Effects Of Light And Organic Material On Iron Reduction In Rice Rhizosphere Soil

After flooding the paddy soil,a series of chemical and biological reactions occur due to the thermodynamic control.The Fe(III)reduction process is the most important process at the end of the electron transport chain in the anaerobic environment.The process of dissimilatory iron reduction in flooded paddy soil is a microbial-mediated biochemical process that plays an important role in the degradation of organic pollutants,the reduction and precipitation of heavy metals,the migration and transformation of radioactive materials,and the inhibition of methane production.Iron is a redox-sensitive metal in the soil.Due to the presence of roots,the rhizosphere is considered to be a key region of the iron biogeochemical cycle.The rhizosphere is a special soil micro-region where rice roots and soil are in close contact.Due to the root radial oxygen loss,there are many facultative and aerobic micro-regions;and the root exudates and root radial oxygen can change the pH and Redox condition of rhizosphere soil environment.In addition,the root radial oxygen loss and released root exudates are different in different growth stages of rice.Therefore,the iron reduction ability and characteristics of rice rhizosphere soil are different from those of non-rhizosphere soil in different rice growth stages.This study used rice root bag soil culture method.Using the paddy soils of different planting areas in China,the rhizosphere and non-rhizosphere iron reduction ability of different paddy soils under rice root oxygen secretion conditions were investigated.The effects of photoperiod on iron reduction in rhizosphere soil were studied by controlling rice light conditions.The effects of organic matter anaerobic fermentation on Fe(III)reduction and dehydrogenase activity were studied by adding different amounts of bio-organic fertilizer from external sources.Combined with the 16 S rDNA PCR enzymatic cleavage technique,the effects of bio-organic fertilizer addition on microbial community structure in rice root zone and non-root zone soil were investigated.The main findings are as follows:(1)The ability of Fe(II)oxidation in the rhizosphere soil was increased by rice root oxygen loss,and the Fe(II)oxidation increased with the increase of rice biomass.The difference in physical and chemical properties of different paddy soils also resulted in the difference in the reduction capacity of rhizosphere iron,which was manifested as the gradual oxidation of Fe(II)in the soil,or the inhibition of Fe(III)reduction.The iron content in the non-rhizosphere soil increased with the increase of the incubation time and the Fe(III)concentration decreased.Dehydrogenase activity in the rhizosphere and non-rhizosphere soil increased gradually in the cultivation system,and the activity of dehydrogenase in the non-rhizosphere soil was higher than that in the rhizosphere.The activity of dehydrogenase is closely related to soil type.After the three paddy soils were flooded,the pH of the non-rhizosphere increased gradually and tended to be neutral.The pH of rhizosphere soil generally showed a downward trend,while the pH of QL paddy soil gradually increased and approached the non-rhizosphere level after 36 d of rice cultivation.Soil dehydrogenase activity is one of the factors affecting iron reduction capacity,and pH can regulate dehydrogenase activity.Dehydrogenase activity was significantly positively correlated with Fe(II)concentration and extremely significantly positively correlated with pH.(2)The concentration of Fe(II)in the rhizosphere soil decreased significantly under continuous light conditions,indicating that light can promote the root radial oxygen loss capacity.The Fe(III)reduction ability of rhizosphere soil increased continuously in the dark light stage,and the Fe(II)concentration gradually increased,which reached the level close to the non-rhizosphere soil in the dark light period of 12 h.The root radial oxygen loss capacity increased with the extension of rice growth period,and the iron oxidation amount of rhizosphere soil increased during the 12 h light period.The iron content in the non-rhizosphere soil was not regulated by the rice photoperiod.The soil Fe(II)concentration was larger than the rhizosphere,and the Fe(III)concentration was lower than the rhizosphere.(3)The addition of bio-organic fertilizer can promote the reduction of Fe(III)in non-rhizosphere soil and inhibit the oxidation of Fe(II)in rhizosphere,and the difference is significantly reduced with the growth of rice.The use of bio-organic fertilizer as a fermentation substrate can increase the activity of dehydrogenase in soil,and it is increasing with the increase of bio-organic fertilizer.The use of bio-organic fertilizer can increase the pH value of the soil.The change of pH value in rhizosphere is more significant than that in non-rhizosphere,which can significantly improve the acidic environment of rhizosphere soil during rice growth.(4)The microbial diversity in the root zone of rice is significantly greater than that in the non-root zone.The use of bio-organic fertilizer can reduce the diversity of bacteria in the soil.As the amount of bio-organic fertilizer used increases,the diversity of soil bacteria in the root zone gradually decreases.However,in non-root zone soils,the 1% application rate has a greater reduction in bacterial diversity than the 3% application rate.