Effects Of Extreme Drought On Soil N₂O Efflux And Underlying Microbial Mechanisms In A Subtropical Evergreen Broad-Leaved Forest

It is predicted that extreme drought event will become more frequent and duration in the context of global warming.As an important ecological function of forest ecosystems,soil nitrous oxide?N₂O?emission is mediated by microbial-driven nitrification and denitrification processes.However,specific microbial mechanisms involved in the processes of N₂O efflux in forest soils under drought remains elusive.In this study,we selected a long-term extreme drought site in a subtropical evergreen broad-leaved forest in Tiantong,Zhejiang Province.Long-term soil in situ N₂O flux were measured by static chamber method to reveal the response of soil N₂O emission to extreme drought.Soil samples were collected seasonally and soil physicochemical properties?soil moisture,pH,total C and N contents,NH₄⁺-N and NO₃^--N contents?were determined.The relative contribution of nitrification and denitrification was distinguished by acetylene inhibition method to illuminate the microbial process of drought affecting soil in situ N₂O flux.In addition,we analyzed the changes in the abundance of nitrification and denitrification functional genes?amoA-AOA,amoA-AOB,narG and nosZ?related to N₂O emission by quantitative PCR method to clarify microbial mechanism of soil N₂O emission under extreme scenario in a subtropical evergreen broad-leaved forest.The main results are as follows:?1?In situ N₂O flux measurements showed that extreme drought significantly decreased soil in situ N₂O flux and annual cumulative flux.?2?The results of laboratory incubation based on acetylene inhibition showed that denitrification was the main source of soil N₂O emission in this subtropical forest,being responsible for 48.4%-77.7%.Extreme drought significantly reduced N₂O emission potential from denitrification,but it had no significant effect on N₂O emission potential from nitrification and N₂O reduction potential from denitrification,indicating that extreme drought reduced the total N₂O emission mainly through the decrease in N₂O production from denitrification.?3?Quantitative PCR results showed that extreme drought significantly reduced the abundance of denitrification functional gene narG,but it had no significant effect on the abundance of nitrification functional gene amoA-AOB and denitrification functional gene nosZ.The results of structural equation modelling showed that N₂O emission potential from nitrification was mainly affected by NH₄⁺-N content,while N₂O emission potential from denitrification was mainly affected by soil moisture and pH.In conclusion,soil N₂O emission in this subtropical evergreen broad-leaved forest will be reduced under the scenario of frequent drought in the future,which is attributed to the lower N₂O production derived from denitrification.Soil environmental factors play a more important role in influencing N₂O production than the abundance of nitrification and denitrification functional genes in this forest soil.Our results provide theoretical support for the changes in soil N₂O emission capacity in subtropical evergreen broad-leaved forests under extreme drought scenario.