| Because of the increasingly serious problem of cadmium pollution in paddy soil,taking effective control measures to ensure the safe production of rice was the focus of attention.Studies have shown that biochar prepared by thermal cracking biomass under oxygen-limited or anaerobic conditions can significantly reduce the bioavailability of Cd in soil.However,due to differences in soil properties,biochar types and Cd fractions,the effect of biochar on Cd immobilization in different types of soils was different,and the mechanism was still unclear.Iron plaque on the rice root surface is an important barrier to inhibiting Cd absorption in rice.Studies have shown that biochar can affect the iron plaque formation and its Cd adsorption effect,but few studies have paid attention to its mechanism.To to explore the mechanism of biochar affecting iron plaque and cadmium immobilization on the rice root surface,this study investigated the effect of S-rich biochar on the iron reduction process in different soils under flooded conditions through pot experiments.In addition,this study analyzed the relationship between iron plaque formation on the rice root surface and the Cd adsorption by iron plaque and the biochar-mediated iron reduction.Moreover,this study clarified S-rich biochar on soil Cd immobilization and Cd uptake by rice,and the mechanism of biochar-induced iron plaque formation on rice roots.This study aims to provide scientific reference for the efficient control on Cd-contaminated paddy soil by biochar.The main results in this study were listed as follows:1.Cd-contaminated red paddy soils(FG,DBS,ZZ,LY)and hydroagric stagnic anthrosol soil(YX)were selected for rice pot experiments.The results showed that soil texture significantly affected the fraction of Cd in soil.Acid-soluble Cd(F1-Cd)in FG and DBS soils with high sand content accounted for 41.6%-55.4%.Compared with different pollution sources,it was found that the proportion of F1-Cd in the mining pollution source soil was significantly higher than that in other pollution source soils.Soil Cd fractions under flooding and drying conditions was different.From flooding to dry conditions,the proportion of F1-Cd in soil increased by 42.1%-401.7%.The results showed that the amount of iron plaque was related to the soil Fe content.The DBS soil with the highest Fe content had a Fe content of 37.0 g/kg in the iron plaque at the tillering stage.Iron plaque formation on the root surface significantly inhibited the Cd uptake by root.The Fe and Cd contents in the iron plaque were significantly positively correlated.2.Rice straw biochar(RSB)and rape straw biochar(S-rich biochar,RASB)were used as soil amendments,and FG(low Fe content)and DBS(high Fe content)soils were selected at 0.5%(RSB0.5,RASB0.5)and 1%(RSB?,RASB?)application rate for rice pot experiments.The results showed that the mechanisms of Cd immobilization by biochar in soils with different properties were different,and RASB with higher sulfur content had a more significant effect on Cd immobilization and iron plaque formation.Biochar decreased soil acid-soluble Cd by 15.9%-25.3%with the increase of soil pH at the maturing stage in FG soil,and 30.1%-59.3%through promoting soil more reductive condition at the heading stage in DBS soil with higher content of Fe and S.In addition,biochar impeded Cd transport from soil to rice roots by increasing the formation of iron plaque at the flooding stage.RASB1 significantly increased DCB-Cd concentration by 99.7%at the heading stage in FG soil and by 2.3 times at the tillering stage in DBS soil.This suggested that biochar might promote Cd immobilization by affecting the cycling of soil iron and sulfur.3.The rice pot experiment was carried out by applying l%(B1)and 2%(B2)rape straw biochar(S-rich)and 80mg/kg(S1)and 160mg/kg(S2)Na2SO4(the amount of application was equal to the content of sulfate in rape straw biochar)to a Cd-contaminated paddy soil.The results showed that rape straw biochar better inhibited the accumulation of Cd in rice plants and significantly reduced the migration of Cd in soil compared with the same amount of sulfur application.Rice grian Cd concentration(GCd)was decreased by 49.6%and 69.8%by B1 and B2,and 57.9%by S2,respectively.The concentration of Cd in the rice root(RCd)was significantly decreased by 63.9%,69.22%,and 75.5%under B2 treatment at the tillering,heading,and maturing stage.Compared with the sulfur application,rape straw biochar could better inhibit the Cd uptake by rice roots,especially at the maturing stage.Compared with the sulfur application,the RCd of rape straw biochar treatment with two application rates decreased by 43.7%and 72.3%,respectively.Soil CaCl2-Cd concentrations decreased by 18.7%-29.6%,22.9%-40.2%,and 19.3%-44.6%under B2 treatment at the tillering,heading,and maturing stage,respectively,and the CaCl2-Cd concentration in 0-1cm soil from the root decreased by 44.6%.4.The results showed that the rape straw biochar significantly increased the relative abundance of functional bacteria related to the transformation of soil Fe oxide fractions and affected the transformation of Fe oxide fractions during the flooding period.The transformation of soil Fe oxide fractions had a significant effect on the migration of Cd.The content of CaCl2-Cd was negatively correlated with the content of Feo and positively correlated with the content of Fec.Compared with CK,the content of Feo in soil under B2 increased by 25.1%-35.5%and 21.8%-32.5%,and the content of Fec decreased by 15.2%25.0%and 12.4%-31.1%at the tillering stage and heading stage.Microbial sequencing results showed that the relative abundance of Bacillus,Magnetospirillum,and Thiobacillus in Fe(Ⅲ)reduction significantly increased by 19.8%,22.9%,and 512.9%under B2 treatment.The relative abundance of Desulfobacterium,Desulfovibrio,Desulfovirga,and Opitutus in sulfate-reducing bacteria increased by 35.6%,157.9%,177.5%,573.8%,and 75.3%under B2 treatment,respectively.Meanwhile,the relative contents of these bacteria were significantly positively correlated with Feo content.5.The results showed that rape straw biochar promoted the iron plaque by promoting root development and radical oxygen loss,and mainly increased the content of amorphous iron oxide in iron plaque.Compared with CK,B2 treatment increased the root length,root surface area,average diameter,root volume,and root tip number by 74.7%,156.6%,47.0%,274.2%,and 83.3%,respectively.B1 and B2 significantly increased root radical oxygen loss by 55.2%and 74.2%,respectively.The concentration of amorphous iron oxide and Cd increased by 101。4%and 43.2%under B2 treatment.In conclusion,S-rich biochar can promote the transformation of soil iron form and the rate of oxygen release in rice roots by influencing the reduction conditions of soil and the relative abundance of functional microorganisms related to iron and sulfur reduction under flooding conditions,thus inducing the the iron plaque formation and reducing the migration of soil Cd to rice roots.The interaction of biochar with iron and sulfur should be concerned. |
PDF Full Text Request