| The aim of this paper was to investigate the influences of silicon application on the migration of cadmium in soil-rice(Oryza sativa L.)system,and to explore the optimal time and ratio of silicon application for alleviating the uptake and accumulation of cadmium(Cd)in Cd-contaminated soil.paddy soils were used as test soils.Cadmium chloride was added to form a polluted soil with Cd content of 100 mg/kg.Based on the same total amount of silicon application(56 mg/kg).Twoexperiments was carried out to study the effects of silicon application at different stages on the fractions of Cd in soil and the uptake,transport and accumulation of Cd in rice.The 2016 test settings are as follows:silicon applied as base fertilizer(100%:0%);silicon applied as basal fertilizer and at jointing stages with the ratio of 1:1(50%:50%);silicon applied at jointing stage(0%:100%).No silicon application was used as control(CK).The 2017 experiment is based on the 2016 test increase two split silicon application treatment: silicon applied as base fertilizer(100%:0%);silicon applied as basal fertilizer and at jointing stages with the ratio of 7:3(70%:30%);silicon applied as basal fertilizer and at jointing stages with the ratio of 1:1(50%:50%);silicon applied as basal fertilizer and at jointing stages with the ratio of 3:7(30%:70%);silicon applied at jointing stage(0%:100%).No silicon application was used as control(CK).The main findings are as follows:1)The application of silicon in different stages significantly affected rice aboveground biomass and yield,increasing the aboveground dry weight of rice,1000-grain weight and seed setting rate,and reduce the number of empty grains per panicle.This shows that the application of Si in 100 mg/kg Cd soil is still conducive to the growth and development of rice plants,which facilitates the accumulation of dry matter and increases the biomass and yield of rice plants.Compare with CK,The 50%:50% and 30%:70%treatments had a significant increase in dry weight,1000-grain weight,and seed settingrate.2)The application of silicon in different stages have a greater influence on the Cd content in soil at mature stage.In the 2016 and 2017 test,the content of Cd in the soil treated with Si were increased compared with CK at mature stage and the soil Cd migration rate was decreased at grouting stage,respectively.The 50%:50%,30%:70%,and 0%:100%Cd content in soils was higher at the maturity stage and the Cd migration rates was lower at the grain-filling stage.This suggests that silicon applied at the jointing stage can effectively prevent the Cd in the soil from transferring to the rice plants,making more Cd stays in the soil.3)The application of silicon in different stages can significantly reduce the exchangeable and reducible Cd content in soil at mature stage,and increase the oxidizable state and residual Cd content,and reduce the bioavailability of Cd in soil,thus reducing Cd migration into the plants.Among them,50%:50%,30%:70% are more effective.Compared with CK,in period of 2016 and 2017,the residual Cd content of each silicon treatment increased at the mature.At the same time,application of silicon can increase soil pH,and the increase of pH can also reduce the effectiveness of Cd in soil.4)silicon applied in different stages had effects on the Cd content and accumulation of various organs of rice plants.Silicon application increase the content and accumulation of Cd in root,husk and aleurone layer at mature stage,and reduce the Cd content and accumulation in stem and polished rice.5)The split Silicon application had little effect on the Cd content of iron films on the root surface of rice at the heading stage.The Cd content of the iron film on the root surface of silicon application treatments was significantly increased compared with CK at mature stage,However,there was no significant difference between the different proportions of silicon application treatments.It shows that the application of silicon makes a part of Cd to be adsorbed in the iron film and preventing Cd from entering rice.6)Si application can reduce the content of Cd in the bleeding sap.Compared with CK,the Cd content of bleeding sap in the 100%:0% was significantly reduced at the heading stage,and the Cd content of bleeding sap in the silicon application treatments was significantly lower than CK at after 15 days of heading,in which 50%:50%,30%:70% Cdcontent of bleeding sap was the lowest,while the Cd content in the bleeding sap was significantly positively correlated with the Cd content in the polished rice at 15 days after heading.It indicated that silicon application could reduce the migration of Cd from the root to the shoot,and deposit Cd on the root of rice,thus reducing the content of Cd in grains.In particular,50%:50% and 30%:7 0% can minimize the transport of Cd into grain.7)Starch grain containing higher Cd were found from the basal stem and the stem below panicle stem node(SEM-EDS).Moreover,the distribution of starch grain decreased as the number of internodes increased.From the mass percent of Cd,the Cd content also decreased with increasing internodes.The mass percentages of CK,100%:0%,70%:30%,50%:50%,30%:70%,and 0%:100% grain Cd were 0.37,0.38,0.09,0.07,0.06,and 0.16,respectively.The percentage of mass of Cd treated with each application of silicon was lower than that of CK,and it was also lower than the mass percentage of Cd in basal stem and the stem below panicle stem node.Grain SEM-EDS found that application of silicon can make starch grains more complete,thus reducing the occurrence of cracks and chalkiness.50%:50%,30%:70%,and 0%:100% have significant effects.8)From the analysis of the correlation between different fractions of Cd in soil and Cd content in rice.The accumulation of Cd in roots was significantly positively correlated with oxidizable Cd and residual Cd,and negatively correlated with exchangeable and reducible Cd.The correlation between the accumulation of Cd in polished rice and different fractions of Cd in soil is just the opposite.The accumulation of Cd in polished rice was significantly negatively correlated with both oxidizable Cd and residual Cd,and was significantly positively correlated with reducible and exchangeable Cd.In summary,silicon applied as basal fertilizer and at jointing stages with the ratio of5:5(50%:50%)is more conducive to the reduction of exchangeable Cd and the increase of reducible Cd in soil.It can effectively increase the dry weight of above-ground at mature stage,more Cd is retained in the soil,which reduces the migration rate of Cd during grouting,and inhibits the transfer of Cd from the soil to the rice.Also increase the content and accumulation of Cd in root to a greatest extent,so that Cd accumulated in rice is largely deposited in the roots,which reduces the Cd content of the bleeding sap,that is,reduces the migration to the various organs in shoots,thus the Cd content and accumulation of polishedrice are significantly reduced,which can be recommended for practical use. |
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