Effects Of Changing Rainfall In Different Periods During The Growing Seasons On Soil Microbial Community Structure And Heterotrophic Respiration

Previous studies have shown that the change of rainfall pattern not only includes the changes of rainfall amount,rainfall frequency,and rainfall intensity at global and regional scales,but also includes the fluctuation of rainfall at inter-annual and intra-annual scales,which is mainly manifested as the change of rainfall distribution in season.The belowground microbial community,which is closely related to the aboveground plant community,is small but responds quickly to environmental changes.Seasonal changes in rainfall distribution can affect the growth and distribution of aboveground plants and subsurface microorganisms by changing soil water availability,and lead to the changes in ecosystem functions.Carbon is the most critical element of ecosystem,and its biogeochemical cycle constitutes the most basic material cycle of ecosystem.Changes in rainfall patterns will affect the ecosystem functions of water-limited plants and soil microorganisms in the temperate steppe of northern China.However,few studies have explored the influence of changes in rainfall timing on microbial community structure and soil carbon emission through influencing microbial-dominated soil heterotrophic respiration,and thus lack of the clear understanding on the effects of changes in relative abundance of dominant microbial groups on soil heterotrophic respiration process.Therefore,it is important to study the general patterns and underlying mechanisms of the effects of rainfall changes in different periods during growing season on microbial community structure and soil heterotrophic respiration for predicting the potential of soil carbon storage under future rainfall change scenarios.A rainfall platform that manipulated different periods of rainfall during the growing season in the semi-arid grassland of northern China was established in 2015.The experiment had seven treatments,including control(C),60% reduction of natural rainfall in the early growing season(DEP),60% increase of natural rainfall in the early growing season(IEP),60% reduction of natural rainfall in the late growing season(DLP),60% increase of natural rainfall in the late growing season(ILP),60% reduction of natural rainfall in the entire growing season(DP),and 60% increase of natural rainfall in the entire growing season(IP).Soil sampling and plant community survey were conducted at the end of June and August in 2020,respectively,to obtain soil and plant community characteristics in the early and late growing season.Meanwhile,soil heterotrophic respiration was measured throughout the entire growing season,and the effects of rainfall changes in the early and late growing season on bacterial and fungal diversity and community composition were studied based on high-throughput sequencing technology,and the mechanisms driving microbial community changes were explored by combining soil physical and chemical properties with plant community indicators.Finally,we explored how soil microbial community structure and soil physical and chemical changes affected soil heterotrophic respiration.The results of the study are as following:(1)Rainfall changes in different periods of growing season had no significant effect on either bacterial or fungal α diversity,but rainfall changes significantly altered community composition of both bacteria and fungi in June and August.In June,soil nitrate nitrogen and dissolved organic carbon accounted for 16.5% and 13.0% changes of soil bacterial and fungal communities,respectively.In August,soil moisture and plant community coverage accounted for 53.4% and 47.7% changes of soil bacterial and fungal communities,respectively.(2)In June,soil heterotrophic respiration was reduced by 39.29% and 41.55% under 60%reduction of natural rainfall in the early and entire growing season,respectively.By contrast,soil heterotrophic respiration was increased by 31.96% and 44.63% under 60% increase of natural rainfall in the early and entire growing season,respectively.In August,soil heterotrophic respiration was reduced by18.36% and 40.09% under 60% reduction of natural rainfall in the late and entire growing season,respectively.Soil heterotrophic respiration was increased by 22.56% and 30.10% under 60% increase of natural rainfall in the early and entire growing season,respectively.By contrast,60% increase of natural rainfall in the late growing season had no significant effect on soil heterotrophic respiration.The results indicate that 60% increase of natural rainfall in the early growing season plays a more important role in soil carbon emission through heterotrophic respiration than 60% increase of natural rainfall in the late growing season.(3)Structural equation modeling results show that,soil heterotrophic respiration in June was directly affected by soil moisture and plant coverage,the relative abundance of Ascomycota,which was negatively correlated with plant coverage and positively correlated with relative abundance of Proteobacteria,had a positive effect on soil heterotrophic respiration;In August,rainfall changes directly affected plant coverage by changing soil moisture and nitrate nitrogen content,indirectly affected the relative abundance of Proteobacteria and Nitrospirobacteria,and ultimately led to changes in soil heterotrophic respiration,the relative abundance of Proteobacteria had a positive effect on soil heterotrophic respiration,while that of Nitrospirobacteria had a negative effect on soil heterotrophic respiration.The results showed that the underlying mechanisms driving microbial community structure and soil heterotrophic respiration were different between June and August.Soil moisture and plant coverage are key factors in the study of soil heterotrophic respiration in June and August.Environmental factors can also affect soil carbon process by influencing microbial dominant groups.Soil heterotrophic respiration significantly increased in the early growing season compared with that in the late growing season,and there was a residual effect in the late growing season.Therefore,60% increase of natural rainfall in the early growing season plays a more important role in soil carbon emissions.The seasonal differences in the effects of rainfall timing on soil heterotrophic respiration will help to better predict the changing process of terrestrial ecosystem function under future rainfall change scenarios.