Analysis On Ecological Characteristics Of Rice Rhizosphere In Response To Alternate Wetting And Drying Irrigation At Grain Filling Stage

Many researches on the effects of the growth of rice plant(overground part)and yield by alternate wetting and drying irrigation at grain filling stage have been reported.However the mechanism of rhizosphere ecological system in response to the variation of soil moisture is still unknown now.As the name implies,alternate wetting and drying irrigation means to control soil water content.As an important component of the soil,water is also recognized as one of the most active fertility factors.Different soil water content not only modulates soil aeration and temperature status,but also has immense impacts on soil nutrient content and enzyme activities.Thus,using the conventional rice line Jinhui-809 with large spikes as experimental material in this study,we explored the ecological factors and the changes in the rhizospheric microbial community structure at the grain filling stage under dry-wet alternate irrigation.The main results were as follows.Combining the field and pot trials,three different irrigation regimes including traditional irrigation(keep water all the time,regarded as CK),alternate wetting and moderate soil drying(rewater when soil water potential dried down to-25kPa,regarded as WMD)and alternate wetting and severe soil drying(rewater when soil water potential dried down to-50kPa,regarded as WSD)were designed to study the rice growth and yield change.It indicated that WMD could dramatically increase root activity and photosynthetic capacity of rice flag leaf and thus promote the grain filling of inferior grains and yield.Furthermore,the ecological factors of rice rhizospheric soil were dynamically monitored in field trial.The results revealed that under WMD treatment,the pH value sharply decreased,but the Eh value was significantly enhanced,and the soil conductivity increased firstly and then decreased with the decrease of soil water potential,then all factors quickly fell back after rehydration.Meanwhile,the soil nutrients under different treating conditions were determined.Overall,the WMD could decrease the soil ammonium nitrogen content,yet sharply increase the contents of nitrate nitrogen,available phosphorus and available potassium.As for soil enzyme activities,WMD could enhance the activities of sucrase,urease and acid phosphomonoesterase in the whole experimental period.The activities of polyphenol oxidase,nitrate reductase and catalase might be more sensitive to soil drought degree,which were quite changeable.Furthermore,the canonical correlation analysis was applied to investigate the relationships between the rhizospheric soil ecological factors and enzyme activities.The results showed that soil Eh value had a positive correlation with acid phosphomonoesterase,and EC value was negatively correlated with nitrate reductase.Finally,the impacts of the WMD irrigation treatment on the microbial community structure in rhizospheric ecosystem were studied by using T-RFLP technique.It was found that the diversity indexes of bacterial community became higher under WMD treatment,but the evenness index reduced.Compared with the Ribosomal Database Project ?,we found that Actinobacteria content gradually increased after WMD treatment,but the relative abundance of Proteobacteria and Firmicutes greatly fluctuated,such as beta Proteobacteria,Bacillibacteria,delta Proteobacteria and Clostrida.In details,the proportion of aerobic bacteria related to the recycling of soil nutrients increased dramatically in the soil dry stage,including Azotobacter(Azospirillum brasilense,Beijerinckia indica subsp,Frankia,Herbaspirillum),nitrifying bacteria(Nitrosospira),Phosporus-solubilizing or Potassium-solubilizing bacteria(Bacillus edaphicus,Agrobacterium),etc.But anaerobic or facultative anaerobic-denifrifying bacteria(Neisseria denitrificans),sulfate-raducing bacteria(Desulfobulbus,Desulfovibrio senezii),cellulose-decomposing bacteria(Clostridium polysaccharolyticum)became much higher after soil rehydration.In conclusion,alternate wetting and moderate soil drying(WMD),the rational regulation method of soil moisture,led to the periodic changes in soil pH,Eh and EC,affected the ratio of aerobic/anaerobic relating to the nutrients recycling,and increased the activities of related soil enzymes and the availability of soil nutrients.Meanwhile,the WMD treatment improved the soil aeration,enhaned the absorptive capacity of rice root system,supported the demands for nutrients at late grain filling stage,and thus promoted the plant physiological activities and grain filling.This study illustrated the physiological and ecological mechanisms of rice rhizosphere in response to alternate wetting and drying irrigation,and enriched the theory of crop cultivation science,which laid the foundation for the next step to set up a scientific irrigation system.