Response Characteristics Of Forest Soil Microbial Community To Nitrogen Deposition Under Urban-Rural Environmental Gradient

Soil microorganism is a sensitive indicator of soil nutrient change,which is a hotspot in the field of ecology in recent years.With the expansion of urbanization,nitrogen deposition has become one of the biggest threats to the global terrestrial ecosystem,and the response of soil to nitrogen deposition is more sensitive.It is necessary to have further studies about whether the nitrogen deposition can affect the microorganism of urban forest soil and whether it can reach the threshold of nitrogen saturation of urban forest soil.The urbanization of Hefei started late,but it has developed rapidly.With the implementation of the strategy for the rise of central China and the regional integration strategy of the Yangtze River Delta,the urbanization level of Hefei has greatly improved.Hefei has became the epitome of urbanization development of major cities in China.In this study,the method of time instead of space was used.Three research sites were established and the natural and geographical conditions are similar:urban forest?Da Shu Mountain National Forest Park?and suburban forest?Zi Peng Mountain National Forest Park?in Hefei City and Rural forest?Wan Fo Mountain?in Shu Cheng county.The Quercus acutissima Carruth.forest widely distributed in Hefei was selected as the research object.We carried out simulated nitrogen deposition experiments(0kg N ha^-1yr^-1,50kg N ha^-1yr^-1,100kg N ha^-1yr^-1)to study the effects of different nitrogen treatments on the composition of forest soil microbial communities,analyze the differences and influencing factors of forest soil microbial biomass and community functional characteristics under the urban-rural gradient.It provide important references and scientific basis to accurately assess the soil health status of urban forest,study the impact of urbanization on urban forest ecosystems,effectively respond to urban nitrogen deposition,improve forest ecosystem functions and.The research results are as follows:?1?The seasonal distribution characteristics of soil microbial biomass carbon?MBC?and nitrogen?MBN?in urban forest and suburban forest are higher in summer and autumn,and lower in winter and spring,and the MBC in rural natural forest is the same,while MBN is high in summer,autumn and winter,and the lowest in spring.In spatial distribution,the MBC and MBC/MBN content of urban forest is higher than that of suburban forest and rural natural forest.?2?Nitrogen deposition significantly increased soil microbial biomass in urban forest in autumn and spring,but inhibited MBC in suburban and had little effect on MBN.Nitrogen deposition promoted the MBC content of rural natural forest in the early stage of nitrogen treatment and suppressed in the later stage,but mainly promoted the MBN in rural.In the early stage of nitrogen addition,both the urban and rural environmental microorganisms MBC/MBN were promoted,while it shows inhibitory effect in the later stage,and the inhibitory effect on the rural and suburban forest is more significant.?3?Nitrogen deposition would increase the average well color development?AWCD?in the spring and autumn of the suburban and rural forest.In the later period of nitrogen application,the soil AWCD value of rural forest and suburban forest were significantly reduced,but the AWCD value of urban forest was significantly increased.?4?The diversity index of soil microorganism in rural natural forest was the highest.The influence of nitrogen deposition on the Mc Intosh diversity index of urban forest is shown as initial growth inhibition and promotion in the peak season but the inhibition was significant in the suburban forest,while the response of rural natural forest to nitrogen deposition was the weakest.At the early stage of nitrogen deposition,it can promote the diversity of soil microorganism Shannon-Winner in the growing season of urban forest,but has no effect on Simpson diversity index.The Shannon-Winner and Simpson diversity indexes of suburban forest and rural forest showed a significant promoting effect in the early stage of nitrogen deposition and a significant inhibiting effect in May 2019.?5?Soil microorganisms in urban-rural environmental gradient tend to metabolize Carboxylic acids,Amino acids and Polymers,while the metabolizability of Amines is the lowest.The metabolic capacity of carbon source is shown as rural>urban>suburban.The effect of nitrogen deposition on carbon source utilization type of rural natural forest was promoted in the early stage and inhibited in the later stage,while the response of urban forest and suburban forest to nitrogen deposition had great fluctuation in plant growth season.The metabolic difference of polyphenols,polyamines and polymers in the soil microorganism in the later stage of nitrogen treatment were the main reasons for the significant difference of soil microorganism in urban and rural environment.The long-term nitrogen deposition would reduce the types of carbon metabolism of soil microorganism in rural natural forest and suburban forest,but increase the types of carbon metabolism of soil microorganism in urban forest.?6?The physical and chemical properties of soil NH₄⁺-N,NO₃^--N,pH,SOC and TN are sensitive indicators affecting soil microbial biomass and population diversity.The characteristics of soil microbial metabolism in urban forest are dominated by the r strategy,which favors the use of external nitrogen.