Effects Of Biochars Assisted By Silicon And Iron On Cd Uptake And Translocation In Rice Plants In Cd-Tained Paddy Soil

Recently,cadmium（Cd）contamination in rice paddy soil in China has become increasing prominent.It has been reported that nearly 70%of rice in heavy metal polluted area in southern China has been contaminated by Cd.The heavy pollution situation in rice paddy soil has seriously threatened China’s rice quality and food safety.Biochar is a solid product synthesized through pyrolysis/carbonization in a condition with little or unavailable air.It has been reported that biochar can reduce Cd bioavailability and Cd uptake in rice through liming effect,sorption and deposition.However,the general application amount of biochar is between 20-60t/ha.The problems of large application amount and high cost have been the main drawbacks of applying biochar in contaminated agricultural soils.It has attracted large attention on how to gain higher remediation effectiveness with lower biochar application amount.In this study,amendments including biochars with different feedstocks,Si supplied with biochar and iron modified biochars were applied to study the effects of different biochar amendments on Cd immobilization in the soil-rice system through field study and pot experiment.This study aims to provide scientific theoretical basis and technical measures to improve biochar effects on preventing and controlling Cd mobility and translocation in the soil-rice system.Main results in this study were listed as follows:1.Wheat straw biochar（BW）,maize straw biochar（BM）and rice straw biochar（BR）were chosen as the amendments and applied once in a Cd-tained rice paddy at 10t/ha and 20t/ha.The field study was continuted for three rice seasons,including late rice season-early rice season-late rice season.The results show that biochar application reduced rice grain Cd concentration in a range of 45.0%-60.7%in the early rice season.In the first late rice season,Cd concentrations in rice grains were reduced by 28.4%,30.1%,30.4%and 47.9%by BM1,BW1,BR1 and BR2,respectively.In the second late rice season,only BR2 significantly decreased Cd uptake in rice grains by 32.1%.Biochar application could significantly reduce CaCl2 extractive Cd（CaCl2-Cd）concentrations in soil.BR2 reduce CaCl2-Cd concentrations in soil by 44.4%averagely over three rice seasons.Statistical analysis results show significantly negative correlation between CaCl2-Cd and soil pH.Compared to the control,the average increment in soil pH was 7.5%with BR2 over three rice seasons.Furthermore,results from FTIR suggest that co-deposition may form between Cd and insoluble SiO₂.Therefore,compared to wheat straw biochar and maize straw biochar,rice straw biochar can significantly reduce Cd mobility and bioavailability in soil.2.BR and sodium silicate（SS）were chosen as soil amendments,which were further applied in a Cd-tained soil in a field study.The field study was continuted for three rice seasons,including late rice season-early rice season-late rice season.The application rates of SS,BR1,BR2,BR1+SS and BR2+SS were 0.75t/ha,10t/ha,20t/ha,0.75t/ha+10t/ha and 0.75t/ha+20t/ha,respectively.Results show that,compared to the control,BR1+SS and BR2+SS reduced Cd concentrations in rice grains by 19.5%-44.5%and 30.2%-73.7%,respectively.Comparetively,treatments with BR1 and BR2 reduced Cd concentrations in rice grains by 8.9%-45.0%and 32.1%-50.2%,respectively.Compared to the control and BR,BR1+SS and BR2+SS reduced total Cd uptake amount in rice plants by 21.7%,21.4%and 40.9%,10.5%averagely,respectively.Compared to the control and BR,BR1+SS and BR2+SS decrease soil CaCl2-Cd concentrations by 53.5%,0.9%and 49.2%,7.1%averagely,respecitvely.Results from FTIR suggest that Si combination enhanced the aging changes of rice straw biochar by increase the strength of-OH（～3690 cm-1）,C=C/C=O（1613 cm-1）and Si-O-Si（～-800 and～700 cm-1）stretches.These changes indicate that additional