| Selenium is an essential micronutrient for human body and animals.The lack of selenium in human body will lead to a series of diseases,such as Keshan disease,Kashin Beck disease,hepatitis,AIDS,hypertension and coronary heart disease.China is one of the countries with serious selenium deficiency in the world.There is a"low selenium belt"from northeast to southwest in China.People living in this area have been affected by their own health due to long-term consumption of selenium deficient food.Therefore,it is urgent for people to supplement selenium.Rice is one of the most important staple foods in China.Increasing the content of selenium in rice is an effective way to alleviate the selenium deficiency of human body from the source,which is particularly important for the selenium deficient mountain residents.The absorption,transportation and accumulation of selenium in rice are affected by many environmental factors,such as the mode of phosphorus nutrition and water management.The chemical forms and ion radius of phosphorus and selenium are similar.Therefore,phosphorus nutrition may affect the absorption and transportation process of selenium in rice,and water management significantly affects soil p H and redox potential,thus significantly affecting the bioavailability of selenium in soil.In this study,pot experiment was used to study the interaction of phosphorus and selenium in rice during the whole growth period,hydroponic experiment was used to study the effects of phosphorus nutrition on the kinetic parameters of selenium absorption in rice,and pot experiment was used to study the effects of different water management methods on the regulation of selenium bioavailability and soil microbial diversity in soil rice system.The main results obtained in this thesis are displayed as follows:（1）Pot experiments were conducted to investigate the effects of phosphorus nutrition on the transformation of selenium forms in paddy soil and the accumulation of selenium in rice.The results showed that the interaction of different Se levels,Se forms and P levels*Se forms had extremely significant effects on the Se content of all forms of soil（P<0.001）.When selenite was applied,the content of available Se in soil decreased with the increase of P application rate.In the whole growth period experiment,the Se content in different parts of rice was in the order of root,leaf,grain and stem.Under different phosphorus and selenium levels,the selenium absorption,transfer and apparent accumulation coefficients of rice after selenate application were significantly higher than those of selenite application.With the increase of phosphorus level,the selenium accumulation and AC,TC coefficients of rice organs after selenite application were promoted by low phosphorus(0-100 mg·kg-1)and inhibited by high phosphorus(100-300 mg·kg-1).The selenium content in rice seeds was the highest(3.85 mg·kg-1)when selenite application was 2.0 mg·kg-1and phosphorus application was 100 mg·kg-1.Selenium content in rice organs,AC and TC coefficients increased at first and then stabilized with the increase of phosphorus level after selenate application.Selenium content in rice grain was the highest(4.87 mg·kg-1)when selenate application was 2.0 mg·kg-1and phosphorus application was 100 mg·kg-1.The uptake of Se by rice roots and the accumulation of Se in various organs reached the maximum at booting stage.（2）The effects of different phosphorus and selenium treatments on selenite and selenate uptake and transport in rice seedlings were studied by hydroponic experiment.The results of time absorption kinetics showed that the absorption rate of selenium in nutrient solution was faster within 5 h after the experiment,and then gradually stabilized.Under the same phosphorus concentration(10 mg·L-1),the selenate uptake and transport capacity of rice seedlings were significantly higher than that of selenite.By calculating the absorption kinetic parameters of rice seedlings under different treatments,the Kmand Imaxvalues under selenite treatment were significantly higher than those under selenite treatment by 68.10%and 87.63%,respectively,and the Cminvalue under selenate treatment was significantly lower than that under selenite treatment by 89.28%（P<0.05）.Therefore,the affinity and absorption capacity of rice seedlings to selenite in nutrient solution were strong,and the absorption potential of selenate was greater than that of selenite.The results of concentration absorption kinetics showed that with the increase of phosphorus level in solution,the transport coefficient of selenite in rice seedlings decreased significantly,but the effect on the transport coefficient of selenate was not obvious;with the increase of selenium level in solution,the transport coefficient of selenite in rice seedlings decreased significantly and the transport coefficient of selenate increased,which was due to the easier fixation of Se（IV）in rice roots,which is not conducive to the transfer of selenium to aboveground.Compared with selenate,the absorption potential of selenite in rice was higher,but the transport efficiency was lower.The increase of phosphorus concentration was not conducive to the absorption and transport of selenite in rice seedlings.（3）For this paper a pot experiment was conducted to evaluate effects of water management varying in pattern on composition of soil selenium,bacterial community diversity in rhizosphere soil and Se accumulation in grains of rice,and further to explore mechanism of water management affecting the absorption and accumulation of selenium in rice,so as to provide considerable theoretical and practical support to effective regulation of selenium transfer from rice field to agricultural products,to meet the demand of human for selenium nutrient and health.The experiment was designed to have three treatments in water management treatments,that is,（I）flooded irrigation（F）;（II）aerobic irrigation（A）;and（III）alternation of flooded and aerobic irrigation（AFA）,and three replicates for each treatment.Starting from D20（20 days after rice transplanting）,soil p H and Eh were measured every 12 days.The selenium in the rhizosphere soil was fractionated at the tillering stage,heading stage and maturity stage of the crop.Content of selenium in rice roots,stems,leaves and grains as well as yield of the crop were measured at maturity.Before rice harvest,the fresh soil in the rhizosphere bag was retrieved and placed in a bag,which was then sealed and stored at low temperature（-80℃）for high-throughput sequencing analysis.Results show that at all the rice growth stages,Treatments A and AFA were higher than Treatment F in soil p H and Eh to a certain extent and in content of soluble and exchangeable selenium as well,thereby in availability of soil selenium.In terms of selenium content,the organs of a rice plant exhibited an order of root(1.38-2.22mg·kg-1)>leaf(0.55-0.85 mg·kg-1)>stem(0.53-0.61 mg·kg-1)>grain(0.15-0.53mg·kg-1).Treatment AFA was the highest in selenium content in rice grains,while Treatment F was the lowest.The difference between the two was obvious.Treatment AFA was significantly or 7.83%higher than Treatment F and 13.51%higher than Treatment A in grain yield.The dominant species of bacteria in the rhizosphere soil of rice were Proteobacteria,Chloroflexi,Bacteroidetes,Acidobacteria,Patescibacteria and Gemmatimonadetes,with Proteobacteria being the most abundant one in rice rhizosphere soil in all the treatments.Obviously,water management had a significant impact on richness of the bacteria.Treatments AFA and A were significantly higher than Treatment F in richness of Proteobacteria.On the level of class,abundance of Gammaproteobacteria was positively related to content of available Se in the soil,so the increase in the former may be deemed as an important reason for higher bioavailability of selenium in the soil.Comprehensive analysis shows that alternation of flooded and aerobic irrigation can not only improve yield and selenium content of rice grains,but also save water for irrigation as compared to normal flooding irrigation.Therefore it can be recommended as preferential water management method in rice production.In conclusion,this study took rice as the experimental object,and set up pot experiment and hydroponic experiment in the whole growth period and seedling stage of rice.It was clear that the phenomenon of low concentration of phosphorus and selenite fertilizer mixed application promoted but high phosphorus inhibited the accumulation of selenium in rice grains.At the same time,the effect of selenium application at booting stage was the best,and selenate was more conducive to the absorption and transportation of selenium in rice seedlings than selenite.The absorption kinetics experiment confirmed that the selenate absorption potential and efficiency of rice seedlings were higher than selenite,and selenite was easy to be fixed in the root,which made the transfer coefficient lower.In practice,AFA irrigation can not only improve rice yield and grain selenium content,but also maintain a healthy soil microecological environment,which is a recommended rice management method. |
