Effect Of Coconut-fiber Derived Biochar On Geochemical Fractionation And Bioavailability Of Pb And Its Accumulation In Rice Plants In Pb-contaminated Paddy Soil

Due to rapid and sustained urbanization and industrialization over past decades in China,the unreasonable disposal of wastes leads to increasingly serious heavy metal pollution in farmland soils.The contamination of farmlands by heavy metals is endangering ecosystems and food safety.However,since limited cultivation area of rice available,large areas of Pb-contaminated paddy soils are inadvertently used to cultivate rice（Oryza sativa L.）.Pb-contaminated paddy soils will inhibit plant growth,reduce yield and quality of rice,and even threaten the health of human being via Pb intake through the food chain.Thus,there is an urgent need to remediate Pb-contaminated paddy soils,and reduce Pb availability in soils,in order to reduce Pb concentrations in the edible parts of crops and ensure safe crop production practices.In the current study,granite-based and basalt-based paddy soils were collected from Hainan Province,Pot experiments were conducted in contaminated soils under different Pb-levels(0,50,250,500,2500 and 5000 mg·kg^-1)amended with coconut fiber-derived biochars（CFB）（3%,w/w）.Tessier sequential extraction method,differential centrifugation,geochemical analysis and synchrotron-basedμ-XRF,were combined aiming to evaluate the influence of CFB on Pb contents in different fractions in soils,and bioavailability and accumulation of Pb in rice plants.The main results obtained were as follows:（1）The contents of all Pb fractions and EDTA extractable Pb in both of the granite-based（Soil _G）and basalt-based（Soil _B）paddy soils were increased significantly with the increasing of Pb contamination.The application of coconut-fiber derived biochars（CFB）improved basic properties of the original soil,and significantly increased soil p H and soil organic matter（p<0.05）.The effect is more obvious that CFB amendments decrease the contents of EDTA extractable Pb in the soils under a higher Pb-level(>50 mg·kg^-1).The CFB amendments decreased the Pb concentrations in exchangeable,carbonate bound and Fe/Mn oxides bound Pb fractions,but increased the concentrations of organic matter bound and residual Pb fractions in studied soils.In addition,immobilization effects of CFB on heavy metals in basalt-based soils are more obvious than that of granite-based soils.In granite and basalt soils,the geochemical fractions of Pb（including EDTA extractable Pb and contents of all Pb fractions）were significantly and negatively related to p H（p<0.01）.The redundancy analysis results show that,the each component of geochemical fractions of Pb in studied soil was greatly affected by p H as Pb-level lower than 500 mg·kg^-1,and slightly influenced by organic matter and CEC;while the effect of basic properties on the each component of geochemical fractions of Pb was decreased under a high Pb level(2500,5000mg·kg^-1),.（2）As for rice growth at entire growth stage,yield and agronomic characters of rice.Pb toxicity did not significantly affect rice plant height at tillering stage.The rice plant height obviously increased at jointing stage,and the rice growth in fertile Soil _B showed lag growth phase at Pb level of 500 mg·kg^-1.With the growth of rice,the apparent differences between different Pb-contain rice gradually increased.The rice agronomic traits and yield both presented“Hormetic effect”（low Pb concentration promotion,and high-concentration inhibition）.At Pb level of 5000 mg·kg^-1.At Pb level of 5000 mg·kg^-1,the rice agronomic characters decreased significantly,and the rice yield decreased by 12.4%and 17.8%in Soil _Gand Soil _B,respectively.Peroxidase（POD）,superoxide dismutase（SOD）and catalase（CAT）both presented“Hormesis effects”as the Pb concentration increasing in Soil_G,and POD and SOD activities showed same regularity,while POD activity in Soil _B was quite the contrary.Compare to that soil without CFB,CFB-amendments reduced the Pb stress in both of Soil _Gand Soil _B at high Pb level(2500 and 5000 mg·kg^-1)on rice plant height at jointing stages,CFB alleviated rice agronomic traits（like total grains,filled grains and 1000-grain weight）.The rice yield of Soil _G+CFB and Soil _B+CFB were rose at Pb levels of 50 and 250 mg·kg^-1,while there was no significantly improvement at other Pb levels.The activities of SOD and POD were improved with CFB amendment,but CAT activities were obviously reduced.（3）In Soil _G and Soil _B,the Pb concentration of rice in shoots and roots at three representative growth stages both were follows as:maturity stage>tillering stage>seeding stage.And the Pb concentration in shoot,root and each subcellular component of rice all increased with Pb concentration,but the trend of increase gradually slowed down.The BCF（biological concentration factor of Pb in rice）and TF（translocation factor of Pb in rice）were presented as:TFr>>TFs and BCFr>>BCFs>BCFbr.At three representative growth stages,translocation factor of Pb and Pb concentration of shoot and root in Soil _G both higger than Soil _B,and each subcellular component of rice in Soil _G was also higher than Soil _B.Compare to that soil without CFB,CFB amendments could reduce Pb concentration in shoot,root and subcellular of rice.CFB could significantly increase the Pb proportion in the cell wall to the total Pb concentration,and significantly decrease the proportion in the soluble fractions to the total Pb concentration.The BCFr,BCFs,BCFbr and TFs of rice plant amendment with CFB were significantly lower than the soil without CFB under different Pb levels,while the TFr amended with CFB was higher than the soil without CFB.μ-XRF Analyses showed that Pb and mineral elements（k and Ca）were mainly in seed coat and aleurone layer in grain rice at Pb level of 5000 mg·kg^-1.CFB amendments significantly reduced the concentration in grain rice,while the content of k and Ca increased.The Pb concentrations in brown rice with the increasing Pb toxicity(≥250 mg·kg^-1)in granite and basalt soils.The regression model showed that Pb concentrations of brown rice in both Soil _G and Soil _B reached the National standard of food safety(0.2 mg·kg^-1),while the concentration of soil Pb（250 and 500 mg·kg-1）was 250 and 447 mg·kg^-1,respectively.At Pb level of 5000 mg·kg^-1,the Pb concentrations in brown rice from Soil _G and Soil _B were 14and 10 times than the National standard of food safety,respectively.CFB could effectively decrease the concentration of Pb in brown rice in both graint and basalt soil,in order to reduce the risk of exceeding food safety standards.The concentration of Pb in brown rice from Soil _B was 0.22 mg·kg^-1,exceeding the National standard of food safety at Pb concentration was 500 mg·kg^-1.Meanwhile,the Pb concentration of brown rice in Soil _B+CFBwas decreased by 38.4%compare with Soil _B,which was in line with the national food safety standard.Coconut-fiber derived biochars（CFB）could:（1）improve basic properties of Pb contaminated soil;increase abundantly soil organic matter and p H in both of granite and basalt soil;decrease the Pb concentrations in exchangeable,carbonate bound and Fe/Mn oxides bound Pb fractions and the EDTA extractable Pb pool,but increase the concentrations of organic matter bound and residual Pb fractions in original soils.（2）alleviate Pb stress on agronomic traits of rice and ease high concentratin of Pb stress(2500、5000 mg·kg^-1)on rice plant height at tillering stage;enhance SOD、POD activities from shoots in rice under low concentraton of Pb stress.（3）increase Pb concentration in the cell wall to the total Pb concentration from roots in rice,and decrease the proportion of soluble fractions;decrease the Pb accumulation in grain rice,roots and shoots in rice,meanwhile increase the risk of Pb uptake from root to shoot（TFr）.Fixate Pb in basalt-based soils effective than that of granite-based soils,including:geochemical fractionation of Pb in soil,bioavailable Pb,growth,yield,quality and accumulaton of rice.