Changing Characteristics Of Iron-reducing Microbial Abundance And Diversity In Paddy Rhizosphere Soil

Alternating changes of redox process in paddy soil has important geochemical and environmental significance for the cycling of elements.Iron is the most important factor in controlling soil redox process,iron oxides can act as electron acceptors,and their anaerobic reduction process is restricted by anaerobic or facultative anaerobic Fe(?)reduction microorganisms.Simultaneously,community succession of Fe(?)reducing microorganisms is also affected by the planting process.Oxygen secretion of roots can change the quantity and function of Fe(?)reducing microorganisms.Systematic study on the effects of oxygen secretion from roots on iron oxide reduction ability,microbial quantity and community structure in soil has important theoretical and practical significance for understanding the succession characteristics of soil biological properties in rice planting process.Through the root-box planting system,the rhizosphere and non-rhizosphere soil samples obtained from different rice growth stages were sliced and layered to distinguish the soil samples at different distances from the root surface.The changes of Fe(?)concentration in different layered soils were detected to verify the effect of oxygen secretion from roots on iron redox capacity of rhizosphere soil.Dehydrogenase activity and pH in different layered soils were detected to verify the effect of oxygen secretion from roots on dehydrogenation and hydrogen production.The abundance and activity of obligative and facultative Fe(?)reducers were detected by extracting soil DNA(RNA)from different layered soils,and the differences of abundance and activity of Fe(?)reduction microorganisms between rhizosphere and non-rhizosphere soils were discussed.High-throughput sequencing technology was used to analyze the microbial community diversity in rhizosphere and non-rhizosphere soils.The major results and conclusions are listed below:(1)The Fe(?) concentration in different rice growth stages decreased gradually with the decrease of the distance from the root surface.Based on the change of Fe(?)concentration,the range of distance between soil and root surface affected by root oxygen secretion was 4.0-6.5 mm.The greater the plant biomass,the stronger the effect on Fe(?)concentration,that is,the distance from the root surface increased,indicating that the redox status of the root zone is sensitive to the oxygen secretion of roots.Dehydrogenase activity and pH in the rhizosphere soil at different growth stages were lower than those in the non-rhizosphere soil.(2)With the decrease of oxygen concentration,the copy number of Anaeromyxobacter decreased;the Geobacteraceae was basically stable;while the copy number of Bacteria,Clostridium and Bacillus increased.The copy number of Bacteria and Bacillus did not change significantly with the increase of growth period.The difference in copy number of Clostridium and Geobacteraceae of rhizosphere soil at different growth stages was more obvious than that of non-rhizosphere.With the increase of distance from root surface,the relative abundance of Geobacteraceae and Anaeromyxobacter decreased;while the relative abundance of Clostridium increased;the relative abundance of Bacillus decreased slightly at first,then fluctuated upward and downward.The contribution of obligative Fe(?)reducers in rhizosphere was greater,and the role of facultative Fe(?)reducers increased gradually with the consumption of oxygen.It was confirmed that the oxygen secretion of roots significantly affected the 16 S rDNA copy number and relative abundance of Fe(?)reducers.(3)Based on the results of high-throughput sequencing,the relative abundance of Geobacteraceae and Clostridium increased with the increase of distance from the root,while Bacillus was opposite,and the relative abundance of Anaeromyxobacter changed slightly.Oxygen released from roots also affected the community structure of these bacteria.The increase of oxygen concentration increased the community diversity of Geobacteraceae and Clostridium,and decreased the community diversity of Bacillus,but had less influence on Anaeromyxobacter.Phylogenetic analysis showed that the dominant groups of these four bacteria mainly existed in paddy soil and sediment environment and had Fe(?)reduction function.Most Geobacteraceae and Anaeromyxobacter were highly associated with uncultured groups,while most Clostridium and Bacillus were closely related to known strains.Correlation analysis found that the change of community structure of Fe(?)reducers caused by the oxygen secretion from roots was closely related to Fe(?)reduction process.(4)Transcriptional activities of Bacteria,Geobacteraceae,Anaeromyxobacter and Clostridium in the rhizosphere soil of different rice growth stages were much higher than those in the non-rhizosphere soil.In the non-rhizosphere soil,the transcriptional activity showed a stable state with the change of rice growth period,while in the rhizosphere soil,with the increase of growth time,the transcriptional activity first increased and then decreased rapidly.Correlation analysis found that the activity changes of Bacteria and Fe(?)reducers caused by the oxygen secretion of roots were closely related to Fe(?)reduction process and microbial hydrogen production in paddy soil.