The Remidation Mechanisms Of Cd-contaminated Soils By Phosphrous-modified Biochar And Its Effect On Microorganisms

The 2014 Bulletin of National Soil Pollution Survey has pointed that the occurrence rate of soil cadmium（Cd）point pollution was 7.0%.Soil Cd pollution is becoming increasingly severe,posing threats to soil quality,plant growth,food security and human health.Therefore,there is an urgent need in taking effective measures to remediate the Cd-contaminated soils.Biochar（BC）is widely applied due its variable sources,low cost and outstanding adsorption property,which is used as a favorable material for immobilizing heavy metals in soil.Pristine BC has been found to be low-efficient,thus BC modification for better adsorption performances of heavy metal removal has become a hot research topic.Phosphate loading on surface of BC to prepare phosphorus（P）-modified BC is an effective method to improve the ability of BC of immobilizing heavy metals.Most of the current studies have focused on the adsorption amounts of heavy metals by P-modified BCs,while the mechanisms of heavy metal adsorption by P-modified BC and its effect on soil microbial properties and plant growth need to be studied in depth.In this study,P-modified BC was prepared from the co-pyrolysis of apple tree branches and potassium phosphates.Modern instrumental analysis techniques,ecoenzymatic stoichiometry,high-throughput sequencing,soil in situ zymography,and root morphology analysis were used to reveal the mechanisms of P-modified BC in remediating the Cd-contaminated soil and its effects on soil microbial properties and crop growth through batch adsorption experiment,root box experiment of wheat,and pot experiment of maize.The main results are as follows.1.P-modified BC of BC-1,BC-2,and BC-3 were obtained through the co-pyrolysis（biomass:P=1:0.05）of apple tree branches with KH₂PO₄,K₂HPO₄·3H₂O,and K₃PO₄·3H₂O,respectively.The modification by potassium phosphates on apple branches increased the BC yield,improved carbon（C）retention and changed the chemical form of P.Orthophosphate（24.8%）,pyrophosphate（73.7%）,and metaphosphate（1.49%）were present in BC-1,while orthophosphate（47.4%and 86.2%）and metaphosphate（52.6%and 13.8%）were present in BC-2 and BC-3,respectively.The maximum adsorption capacities of BC-1,BC-2,and BC-3 for Cd were 88.5,95.8,and 116 mg g^-1,respectively,which were 8.49,9.18,and 11.2 times higher than the pristine BC(10.4 mg g^-1).BC-1 removed Cd mainly by cation exchange（mainly potassium ion exchange）,while the main mechanism of Cd removal by BC-2 and BC-3 was the co-precipitation of Cd and phosphate.BC-3 was the most efficient P-modified BC due to its highest loading of P.2.The mechanism of BC-3 in soil Cd immobilization and its effects on P-related functional microbes were investigated based on wheat root box experiment.The results showed that the addition of pristine BC and BC-3 to the Cd-contaminated soil reduced the available soil Cd content by 26.91%and 69.77%,respectively,indicating that BC-3 was also efficient in immobilizing Cd-contaminated soil.The Cd-removal effect of BC-3 was much higher than that of pristine BC,mainly due to the fact that BC-3 increased soil p H,total P,and available P content while it also promoted the co-precipitation of Cd with phosphate.Soil bacterial community analysis demonstrated that BC-3 addition increased the abundance of the gene（phoD）encoding alkaline phosphatase（ALP）,and improved the complexity and stability of soil phoD-harboring bacterial communities.It was also revealed that soil ALP activity was not exclusively regulated by phoD gene abundance;soil phoD gene-harboring communities of Priestia and Massilia genera dominated the ALP activity in the rhizosphere of wheat and bulk soil,respectively.Soil available Cd stimulated the abundance of Priestia and Massilia genera and ALP activity.3.The remediation effect of BC-3 on multiple heavy metal contaminated soil and the responses of soil microbial properties were studied by pot experiment during the whole growth period of maize.The addition of BC-3 reduced the soil bio-available Cd,lead,copper,and zinc by 34.68%,31.91%,12.42%,and 15.29%,respectively,which could be attributed to co-precipitation,ion exchange,and complexation as well as the soil p H increase.At the same time,heavy metal concentrations in all organs of maize were significantly reduced,with the Cd,lead,copper,and zinc contents of the seeds reduced by 36.52%,61.82%,36.93%,and 29.35%,respectively.The addition of BC-3 significantly increased soil microbial biomass and C-and N-related acquisition enzyme activities.The ecoenzymatic stoichiometry and random forest model analysis showed that BC-3 significantly alleviated the P limitation during soil microbial metabolism due to the increased soil P content and its bioavailability.At the same time,the increase in p H and the change in nutrient status caused significant decreases in the abundance and diversity of soil bacteria.The co-occurrence network module analysis demonstrated that modules 1 and 3 had important and reverse roles in soil N and P cycling processes.Most of the bacteria in both modules belonged to Actinobacteria and Proteobacteria,and the fungi belonged to Ascomycota,which were the key microbial taxa in regulating soil nutrient cycling.Their activities were significantly affected by soil total P,N/P,and available Cd and lead.4.To compare the effects of P-modified BC prepared by different methods on Cd-contaminated soil,a new P-modified BC（BCP）was further prepared by immersing the pristine BC into the phosphate solution;and its effect on soil Cd remediation and wheat P acquisition strategy were investigated.The addition of BCP reduced the soil available Cd by79.4%,significantly reduced the leaf Cd concentration of wheat seedlings,and increased wheat biomass,mainly through accelerating the co-precipitation of Cd with phosphate due to the rapid release of phosphate of BCP prepared by the immersion treatment.Compared with the BCP addition,the remediation effect of pristine BC was not only much lower,but also it has caused the soil nutrient imbalance by increasing C/P and N/P,thus a relative shortage of soil P supply.Soil zymography and root morphology analyses suggested that,on one hand,longer and finer roots of wheat were formed to search for and obtain more available P.On the other hand,more phosphatase was released in order to increase the bioavailability of P at the root-soil interface under the P limitation situation.Moreover,soil available Cd also affected the P acquisition strategy of wheat by stimulating the release of more phosphatases from the roots and microorganisms.Thus,compared to pristine BC,BCP had advantages in serving multiple functions of remediating Cd-contaminated soil and regulating soil nutrient cycling.In summary,the study clarified the mechanism of Cd removal by P-modified apple tree branch BC,elucidated the effects of P-modified BC on the remediation of Cd-contaminated soil and plant growth,and revealed the responses of soil microbial properties under heavy metal stress with P-modified BC addition.The results provide a scientific basis for the application of P-modified BC in natural heavy metal contaminated environments.