel Test and partial Mantel Test showed that NH₄⁺-N?r=0.676,P<0.001?,temperature?r=0.264,P<0.05?,NO₃^--N?r=0.210,P<0.05?,moisture content?r=0.164,P<0.05?were significantly related to the succession of diazotroph community in BC,while NH₄⁺-N?r=0.416,P<0.001?,moisture content?r=0.242,P<0.05?and p H?r=0.166,P<0.05?were significantly correlated to diazotroph community structure in CK.Biochar increased the correlation between temperature,NH₄⁺-N,NO₃^--N and diazotroph community composition.Spearman correlation indicated that biochar improved the correlation between Pseudoxanthomonas and nitrogen.Correlation between Psychrobacter,Azotobacter,Bradyrhizobium and inorganic nitrogen?NH₄⁺-N and NO₃^--N?were decreased by biochar,while the correlation between Streptomyces,Azoarcus,Azospira and total nitrogen were increased.?5?Based on Stepwise regression model,path analysis and Spearman correlation,Rhodopseudomonas were the predictive variables determining NH₄⁺-N transformation in CK,and Pseudoxanthomonas were observed closely correlated with NH₄⁺-N in BC.Clostridium and Bradyrhizobium were major diazotrophs communities influencing NO₃^--N accumulation and consumption in CK,respectively,and Methylocystis and Azospira were key communities influencing NO₃^--N and total nitrogen in BC.Biochar altered the key diazotrophic communities influence nitrogen transformation,and regulated the transformation of NH₄⁺-N and NO₃^--N in compost.