Response Of Soil Microbial Community To Simulated Freeze-thaw Events And Its Relationship With Multifunctionality In Larix Gmelinii Forest

Global climate change has altered soil freeze-thaw(FT)patterns via thinning of snow cover,especially in high la titude and high al titude ar eas.The changes of FT patterns may alter soil hydrothermal conditions,which would further lead to the shift of microbial community characteristics and the soil biogeochemical cycle processes mediated by them.However,less is known about the response of soil carbon and nitrogen turnover,soil microbial diversity and soil multifunctionality to FT events,the knowledge of relationship between soil microbial diversity and soil multifunctionality also needs to be improved.Here,Larix gmelinii forest,the zonal vegetation inDaxing'an mountains,was selected as the research object.Simulated FT conditions were used to determine:(1)the impact of the number of FT cycles(FTN)and FT temperature fluctuation(FTF)on soil ph ysical and chemical properties,carbon and nitrogen fractions and turnover;(2)the responses of soil microbial biomass,metabolic activity,diversity,species connectivity and community composition to FT events;(3)the change rules of soil multifunctionality during FT and the driving effect of soilmic robial diversity on soil multifunctionality.The main results are as follows:1)FTF had a significant effect on soil nitrogen turnover rate(net nitrogen mineralization rate,net nitrification rate,net ammoniation rate,p<0.05).The soil nitrogen turnover rate under deep FT treatment was significantly higher than that under mild FT treatment and increased significantly with the increase of FTN(p>0.05),but it had no significant difference among different FTN under mild FT tr eatment(p>0.05).S tepwise linear r egression analysis s howed t hat s oil di ssolvable or ganic nitrogen(DON)ha d a s ignificant a nd pos itive effect on soil n et n itrogen min eralization r ate,n et nitrification rate,and net ammoniation rate,with the standardized coefficients of 0.348,0.266 and 0.451,respectively.S oil total nitrogen(TN)decreased significantly with the increase of FTN(p<0.05).2)The effects of FTN and FTF on soil carbon mineralization rate were not significant(p>0.05).The total organic carbon(TOC)pool and active organic carbon,recalcitrant organic carbon and their ratio to TOC also did not change significantly(p>0.05)during FT events.Recalcitrant organic carbon accounted for 65.7-77.0%of TOC.There was no obvious loss of carbon pool in short-term FT,and soil carbon pool can maintain stable.Soil water content(SWC)had significant and positive correlation with TOC and carbon pool fractions(p<0.05).SWC was the most important environmental factor contributing to the change of TOC and carbon pool fractions.The main driving factors of s oil carbon mineralization rate were microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN).SWC and microbial biomass were the main factors affecting the stability of soil carbon pool.3)The results of multiple regression tree analysis showed that FTN hada greater effect on microbial communities than FTF.FTN and FTF explained 20.9 and 10.8%of the variation in microbial community structure,respectively,and 22.9 and 11.6%of the variation in enzyme activities,respectively.The total and subgroup microbial biomass,the ratio of fungi to bacteria(F/B),and C-and N-hydrolyzing enzyme activities all decreased with an increase in FTN.Among all microbial groups,arbuscular mycorrhizal fungi(AMF)were the most sensitive to FT events.SWC,TOC,and TN were the major factors affecting microorganisms in FT events.In the forest ecosystem,soil water and fertilizer were important factors to resist the damage of FT to microorganism,and sufficient water and fertilizer can lighten the damage of FT events to microorganisms.4)The effect of FT on the community composition and diversity of fungi and bacteria was different.Compared with bacteria,fungal community was more sensitive to FT.Community composition,?-diversity index,? diversity,and co-occurrence ne twork complexity of fungi significantly changed during FT events,whereas the same parameters did not exhibit significant alterations for bacteria.Co-occurrence network analysis showed that the complexity of fungal community increased significantly a fter 7th and 12th FTN treatments,while the complexity of bacterial community hardly changed.The results of Linear discriminant analysis effect size showed 13 and 8 fungal subgroups significantly changed in a bundance a mong different FTN and FTF,respectively,while bacteria subgroups did not have significant changes in abundance.5)FT events significantly affected soil multifunctionality.The soil multifunctionality under the FT treatments was significantly lower than that of the control after the 1st FTN(p<0.05),and then increased with the increase of FTN,with the maximum at the 7th FTN.FTF and FTN ex erted different impact paths on soil multifunctionality.The effect of FTN on soil multifunctionality(0.289)was greater than that of FTF(0.080).A random forest analysis,analysis of covariance and structural equation modeling showed that the fungal diversity index was the main predictor of soil multifunctionality.The changes of soil abiotic factors(SWC and TOC)also have positive and negative feedback on the relationship be tween soil microbial diversity and multifunctionality.Moreover,changes in soil abiotic factors also affected the relationship between soil microbial diversity and multifunctionality.