Study On The Bounding Characteristics Between Soil Iron And Carbon And Microbial Interactions In Typical Paddy Fields Of China

Paddy ecosystems play a dual role as a carbon source and carbon sink in the global carbon cycle,and iron oxides play an important role in promoting the stability of soil organic carbon.In order to explore the bounding relationship between iron oxides and organic carbon and its microbiological characteristics at a large scale,we took typical paddy fields in China as the research object.Based on the chemical stabilization mechanism and biological protection mechanism of iron cycle,the soil physicochemical properties,iron phase distribution characteristics,Fe-bound organic carbon content and the distribution of quantitative parameters in different rice cropping areas were analyzed and compared,and with the help of high-throughput sequencing technology,the iron-carbon bounding relationship mediated by iron cycling microorganisms and its material cycle laws were explored.It is expected to provide a reference for sustainable soil utilization and scientific management,and to achieve the agricultural emission reduction target of"carbon peaking in 2030 and carbon neutrality in 2060".The main findings are as follows:（1）The basic physicochemical properties and soil organic carbon（SOC）content in different rice cropping areas were different;the Fe（II）and Fe（III）contents were between0.01～3.62 g·kg^-1and 0.01～6.29 g·kg^-1,respectively.Fe（II）was positively correlated with water content（WC）（p<0.01）,and positively correlated with p H（p<0.05）;Fe（III）was positively correlated with WC and SOC,and negatively correlated with p H（p<0.01）.（2）The contents of free iron oxide（Fe_d）,amorphous iron oxide（Fe_o）,and complex iron oxide（Fe_p）are 5.75～24.71 g·kg^-1,0.93～19.16 g·kg^-1,0.03～1.27 g·kg^-1,respectively.From the overall spatial distribution characteristics,the soil Fe_dcontent in paddy field decreased from south to north,while the Fe_pcontent and iron complexation degree decreased from northeast to southwest.Soil iron oxide contents were significantly positively correlated with SOC（p<0.05）,and significantly negatively correlated with soil p H and electrical conductivity（EC）（p<0.01）.Soil iron activation degree and complexation degree were significantly positively correlated with Fe_pand WC（p<0.01）,and significantly negatively correlated with soil p H and EC（p<0.05）.（3）The content of Fe-bound organic carbon（Fe-OC）in the surface soil of the rice cropping area varied from 0.20 to 3.02 g·kg^-1,with an average value of（1.29±0.06）g·kg^-1.From the overall spatial distribution characteristics,soil Fe-OC content showed a decreasing trend from northeast to southwest;on average,（11.24±6.25）%of SOC in all paddy soil samples was in the form of Fe-OC,indicating that the bounding and immobilization of iron oxides play an important role in the stabilization of SOC.（4）The molar ratios of carbon to iron（OC:Fe）in the paddy fields of Yancheng City in East China and Wusu City in Northwest China were 1.30 and 1.21,respectively,but90%of the soil samples were found to have OC:Fe less than 1 through analysis,indicating that Fe-OC of the paddy fields were mainly formed by surface adsorption,while co-precipitation contributed less to its formation.（5）Fe（III）was significantly negatively correlated with the proportion of Fe-bound organic carbon(f _Fe-OC)and OC:Fe（p<0.01）,indicating that Fe（III）was more conducive to the formation of soil Fe-OC,while Fe（II）limited effect on its formation.There was no significant correlation between the type and content of soil iron oxides and Fe-OC,while Fe-OC was significantly positively correlated with SOC.（6）A total of 9 bacterial phyla and 86 genera were identified in the soil iron-reducing bacteria community in different rice cropping areas.The diversity and richness of the iron-reducing bacteria community in the soil of HLJ were significantly higher than those in the soils of the other four regions（p<0.05）.Proteobacteria,Firmicutes,Myxococcota,Desulfobacterota and Acidobacteriota are the dominant phyla in paddy fields;Sphingomonas,Bacillus,Bradyrhizobium and Thiobacillus are the dominant bacterial genera in paddy fields.The community structure of iron-reducing bacteria in the soils of HN and JX was similar,while the soil community structure of SD,HLJ and YN was quite different.（7）A total of 11 bacterial phyla and 37 bacterial families were identified in soil iron-oxidizing bacteria communities in different rice cropping areas.The soil samples from HLJ had the highest community diversity and richness of iron-oxidizing bacteria,and the soil samples from YN had the lowest.Among them,the relative abundance of Chloroflexi,Proteobacteria,Gemmatimonadota and Acidobacteriota is as high as71.25%～98.43%,which are dominant phyla in the iron-oxidizing bacteria community in paddy fields.The dominant bacterial families of each soil sample were similar and different;the community structure of iron-oxidizing bacteria in SD soil was quite different from that in other regions,while the soil community structure in HN and JX was similar.（8）Environmental factors had significant effects on the relative abundance of soil microorganisms in paddy fields.Sphingomonas was positively correlated with soil total phosphorus（TP）;Thiobacillus and Pseudomonas were positively correlated with p H,EC,BD and silt,and negatively correlated with total nitrogen（TN）;Anaeromyxobacter and Haliangium were positively correlated with p H,silt and clay;Bradyrhizobium and Bacillus were positively correlated with TN and TP.Sphingomonadaceae was positively correlated with soil TP and EC;Gemmatimonadaceae was positively correlated with TP and TN;Anaerolineaceae was positively correlated with TN,silt and sand,and negatively correlated with EC,TP.Both SJA-15 and KD4-96 were positively correlated with soil EC,BD,p H and silt,and negatively correlated with TN,TP and sand.（9）There were significant correlations between soil microbes and iron and carbon components in paddy fields.SOC,Fe（III）,Fe_o,and Fe_pwere significantly positively correlated with Haliangium,Anaeromyxobacter,Geothrix and Sideroxydans（p<0.05）,extremely significant negative correlation with Sphingomonas（p<0.01）;Fe（II）was significantly positively correlated with Geothermobacter,Thiobacillus,Pseudomonas,Geobacter and Desulfuromonas（p<0.01）.SJA-15 and KD4-96 were positively correlated with soil Fe（Ⅲ）,SOC and Fe-OC,and negatively correlated with Fe（Ⅱ）;Gemmatimonadaceae and Sphingomonadaceae were positively correlated with Fe_d,but negatively correlated with Fe（Ⅱ）.