Effects Of Freeze-thaw On Carbon Emissions From Peatlands And Their Microbial Mechanisms

Freezing-thawing cycling（FTC）is a common physical process in soil,which occurs widely in the middle and high latitudes of northern hemisphere.FTC directly or indirectly affects soil physicochemical and biological properties,and is an important factor affecting soil carbon and nitrogen biogeochemical processes.Global warming is an indisputable fact,which will change the pattern of FTC,and then affect the stability of soil carbon pool.Therefore,taking Jinchuan peatland in Changbai Mountains as the research area,this study revealed the characteristics of soil active carbon components,microbial enzyme activities,phospholipid fatty acid concentrations,and soil CO₂and CH₄ emissions under different FTC pattern by indoor FTC simulation experiment,,chloroform fumigation,phospholipid fatty acid,and meteorological chromatography method.This study explored the key factors affecting soil microbial communities and enzyme activities under FTC combined with the soil nutrients and soil microbial biomass using the redundancy analysis method and the microbial mechanism of carbon emissions under FTC using path analysis method.The main conclusions were as follows:（1）FTC promoted the release of soil soluble organic carbon and nitrogen（DOC,DON）.There was heterogeneity in the responses soil DOC concentrations to different FTC amplitudes.The DOC concentations decreased firstly and then increased during the period of-5-℃5 FTC,while it increased firstly and then decreased during the period of-10-10℃FTC.Soil DON concentrations fluctuated during the period of-5-℃5 FTC,but increased firstly and then decreased during the period of-10-10℃FTC.The effects of different amptitude of FTC on soil DON concentration of different depth were-10-10℃>-5-5℃and 0-15cm>15-30 cm.Soil DOC and DON were positively correlated with soil total carbon（TC）cencentration,but negatively correlated with soil water content（SWC）（p<0.05,p<0.01）.（2）Soil microbial biomass carbon（MBC）concentrations showed an increasing trend with FTC times increasing during the periods of both-10-10℃and-5-5℃.The effects of different amptitude of FTC on soil MBC concentrations of different depths were,-10-10℃>-5-℃5 and 15-30 cm>0-15 cm.FTC reduced soil microbial biomass nitrogen（MBN）concentrations.Under the two FTC amptitude,soil MBN of 0-15 cm fluctuated with the increase of FTC times but decreased first and then increased in 15-30 cm depth Soil MBN was negatively correlated with DOC,DON,and soil total nitrogen（TN）,but positively correlated with soil total phosphorus（TP）and SWC（p<0.05,p<0.01）.（3）Soil CO₂ emission rates showed a gradual downward trend with the increase of FTC times both during the periods of-5-℃5 and-10-10℃FTC.The CO₂ emission rates of-5-℃5 FTC had an obvious emission peak after 7 FTC times.The effects of different amplitude of FTC on soil CO₂ emission rate were-10-10℃>-5-5℃.After FTC,the variation in soil CO₂ emission rates of different depths was 15-30 cm>0-15 cm.The oxidation rates of CH₄decreased with the increase of FTC times both in-10-10℃and-5-℃5 FTC.The effects of different amptitudes of FTC on the CH₄ oxidation rate of different depths were-10-10°C>-5-5°C and 15-30 cm>0-15 cm.（4）FTC significantly reduced the microbial phospholipid fatty acid（PLFAs）concentrations and changed the composition of microbial community.Soil PLFAs concentrations in 0-15 cm and 15-30 cm depth did not change significantly in the early stage of FTC（5FTC）and gradually decreased after 5 FTC times during the period of-5-℃5 FTC.However,during the period of-10-10℃FTC,soil PLFAs concentrations began to decrease at the initial stage of FTC.The variation in soil microbial concentrations of-5-℃5 FTC was greater than-10-10℃FTC,and the influence FTC on soil microbial concentrations at 15-30cm was greater than that at 0-15 cm.Soil F/B increased first and then decreased in 0-15 cm depth with FTC times increasing but increased significantly in 15-30 cm depth during the period of-5-℃5 FTC,indicating that FTC significantly changed soil microbial community structure.The results of redundancy analysis showed that soil MBC,SWC,DOC,DON,MBN,and TN were the key factors affecting the changes of soil microbial community under FTC.（5）The soil hydrolase and oxidase activities significantly decreased after FTC.The effects of different amplitude of FTC on soil hydrolase activities were-5-5℃>-10-10℃,but were-5-5℃<-10-10℃for soil oxidase enzyme activities.After FTC,the variation in soil hydrolase activityies of different depths was 0-15 cm<15-30 cm,but was 0-15 cm>15-30cm for soil oxidaseactivities.Correlation analysis and redundancy analysis showed that FTC promoted the death of soil microorganisms and reduced the release of soil enzymes.At the same time,decomposition of microbial residues released large amount of available carbon and nutrients,which ultimately leading to the decrease in soil enzyme activities.（6）Soil CO₂ emission rate was positively correlated with soilβ-1 4-glucosidase（βG）activities,peroxidase（PER）activities and TP concentrations（p<0.01）.The oxidation rate of CH₄ was negatively correlated with the different groups of PLFAs concentrations and the activities ofβG,β-1,4-N-acetylglucosamine glycosidase（NAG）,acid phosphatase（AP）,and PER（p<0.01）.The results of path analysis showed that freezing temperature reduced soil oxidase activities through influencing soil water content and TP content,and finally reduced soil CO₂ emission.In the CH₄ oxidation pathway,on the one hand,lower freezing temperature directly affected microbial PLFAs and community structure;On the other hand,FTC decreased soil hydrolase activities and finally increased CH₄ oxidation rate by changing soil moisture content and TP concentrations.In conclusion,FTC reduced soil microbial PLFAs and enzyme activities and changed the composition of microbial community by changing soil nutrient concentrations,which finally reducd soil carbon emissions in peatland during the FTC.This study has important scientific significance for revealing the microbial mechanism of the effect of FTC on CO₂ and CH₄emissions from peatland soils under the background of climate change.