Microbial Mechanisms Of The Soil Iron-Enzyme-Carbon Relationship In Degraded Peatlands During Water Table Restoration

Peatland ecosystem is rapidly degraded under the influence of climate change and human drainage.The organic carbon decomposition mechanism of the peatland become a hot topic during the drained and rewetting processes.Drainage/rewetting process in peatlands significantly changes soil redox conditions,which in turn changes soil microbial community and enzyme activities,drives iron redox process,and finally affects soil organic matter decomposition.Soil enzymes and iron,which are both affected by redox conditions,act as‘enzyme latch’and‘iron gates’,respectively,in the decomposition and storage of soil organic carbon.However,there is a urgent need for further research on the iron-enzyme-carbon interaction and microbial driving mechanism during drainage/rewetting processes of peatlands.Therefore,this study was carried out in the drainage peatland and the restoration peatland in Changbai Mountains.The first aim was to reveal the changes in organic carbon（SOC）,active organic carbon,iron-bound organic carbon（Fe-SOC）content,enzyme activities,and bacterial diversity and community structure during the water level restoration process in the drainage peatland through in situ water table restoration controlling experiments,soil enzyme activity fluorescence method,and high-throughput sequencing.Secondly,clarify the relative importance of the‘enzyme latch’and‘iron gate’mechanisms in regulating the storage of SOC and its active components during water table restoration in drained peatlands using the structural equation model.Finally,reveal the microbial mechanism of the iron-enzyme-carbon relationship during the water table restoration process of drained peatland and provide a scientific reference for the restoration of carbon‘sink’function of the drainage peatland in China.The main conclusions were as follows:（1）Soil physico-chemical properties changed significantly with the raising of water table afte restoretion.Soil p H and water content（SWC）showed a trend of restoration area>drainage area（p<0.05）from 0 to 20 cm,but showed of drainage area>restoration area（p<0.05）from 20 to 100 cm.Soil total nitrogen（TN）and redox potential（Eh）showed a trend of the drainage area>the restoration area（p<0.05）.Soil total phosphorus（TP）and C/N showed a trend of the restoration area>the drainage area（p<0.05）.（2）There were differential effects of water table restoration on different forms of iron contents.Soil ferrous iron(Fe²⁺)content showed a trend of restoration area>the drainage area（p<0.05）.The trivalent iron(Fe³⁺)and complexed iron（Fe_p）contents showed trends of the drainage area>the restoration area（p<0.05）.Total iron（TFe）,amorphous iron（Fe_o）,and free iron（Fe_d）showed trends of the restoration area>the drainage area from 0 to20 cm（p<0.05）,but showed of the drainage area>the restoration area from 20 to 100 cm（p<0.05）.Correlation and redundancy analyses showed that the variation in the different forms of Fe contents was mainly influenced by soil water content and iron oxidising bacteria（Fe OB）.Among them,the Fe contents of the drainage area was mainly influenced by soil Eh and SWC,but was by soil p H and SWC.（3）There were significant differences of SOC and Fe-SOC contents between the drainage and restoration peatlands.SOC content showed a trend of the restoration area>the drainage area（p<0.05）.Soil soluble organic carbon（DOC）content showed a trend of drainage area>the restoration area from 0 to 20 cm（p<0.05）,butr showed of the restoration area>the drainage area from 20 to 100 cm（p<0.05）.Soil microbial mass carbon（MBC）and Fe-SOC content showed a trend of drainage area>the restoration area（p<0.05）.Correlation analysis showed that soil water content and C/N were the main soil physico-chemical properties influencing the different soil carbon forms and Fe-SOC content.Soil carbon of the drainage area was mainly influenced by p H,soil water content,and C/N,but was by p H,Eh,and C/N In the restoration area.（4）Significant changes in soil oxidase and hydrolase activities occurred from 0 to 100cm after water table restoration.Overall,soilβ-1,4-glucosidase（BG）andβ-1,4-N-acetylglucosaminoglycosidase（NAG）activities showed a trend of restoration area>the drainage area（p<0.05）,but there was no significant difference for soil acid phosphatase（AP）activities.Soil polyphenol oxidase（PPO）and peroxidase（PER）activities showed a trend of restoration area>the drainage area in 0-20 cm soils（p<0.05）,but showed of the drainage area>the restoration area in 20-100 cm soils（p<0.05）.Correlation and redundancy analyses showed that soil TFe,C/N,and water content had significant effects on soil enzyme activities（p<0.05）.In particular,soil enzyme activities in the drainage area was mainly influenced by soil p H and TP as well as Fe_oand Fe_d.Soil enzyme activity in the restoration area was mainly influenced by soil p H,water content,C/N,and different forms of Fe.（5）Soil bacterial diversity and community composition responded rapidly to water table restoration.Soil bacterial diversity and the number of endemic OTU taxa increased significantly after water table restoration（p<0.05）.Soil bacterial community structure was significantly different between the drainage area and the recovery area,and the difference was most significant above the water table（0-20 cm）.The abundance of the dominant soil bacterial group Acidobacteria decreased,but the abundances of Bacteroides,Desulfovibacteria,and Actinobacteria tended to increase after the water table restoration.The relative abundance of soil iron-oxidising bacteria（Fe OB）showed a trent of the drainage area>the restoration area,but showed of the restoration area>the drainage area for the relative abundance of soil iron-reducing bacteria（Fe RB）（p<0.05）.Correlation and redundancy analyses and Monte Carlo ranking tests showed that soil bacterial community structure was significantly influenced by soil water content and C/N（p<0.05）,while soil Fe OB and Fe RB community structure was significantly influenced by water content（p<0.05）.（6）The response mechanism of SOC and its active components to water table recovery is controlled by water table depth.The results of correlation and path analysis showed that soil SOC above the water table depth was mainly controlled by SWC,C/N,and Fe-SOC with direct effect coefficients of 0.570,0.156,and 0.341,respectively.Soil DOC was mainly influenced by SWC,oxidase,TFe,and Fe³⁺with direct effect coefficients of-0.875,-0.638,1.035,and 0.966,respectively.Soil MBC was mainly influenced by bacterialα-diversity,Fe²⁺,and SWC with direct effect coefficients of-0.838,-0.719,and-0.748,respectively.SOC below water table depth was mainly influenced by Fe²⁺,Fe³⁺and Fe-SOC with the direct effect coefficients of-0.744,-0.438,and 0.406,respectively.Soil DOC was mainly influenced by oxidase,Fe²⁺,and C/N with direct effect coefficients of-0.628,0.467,and 0.384,respectively.Soil MBC was mainly influenced by bacterialα-diversity,hydrolase,bacterialβ-diversity,and Fe²⁺with direct effect coefficients of-1.012,0.556,0.408,and-0.588,respectively.In summary,soil bacterial and enzymatic activity quickly responded to the water table restoration of degraded peatlands.However,the increase in SOC content following water table restoration did not regulated by an‘enzyme latch’mechanism,but was controlled by soil water content and C/N.At the same time,Fe-SOC could contribute to the increase in SOC.Soil MBC and DOC content decreased above the water table depth after water table restoration.Soil DOC content increased,but soil MBC content decreased below the water table depth.Abiotic factors such as soil water content played an important role in the variations in SOC after water table restoration,while biotic factors such as bacterial diversity,community structure,and enzyme activity played important roles in the variations in soil active organic carbon fraction.The results of the study provided a scientific basis for investigating the mechanisms of soil organic carbon decomposition in peatlands after water table restoration,and also provided important parameters for assessing the status of peatlands in global carbon cycle models.