Effects Of Extreme Drought On Soil Aggregation,Soil Enzyme Activities And Associated Bacterial Community Structure In A Subtropical Forest

Subtropical forests are considerable carbon sinks in eastern Asia and play an important role in regulating regional carbon budgets,yet are facing the threat of drought with increased frequency and prolonged duration under global warming.However,the effect of drought on the carbon cycle of subtropical forests and related microbial processes remain unclear.In this study,we conducted our research based on a long-term drought experiment site in a subtropical evergreen broad-leaved forest of eastern China,and the drought treatment in this site had last five years through 70%of throughfall reduction.Through measurements of the soil physicochemical properties and soil enzyme activities,together with 16 S rRNA gene-based high-throughput amplicon sequencing,the effects of long-term drought on soil aggregation,organic carbon composition and content,soil bacterial community structure and enzymatic functions were comprehensively studied.The purpose of the study is to reveal the response of soil organic carbon and its components to long-term drought and the underlying microbial ecological mechanisms.The major findings are summarized as follow:(1)Drought changed the distribution of soil aggregate fractions,reducing the stability and carbon protection ability of large aggregates.Compared to control,drought resulted in loss of a third of large macroaggregates(>2 mm),and doubled the proportion of microaggregates(<0.25 mm).Soil mean weight diameter(MWD)and geometric mean diameter(GMD)also decreased significantly.In addition,the total carbon,total nitrogen,and non-hydrolyzed carbon content in large macroaggregates(>2 mm)decreased significantly under drought,indicating that extreme drought destroyed the stability of soil structure and lowered the capacity of soil carbon fixation and storage.(2)Different bacterial phyla responded differently to drought stress,and the overall trend reflected changes from rapidly growing Gram-negative bacteria toslow-growing Gram-positive bacteria.The dominant bacterial phyla in subtropical forest soils were Acidobacteria,Proteobacteria and Actinobacteria.The majority of the affected taxa showed reduced abundances,including Proteobacteria,while that of Actinobacteria,a group commonly associated with recalcitrance C degradation,significantly increased.(3)Drought significantly affected the structure and function of soil microbial communities,thereby changing the microbially mediated carbon conversion mechanisms.The soil bacterial community structure at different sampling times was significantly different between drought and control,which may be indirectly affected by drought-induced substrate reduction.The community diversity and soil hydrolase activity decreased significantly,indicating that drought reduced the decomposition rate of forest soil organic matter.However,the soil total potential peroxidase(PER)activity related to stable carbon decomposition was significantly increased,which showed a significant positive correlation with Actinobacteria,indicating that the structure and function of the soil bacterial community may turn to decompose stable carbon.(4)Temporal variation(seasonal change)of soil bacterial community diversities,community structures and soil enzyme activities are more significant than that of the drought treatment.Soil total carbon,total nitrogen,non-hydrolyzed carbon,and carbon-to-nitrogen ratio are the most important factors affecting soil bacterial communities,indicating that changes in soil nutrient availability caused by plant phenology are the key drivers of the seasonal characteristics of forest soil bacterial community structure and functions.The effect of drought on bacterial community structure and soil enzyme activities were most significant in autumn and winter,probably because the soil substrate content was significantly reduced compared to the control at that time.These findings indicated that five years of rainfall isolation has negatively impacted the carbon balance of the ecosystem and may cause forestdegradation in the long run.In summary,this study revealed that long-term drought may lead to a decrease in the stability of the forest soil carbon pool,not only through macroaggregate disintegration and changes in soil chemical characteristics,but also shifted microbial communities in both composition and activities toward enhanced abilities of recalcitrance carbon conversion.This study highlights the importance of microorganisms in mediating the soil carbon conversion process and helps us better predict the response of forest ecosystems or regional carbon cycles to future climate changes.