input Si may enhance Cd depostion in biochar aging process.3.Rice straw,rice husk and rape straw were chosen as feedstocks,which were further modified by FeSO4.In the Cd-tained soil,rape straw biochar（BRE）,rice straw biochar（BRS）,rice husk biochar（BRH）,Fe modified rape straw biochar（BRE-Fe）,Fe modified rice straw biochar（BRS-Fe）and Fe modified rice husk biochar（BRE-Fe）and rape straw biochar（BRE）were applied at 10t/ha in a pot experiment.Compared to the control,BRE,BRS and BRH reduced Cd concentrations in rice grains by 23.8%,22.3%and 46.1%,respectively.While the reductions in Cd concentrations in rice grains were 2.7%and 46.7%for BRS-Fe and BRE-Fe,respectively.Compared to the control,BRS notably decreased Cd content in rice stem by 15.3%,while BRH significantly reduced Cd concentrations in both rice roots and stems with 25.0%and 42.6%,respectively.Furthermore,the translocation factor of Cd from roots to stems was reduced by 23.9%with the treatment of BRH,compared with the control.With the treatment of BRH,Cd concentrations from tillering stage to maturing stage was reduced by 80.8%.Compared with the control,BRH significantly increased Fe and Si uptake in rice leaves,and the increments were 13.4%and 90.5%,respectively.In soil,BRE and BRS had no effect on CaCl2-Cd concentrations,while BRH increased CaCl2-Cd concentrations by 181.9%.However,BRS and BRH notably decreased the translocation factor of Cd from soil to root.Compared to the control,BRS reduced Cd translocation factors from rhizosphere and bulk soil to roots by 33.0%and 13.9%,respectively,while that was 73.4%and 39.6%for BRH,respectively.The pristine biochars significantly increased soil pH with 19.4%,11.2%and 6.2%by BRE,BRS and BRH,respectively.4.While BRH-Fe increased Cd concentrations in rice grains by 4.8%.Compared to BRE,BRE-Fe significantly increased Cd translocation factor from stems to leaves（TFS/L）by 74.3%.Meanwhile,BRE-Fe largely reduced Cd translocation factors from leaves to grains（TFL/G）and stems to grains（TFS/G）by 66.8%and 38.5%,respectively,compared with BRE.Furthermore,BRS-Fe and BRH-Fc had no significant effects on TFS/G but BRH-Fe significantly increased TFL/G.Compared with BRH,the increment in TFL/G is 158.1%by BRH-Fe.Results of mineral elements concentrations in rice tissues show that,compared with BRE,the increments in Zn concentrations in leaves and Fe concentrations in stems are 16.2%and 51.5%,respecitvely.Fe modified biochars significantly increased S uptake in rice leaves,compared to BRS-Fe and BRH-Fe,BRE-Fe increased S concentrations in rice leaves by 1.7 times and 1.8 times.BRE-Fe significantly increased Cd,Zn and S uptake in rice leaves.Meanwhile,BRE-Fe inhibited Cd remmobilization from leaves to grains and Cd translocation from stems to grains.5.Biochars application could impact iron plaque formation.Compared to the control,iron plaque formation amount reduces in a range of 36.2-59.6%with BRE,BRS and BRH.Compared to BRE,BRE-Fe increased Fe concentration in iron plaque by 290.5%.Meanwhile,BRE-Fe significantly reduced the translocation factor of Cd from soil to root.Compared to the control and BRE,the reductions in Cd translocation factor from rhizosphere soil to root were 80.6%and 83.0%,while that were 56.7%and 54.2%in bulk soil,respectively.X-ray photoelectron spectroscopy（XPS）analysis of the surface functional groups on aged BRH and BRE-Fe particles showed that new complexation of Fe/S/O may form on the the surface of BRH.While the aging process of BRE-Fe happened with the release of S2-and Fe2+.The aged BRE-Fe particles had higher degree of aging with higher contents of O=C-O/O=C-N functional groups.These results suggest that compexation similar with iron plaque may form on the surface of aged BRH,which may further reduce iron plaque formation on root surface.The highly developped aging process of BRE-Fe may enhance the formation of iron plaque and inhibitation effect of mineral elements on Cd translocation.