Effects Of Exogenous Selenite On The Se Speciation In Soil-rice System

Objective: Selenium(Se)is one of the essential trace elements for humans,and both its deficiency and excess are harmful to human health.Diet is the major source of human Se intake,and the Se levels in food depend on the Se concentrations and speciations in soil.In this paper,we focused on the fractions and bioavailability of Se in the paddy soil after exogenous Selenite addition,which provided scientific basis for enhancing rice Se contents and then gradually improving human Se intake.Method: The study with the material of high-and low-selenium rice cultivars and by the way of rhizo-box experiment with 4 soil Se(Na2SeO3)treatments(0,0.5,1.0,5 mg·kg-1).Plant height during the growth of rice,leaf chlorophyll relative content(SPAD)in the early filling stage,grain yield and shoot fresh weight after harvest were measured;the total Se content in various parts of rice plants(root,shoot,leaf,brown rice,husk)and soil were detected with hydride generation-atomic fluorescence spectroscopy(HG-AFS);the organic Se in brown rice were determined by continuous dialysis method(8 ~ 12 kDa cutoff molecular weight);the sequential chemical extraction technique were used to extract and determine each Se fraction(water soluble,exchangeable,Fe-Mn oxides bound,organic bound and residual)in rhizosphere and non-rhizosphere soils.Results:(1)The plant height increased with the rice growing,compared to the control,there are remarkable promoting effect for the growth of rice in the middle and later periods(after 42 d)by Se treatment,meanwhile,plant height of high-Se rice cultivar were significantly higher than the low-Se rice cultivar after sowing 64 days(P < 0.05)in the control and same Se treatment.The shoot fresh weight and grain yield of two rice cultivars did not affect by Se application(excluding 5.0 mg·kg-1).The leaf chlorophyll relative content(SPAD)and shoot fresh weight of high-Se rice cultivar were significantly higher than the low-Se rice cultivar(P < 0.05),whereas contrary to it in grain yield.(2)The Se content in various parts of rice plants and the organic Se in brown rice increased with the increment of Se applied in soil and the total and organic Se contents in brown rice of high-Se rice cultivar were significantly higher than the low-Se rice cultivar for the same Se treatment(P < 0.05).The Se contents in husk and shoot of high-Se rice cultivar were higher than the low-Se rice cultivar for the same Se treatment while there was almost no significant difference on Se of leaf and root between two rice cultivars(P > 0.05).The application of Se increased the Se translocation factors in root-shoot,leaf-grain,and the Se translocation factors in root-shoot,leaf-grain of high-Se rice cultivar were significantly higher than the low-Se rice cultivar for the same Se treatment(P < 0.05),whereas contrary to it in shoot-leaf Se translocation factors.The main form of Se species in brown rice were organic Se,about 72% of total Se,and no significant differences existed for the ratio of organic and total Se in brown rice among different treatments or between two rice cultivars(P > 0.05).(3)In control,compared with soil before planting,the total Se concentration in soil(rhizosphere and non-rhizosphere)significantly decreased after rice harvest(P < 0.05).Therein,the residual Se content changed little(P > 0.05),exchangeable and Fe-Mn oxides bound Se content significantly decreased(P < 0.05),and the bigger decline occurred in Fe-Mn oxides bound Se content.However,the organic bound Se content increased.(4)The application of exogenous selenite into soil significantly increased the concentration of total Se and each Se fraction in soil,and then improved the Se content in rice.Whether application of Se or not,the main form of species in soil(rhizosphere and non-rhizosphere)were Se-bound to organic matter(more than 60%)and Fe-Mn oxides(about 20%).However,the percentage of exchangeable Se increased and the percentage of organic bound Se decreased with the increment of Se applied in the soil.The concentrations of total and fractions of Se(exchangeable,Fe-Mn oxides bound,organic bound and residual)in rhizosphere soil were significantly lower than bulk soil(P < 0.05).Moreover,under the same Se treatment,the ratios of organic bound Se and total Se in rhizosphere soil were higher than in non-rhizosphere soil,while the ratios of Fe-Mn oxides bound Se and total Se in rhizosphere soil were lower than in non-rhizosphere soil,the percentage of exchangeable and residual Se in total Se were accordance between rhizosphere and non-rhizosphere soils.The total Se and each Se fraction in rhizosphere soil of high-Se rice cultivar were significantly greater than those of low-Se rice cultivar at the higher Se levels(1.0 mg·kg-1 and 5.0 mg·kg-1),whereas contrary to it in non-rhizosphere soil.Conclusion: Optimal Se does can stimulate growth of rice plant.The application of Se can not only significantly increased the total and organic Se content in rice grain,but also more benefit for Se translocation form root to shoot and the accumulation in grains of high-Se rice cultivar.The addition of exogenous selenite,the plant uptake significantly influenced the transformation of exchangeable,Fe-Mn oxides bound,organic bound Se,therefore,the transformation in various Se fractions in soil of great important for assessment Se bioavailability in paddy soil.