Study On The Coupling Relationship Of Soil Carbon And Iron Oxides And Its Microbiological Characteristics Under Long-term Rice Cultivation

In paddy soil,microbially mediated dissimilatory iron reduction is one of the most important geochemical processes,and also one of the indispensable strategies of microbial iron metabolism.It plays an irreplaceable role in the processes of material cycling and energy flow for rice ecosystem.However,little is known about the interaction among iron reduction,organic carbon accumulation and microbial community succession under long-term rice cultivation.Soil chronosequence is a unique tool for investigating the rate and direction of biogeochemical changes in paddy soil,which can provide a valuable model system for investigating pedogenesis process and element coupling.In this study,a rice chronosequence in Hangzhou Bay,Cixi,Zhejiang Province was used to assess the effect of long-term rice cultivation on iron oxides,organic carbon accumulation and coupling relations mediated by microorganisms.We used ICP to determinate the content of total Fe,free total Fe oxides,amorphous Fe oxides and organically complexed iron oxides.HCl-extracted Fe(II),Fe(III)and total Fe were measured by Phenanthroline colorimetry.The gene abundance and community structure of iron reducing microorganisms were determined by q PCR(Geobacter,Shewanella and Anaeromyxobacter)and 454 high-throughput sequencing.Anaerobic incubations of paddy soils were conducted with a constant temperature to determinate the change of iron oxides reducing and carbon emission with the chronosequence.The aim of this study is to investigate the dynamic characteristics of iron-oxide and the structure of iron reducing microbial community,and the potential of dissimilation iron reduction,to illustrate succession mechanism of iron reducing microbial community and the effect of iron oxides on the accumulation of organic carbon under the long-term rice cultivation,and to explore microbial mediated coupling mechanism of organic carbon and iron oxides.The major results and conclusions are listed below:(1)With the increase of rice planting years,SOC and TN increased about twice.During the initial 50 years of paddy rice cultivation,total Fe,free iron oxides and poorly crystalline iron oxides contents declined.After that,total Fe contents decreased at a rate of 10 kg ha-1 year-1 until 1000 years of rice cultivation.Free iron oxides violently changed,whereas poorly crystalline iron oxides contents increased with a low rate during the 50 years to 300 years of paddy rice cultivation.And thereafter,both of them have no significant change.These results indicated prolonged rice cultivation leaded to the decreasing of total Fe and the redistribution of iron oxides.At the same time,free iron oxides and poorly crystalline iron oxides would transform to other forms in the early stage of rice cultivation.The chemical combination of organically complexed iron oxides and OC was one of the important reasons for the accumulation of organic matter in paddy soil.Organically complexed iron oxides contents and Fe complexing index increased 3 folds and 5 folds during 1000 years of rice cultivation,respectively.In the chronosequence,the SOC and DOC contents were positively linearly correlated with HCl-extracted Fe(III).This finding suggested HCl-extracted Fe(III)contributed to the OC accumulation in paddy soil chronosequence.Besides,SOC/Fe molar ratio had a strong positive correlation with prolonged rice cultivation(correlation coefficient is 0.98).It demonstrated there was a strong stoichiometric relationship between SOC and total Fe contents under the paddy chronosequence.(2)Gene copy numbers of Geobacter and Anaeromyxobacter increased 10 folds and 80 folds with paddy development,and Shewanella decreased 80%.The gene copy number and relative abundance of iron reducing microorganisms were stable after 300 years of rice cultivation.The succession of Fe-reducing bacterial groups were primarily driven by the change of physical and chemical properties with long-term anthropogenic pedogenesis process.Structural equation modeling(SEM)showed there was a complex coupling relationship among C,Fe,and Fe-reducing microorganisms in paddy soil.In the initial 300 years of chronosequence,soil organic carbon accumulated rapidly,and was mainly affected by the content of free iron oxides,poorly crystalline iron oxides,organically complexed iron oxides and HCl-extracted ferric oxides.Thereafter,the same management inducted slow accumulation of OC which was associated with changes in organically complexed iron oxides and HCl-extracted ferrous oxides.In conclusion,with the paddy ecosystem gradually stabilized over the long-term rice cultivation,the complexed coupling relationship of C,Fe and Fe-reducing microorganisms became simple and clear.(3)With the addition of ferrihydrite,goethite,hematite and magnetite,the 700 years aged soil has the strongest iron reduction ability with net increase rate of change in Fe(II)was 19.1%~26.5%,while marsh soil and the 50 years aged soil had the weaker reduction ability with net increase rates of change in Fe(II)were 0.6%~7.7% and 5.5%~8.3% respectively.These results demonstrated there were the different effects on the Fe-reducing capacities in the chronosequence of paddy soils after the addition of iron oxides.Besides,the net increase rate of change of HCl-extracted total Fe was 12.2%~21% in the chronosequence with the addition of ferrihydrite,which showed ferrihydrite significantly promoted the activity of iron.The 700 years aged soil had stronger iron oxides activation ability than other soils with four exogenous iron oxides addition(the net increase rates of change are 7.4%~19.9%).The decrease in methane emission from paddy soil(P50~P700)was directly related to the peoduction of ferrous iron,which showed a significant negative correlation.Overall,the dissimilated iron reduction process was one of the important reasons to curb methane emission in paddy soil.