Research On The Effects And Mechanisms Of Biochars On Micronutrients(Fe,Mn,Cu,Zn) Bioavailability

With the development of global industrialization and agricultural modernization,soil pollution and degradation are becoming more and more serious in China,and food safety problems are frequent.As a soil conditioner,biochar has great potential for remediation of soil pollution,,which has attracted wide attention globaly in recent years.Biochar is alkaline,porous,with large specific surface area and high cation exchange capacity.It can improve soil physicochemical properties,effectively absorb soil heavy metals,and affect the migration,transformation and bioavailability of heavy metals.When applied to soil,biochar may also change the speciation of soil trace elements to some extent,affecting their bioavailabilities and therefore soil productivity.In this study,biochar was prepared from common agricultural wastes.Focused on the adsorption characteristics of biochar for micronutrients in solution,this work studied the relationship between the structure of biochar and its ability to immobilize micronutrients by means of FTIR,XRD,SEM,XPS characterization.The micronutrients adsorption characteristics and mechanisms of biochar,soil physicochemical properties and soil micronutrients bioavailabilities after biochar application were discussed.The main research contents and results are as follows:(1)Using biochars developed from rice straws and reed straws as adsorbents,the characteristics and mechanisms of two biochars in adsorbing trace elements in solution were studied,and the contribution of each mechanism was evaluated.The results showed that the adsorption capacity of rice straw biochar to Fe,Mn,Cu,Zn in aqueous solution was greater than that of reed straw biochar.The adsorption capacity of rice straw biochar was in the order of Fe2+>Cu2+>Zn2+>Mn2+.The adsorption mechanisms include non-specific adsorption(mainly electrostatic adsorption and physical adsorption)and functional group complexation,mineral precipitation,ion exchange and π electron coordination.The applicability of adsoprtion mechanisms depended on both biochar type and element type,with the contribution of mineral precipitation to total adsorption being the largest,which was 35%～56.8%.(2)Rice straw biochar was selected to study the effects of biochar on the chemical forms and bioavailabilities of different soil trace elements.The results showed that biochar increased the pH and organic matter content in both neutral and acid soils,but reduced the content of DTPA-extractable Fe.The application of biochar also increased the content of oxidizable Fe,extractable Mn and weak acid Mn,but had no effect on the contents of DTPA extractable Cu and Zn.Biochar application increased the richness and diversity of soil microbial community The precipitation and complexation of Fe in soil were increased by the rises in soil pH and SOM,and the foramtion of organic-inorganic complexs with Fe was increased due to aging of biochars,both of which contributed to the decline in soil Fe availability.(3)Rice straw and reed straw biochar were adopted in a pot experiment to study the growth of Brassica chinensis L.for two continous seasons and the change of micronutrients contents in plant with biochar amendments.The results showed that biochar application decreased the contents of soil DTPA-Fe and DGT-Fe in both two seasons.Rice straw biochar increased the content of soil available-Mn,while reed straw biochar reduced the content of soil available Mn.Biochar application decreased the contents of available Cu and Zn.The contents of Fe,Mn,Cu and Zn in Brassica chinensis L.were decreased by biochar application except that Mn content in Brassica chinensis L.was increased by rice straw biochar.In addition,biochar application ratio determined the extent of the changes in the contents of micronutrients in soil and vegetables,and the effect of rice straw biochar was greater by comparison to reed straw biochar.The contents of micronutrients in soil measured by DTPA and DGT were positively and linearly correlated to the contents in Brassica chinensis L.,with the DGT method having a better correlation than DTPA.