Effectsof Aged Biochar On The Bioavailability Of Heavy Metals And Nitrogen In Soil

Biochar is an important soil improver,and often used to improve soil properties,enhance soil functions,or remediate inorganic or organic contaminated soils.It can be aged biochar after physical freeze-thaw,chemical oxidation or natural aging treatment.Compared with original biochar,aged biochar changed the oxygen-containing functional groups,the contents of various elements,and the pore structure of biochar,thereby affecting the contents of pollutants and nutrients(nitrogen)in the soil.However,long-term application of biochar in soil may change its impact on soil,plants and nitrogen.Therefore,it is extremely necessary to study the effect of aged biochar on the bioavailability of heavy metals and nitrogen changes in contaminated soils.This study used corn straw-derived original biochar(BC)and oxidized biochar(OBC).Aged biochars was mixed with the soil at 0,2% and 5%,and balanced for 2 years.Through the pot experiment of wheat,the residual and bioavailable of three kinds of heavy metals Pb,Cr,and Cd in rhizosphere soil and non-rhizosphere soil were compared between the original biochar and aged biochar,as well as the accumulation of Wheat shoots and roots.The effects of biochar on available nitrogen(NH4+-N and NO3--N)in heavy metal contaminated soil were discussed in order to provide a theoretical basis for the long-term application of biochar in contaminated soil remediation.The main findings are as follows:The original biochar and aged biochar have strong adsorption and retention capacity for heavy metals Pb,Cr and Cd.Compared with the control without biochar,both biochars can significantly increase the total residual amount of the three metals in the soil(rhizosphere soil,non-rhizosphere soil),but the adsorption of Cd in soil by the original biochar was weak,and only the high-dose aged biochar(5OBC)treatmentsignificantly increased the residual of Cd in the soil.Under high-dose aged biochar(5OBC)treatment,the residual of Pb,Cr,and Cd in rhizosphere soil increased by 11.94%,11.18%,and 11.22%,which increased the residual of the three metals in non-rhizosphere soil by 6.03%,6.42%,11.81%,so the three heavy metals Pb,Cr,and Cd are more easily fixed by 5OBC.The residual of Pb,Cr,and Cd in the soil increased with the increase of the amount of biochar application..Both kinds of biochar can significantly reduce the bioavailable of Pb and Cr in soil(rhizosphere soil,non-rhizosphere soil).The original biochar has no significant reduction in the bioavailable of Cd in rhizosphere soil,andthe bioavailable of Cd in the non-rhizosphere soil was significantly reduced during applied the high dose,but both doses of aged biochar significantly reduced the bioavailable of Cd in the soil.Under high-dose aged biochar(5OBC)treatment,the bioavailable of Pb,Cr,and Cd in rhizosphere soil were reduced by 13.89%,22.43%,and 15.75%,respectively,which reduced in non-rhizosphere soil by12.52%,18.89%,12.54%.In addition to the aged biochar(2OBC,5OBC)treatment,the bioavailable of Cr in the soil(rhizosphere soil,non-rhizosphere soil)decreased with the increase of the application amount,and the bioavailable of Pb,Cr and Cd in soil did not decrease with the increase of the application amount under the other treatments.Compared with the control,the biochar treatments can significantly increase the biomass of wheat seedling stage,and the biomass of wheat seedling stage under agedbiochar treatmentsare higher.Both biochars significantly reduced the accumulation of Pb,Cr,and Cd in wheat.At the same dose,aged biochar could reduce the accumulation of Pb,Cr,and Cd in wheat.Among them,5OBC treatment had the most obvious effect.The accumulation of Pb,Cr,and Cd in wheat shoots under high-dose aging biochar(5OBC)treatments decreased by 56.59%,47.91%,and 80.60%,respectively,which reduced the accumulation of Pb,Cr,and Cd in wheat roots by 61.83%,43.43%,and77.03%,respectively.Therefore,the 5OBC treatment can significantlyreduce soil bioavailability.The original carbon can significantly reduce the transfer coefficients of Pb and Cd,and the aged biochar can significantly reduce the transfer coefficients of Cr and Cd,and inhibit the transfer of heavy metals from roots to shoots.After the application of nitrogen fertilizer,the content of NH4+-N in the soil gradually decreased and the content of NO3--N gradually increased with the time.However,the application of biochar will significantly reduce the content of NH4+-Nand NO3--N in the soil,and at 5OBC treatment,the content of NH4+-N and NO3--Nin the soil can be significantly reduced.In the pot experiment with original biochar and biochar that has been aged for two years in soil,the two kinds of biochar can significantly increase the residue of heavy metals in rhizosphere soil and non-rhizosphere soil after being applied to the soil contaminated by heavy metals,reduce the bioavailable of heavy metals in rhizosphere soil and non-rhizosphere soil,reduce the accumulation of heavy metals in wheat shoots and roots,and reduce the content of NH4+-N and NO3--N in the soil.Among them,the5 OBC treatmenth is the best effect on reducing the bioavailability of heavy metals by adsorbing and fixing heavy metals Pb,Cr,and Cd in soil,and has the best effect on reducing the contents of NH4+-N and NO3--N.