Effects Of Millipedes On Microbial Community Structure And Oxidase Activity Under The Nitrogen Deposition

Atmospheric nitrogen deposition has increased dramatically since the industrial revolution.With rapid economic development in the last four decades,China now is experiencing the most serious nitrogen deposition,which is predicted to exacerbate in the future.In the terrestrial ecosystems,excess nitrogen inputs can greatly affect soil chemical properties,plant growth and soil biological activities.Millipedes are arthropods that mainly live in the plant litter layer and top layer soils.This soil invertebrate can fragment,consume and transform litter to accelerate its decomposition.Millipedes also can regulate nutrient cycling and affect soil fertility through feeding and excretion activities.Moreover,millipedes can exert a top-down control on microorganisms.As excess nitrogen inputs can change the physical and chemical properties of soil,which in turn my affect the growth and survival of the millipedes.In this study,we designed a microcosm experiment to assess the influence of nitrogen deposition,millipedes and their interactions on microbial community structure and function.Four treatments were included:control?CK?,nitrogen addition?N?,millipede addition?M?,and nitrogen deposition plus millipede addition?N+M?.The pH?organic carbon,total nitrogen,total phosphorus,dissolved organic carbon,dissolved total nitrogen,dissolved total phosphorus,NH₄⁺-N,NH₃^--N,microbial biomass carbon and nitrogen of litter,millipedes faeces and soil were measured;PLFA profile and oxidase activities in litter,millipede faeces and soil were also measured.Correlation between soil microbial properties and chemical properties were assessed by bivariate correlation analysis and redundancy analysis.We found that:?1?In litter,nitrogen addition,millipede addition,and nitrogen deposition plus millipede all decreased the pH value.The content of total phosphorus was increased but the content of dissolved organic carbon and dissolved total nitrogen were decreased by nitrogen addition.Litter mass loss was increased but the content of total nitrogen,dissolved total nitrogen and dissolved total phosphorus were reduced by millipede addition.Bacteria were the dominant microbial group in litter.Nitrogen addition decreased the gram negative bacteria.The total microbial biomass,bacteria biomass,gram negative bacteria,fungi and protozoa were reduced by millipede addition.No significant interactive effects between nitrogen addition and millipede were observed.?2?In millipedes faeces,microbial community structure was altered by nitrogen addition.Nitrogen addition significantly reduced the biomass of fungi and actinomycetes.Furthermore,nitrogen addition suppressed oxidase activity.Microbial community structure and oxidase activities were significantly correlated to organic carbon and dissolved total nitrogen of faeces.?3?In soil,nitrogen addition,millipede addition,and nitrogen deposition plus millipede addition all increased NH₃^--N content.The content of soil organic carbon and total phosphorus were reduced by nitrogen addition.The ratio of soil C/N was increased by millipede addition.The ratio of soil N/P,the content of dissolved total nitrogen and ammonium nitrogen were increased but total phosphorus was decreased by nitrogen deposition plus millipede addition.Nitrogen addition promoted the release of some available nutrient in litter and faeces.Millipede may exacerbate litter decomposition through feeding activity.These processes may also drive the release of some nutrient elements from litter.Ultimately,all of these nutrients may return to soil and thus may increase available substrates for soil microorganisms,which may induce slightly higher biomass of total microbial community,bacteria,gram negative bacteria,gram positive bacteria and actinomycetes in soil under millipede addtion.Bacteria dominated in soil.The microbial biomass carbon,fungi and actinomycetes were reduced but microbial biomass nitrogen was increased by nitrogen addition.The biomass of fungi was decreased by millipede addition.The microbial biomass nitrogen,total microbial biomass,bacteria,gram negative bacteria,actinomycetes and protozoa were increased,but the biomass of fungi was decreased by nitrogen deposition plus millipede addition.The nitrogen and millipede addition treatment also altered the soil microbial community structure.The biomass of actinomycetes and protozoa were increased but fungi were reduced by the nitrogen deposition plus millipede addition.Oxidase activity was suppressed by millipede addition.Peroxidase activity was suppressed by nitrogen deposition plus millipede addition.In summary,both millipedes and microorganism occupy significant ecological niche in ecosystem.They both decompose and transform litter and is an important factor to maintain the stability of the ecosystem.For example,the cycling of soil carbon and other key nutrients can be regulated directly or indirectly by feeding and excretion activities of millipedes,microbial biomass and enzyme.Therefore,it is noteworthy to pay more attention to the function of saprotrophic invertebrate in the future N deposition studies.