Effects Of Sulfur And Selenium On Cd And As Speciation And Bioavailability In Rhizosphere And Non Rhizosphere Of Rice

Pot experiments using exogenous Cd-polluted and As-polluted paddy soils were carried out to investigate the influence of different forms of sulfur and selenium fertilizers(gypsum and sodium selenite)on Cd and As chemical speciation in the rhizosphere and bulk soil and uptake by rice.The results showed:(1)The whole rice growth period that the redox potential(Eh)was about 93-406 m V and 100-407 m V,respectively,the soil solution p H was 6.2-8.6 and 6.1-9.1,respectively,pe+p H was 8.40-14.80 and 8.58-14.71,respectively,in the rhizosphere and bulk soil.Solution p H in the rhizosphere was generally lower than that in the bulk.(2)The highest amounts of Fe and Mn plaque at boot stage,and the lowest one at mature stage.The mass fractions of Fe and Mn plaque on rice root were 8057.51 mg·kg-1-29327.89 mg·kg-1 and 91.85 mg·kg-1-655.59 mg·kg-1,respectively.The high sulfur treatment led to the formation of more Fe plaque than the low sulfur treatment in the boot stage and mature stage.Application of S induced the formation of iron plaque in the root surface.The high selenium treatment led to the formation of more Fe plaque than the low selenium treatment in the tiller stage and boot stage.(3)The mass fractions of Cd adsorbed by rice roots surface plaque decreased with the increase of growing stages.The mass fractions of Cd adsorbed on the roots surface plaque reached the maximum value at 14.62 mg·kg-1-35.16 mg·kg-1 in the tiller stage,6.47 mg·kg-1-17.87 mg·kg-1 in boot stage,2.18 mg·kg-1-10.82 mg·kg-1 in flower stage,and 2.54 mg·kg-1-13.10 mg·kg-1 in the mature stage.The high sulfur treatment led to the formation of more Cd plaque than the low sulfur treatment.(4)Rice roots surface plaque colud effectively prevent the migration of Cd by rice roots.The mass fractions of Cd in the different parts of rice followed the order of roots>stem leaf>grain.The highest accumulation of rice roots were 41.16mg·kg-1,stem leaves were 5.785 mg·kg-1,grains were 0.9609 mg·kg-1.The sulfur and selenium fertilizers applied significantly reduced the mass fractions of Cd uptake in different parts of rice.(5)In rhizosphere and non rhizosphere soil,where Cd existed mainly in ion exchange state and carbonate combined state,followed by Fe-Mn oxide bound state and weakly organic state and strongly organic state,while residual state accounted for the least.The mass fractions of rhizosphere soil of exchangeable Cd is higher than that in non rhizosphere soil.(6)The amount of As plaque of rice roots and stem leaf increased with the increase of growing stages.The As uptake by rice was mainly concentrated in the roots,the mass fractions of As adsorbed on the roots were 116.76-445.94 mg·kg-1,stem leaves were 5.11-29.86 mg·kg-1,grains were 1.005-2.922 mg·kg-1.The sulfur fertilizers applied significantly reduced the mass fractions of As uptake in different parts of rice.The sulfur and selenium fertilizers applied significantly reduced the mass fractions of As grains of rice.(7)In the rhizosphere and non rhizosphere soil,where As existed mainly in specific adsorption state and amorphous Fe-Mn oxide binding state,followed by crystalline Fe-Mn oxides and the residual state,the nonspecific adsorption state is the least.Application of S or Se induced on As chemical speciation in the rhizosphere and bulk soil and uptake by rice.