Effects Of N Deposition On Soil Microbial Community Structure And Greenhouse Gas Fluxes In A Natural Castanopsis Carlesii Forest

Nitrogen?N? deposition has become more serious in subtropical China and it has affected the fluxes of greenhouse gases?e.g.CO₂,CH₄,N2O?in forest soils.This study selected a natural forest of Castanopsis carlesii in Sanming,Fujian China and nitrogen addition has been applied in this forest for 5 years.The experiment was designed with three N treatment levels?four replicates for each treatment?(0,40 and 80 kg N ha^-1·yr^-1 for control,low-N,and high-N treatment,respectively)to explore the effects of N deposition on fluxes of CO₂,CH₄ and N2O and microbial community structure in this natural forest.The results showed that soil organic carbon?SOC?content in summer was significantly higher than those in winter and spring under high N treatment.Soil pH value tended to be higher in control treatment than in low-N and high-N treatments,which indicated that soil was gradually acidified by N application.Soil NH₄+ content was highest in summer and lowest in winter,which revealed that there was a clear seasonal difference under N addition.Soil NO3-content tended to be higher in low-N and high-N treatments than in control treatment,and significantly higher in winter than in summer.The lowest soil C/N ratio was in HN treatment.N addtion also changed soil microbial community structure in the 0-10 cm soil layer.The abundances of gram-positive bacterial PLFAs?GP PLFAs??21.9 nmol·g-1?,gram-negative bacterial PLFAs?GN PLFAs??13.9 nmol·g-1?,actinomycete PLFAs?8.6 nmol·g-1?,fungi?5.4 nmol·g-1?and total PLFAs?52.2 nmol·g-1?in surface soil were highest in summer under LN treatment.In the winter,the microbial PLFAs mentioned above were all lower than the other three seasons under HN,LN and CT treatments.In the three treatments,the ratio of GP:GN was the lowest in winter,while F:B ratio was the opposite.The annual flux of CO₂ emission from soil was obviously lower under HN treatment(1081 g C·m^-2·yr^-1)than those under LN(1296 g C·m^-2·yr^-1)and CT?1295 g C·m-2 ·yr-1?treatments.However,there was no significant difference between LN and CT treatments.The annual flux of N2O emission from soil in CT were 43%and 42%of the HN and LN treatment,respectively.But the difference between HN and LN was not significant.The annual flux of CH₄ under HN,LN and CT treatments were-114 mg m^-2·yr^-1,-193 mg m^-2·yr^-1 and-225 mg·m^-2·yr^-1 respectively.This indicated that N application promoted N2O emission from forest soil and inhibited atmospheric CH₄ absorption and CO₂ emission,and the degree of suppress or stimulation increased with the amount of nitrogen applied.At the same time,N application significantly elevated the global warming potential of CH₄ and N₂O.