Microbial Population Characteristics And Influencing Factors In In Heavy Metal Contaminated Soil Remediated By Biochar And Compost

At present,the situation of soil pollution by heavy metals in China is not optimistic,especially the increasingly serious heavy metal pollution,which has posed a serious threat to our living environment and health problems.Biochar and compost are often added to soils as remediation agents to reduce heavy metal pollution and improve soil function and fertility.The certain interactions between biochar and compost can be used as a combined remediation strategy to remediate improved soils.However,there are few studies on the effectiveness of biochar and compost combined remediation in reducing soil heavy metals,and its impact on soil biological properties is often ignored.Therefore,the As,Cu,Cd,Zn contaminated soil collected in the field was used as the research object in this study,and biochar,compost,and their combined addition were used as remediation agents to（1）compare the basic physicochemical properties of soil and soil aggregates under different treatments and study the correlation between soil physicochemical properties;（2）the availability of heavy metals（As,Cu,Cd,Zn,etc.）was studied;（3）explore the effects of the different treatments on the abundance and community structure of microorganisms in the contaminated soil;（4）effects of biochar combined with granular organic fertilizers on heavy metal species and microbial characteristics in the rhizosphere and non-rhizosphere soils after Cabbage growing.In this study,the soils with different treatments were divided into 4 groups（5-2 mm,2-1mm,1-0.25 mm,<0.25 mm）by the dry sieving method.The physicochemical parameters of aggregates with different particle sizes by biochar and compost remediation were systematically determined.The results showed that the combined addition of biochar and compost was more beneficial to increase electrical conductivity（EC）and cation exchange capacity（CEC）,while biochar could significantly increase soil pH.The utilization of biochar and compost was beneficial to the formation of macro-aggregate,promote the stability of aggregates,and keep soil fertility.The larger the soil aggregates,the higher content of nutrients（C,P,K）,the stronger the CEC.Additionally,the content of available nitrogen increased with the decrease of soil aggregates size,and the content of iron was mainly concentrated in the aggregates with the particle size of 1-2 mm.Biochar and compost reduced the toxicity of heavy metals Cd and Cu.Therefore,biochar and compost have significant effects on the particle size distribution,nutrient content,and cation exchange capacity.The effects of biochar,compost,and their combined addition on the availability of heavy metals,physicochemical properties,and microbial communities in heavy metal contaminated soil were determined by using the method of redundant analysis and principal component analysis.The results showed that the soil physicochemical properties changed significantly after the addition of amendments.The combination of biochar and compost application increased soil pH,organic matter（OM）,available potassium（AK）,available phosphorus（AP）and total organic carbon（TOC）more effectively than biochar or compost addition.The results indicated that Cd and Zn could be inhibited by biochar and compost,but slightly activated As and Cu.Moreover,there is a significantly relationship between the availability of As,Cu,Cd,Zn and EC,AP,AK.Biochar showed slight inhibition of microbial PLFA,while compost and the mixture of biochar and compost significantly promoted microbial proliferation in soil.Moreover,the interaction between compost and biochar will affect the effect of the improver.EC,AP,and AK in soil were the main physical and chemical factors affecting the microbial biomass.AK and NO₃^--N significantly affected soil biological composition.Soil microorganisms were significantly affected by the availability of As,Cu,Cd and Zn.The results showed that As,Cu and Zn were positively correlated with soil microorganisms,while the negative correlation between Cd,Zn and soil microorganisms.All of these indicated that the soil physicochemical properties,metal availability,and microbial communities were significantly affected by the utilization of biochar and compost in heavy metal-contaminated soils.The changes inβ-glucosidase（BG）,β-N-acetylglucosaminidase（NAG）,leucine aminopeptidase（LAP）,and acid phosphatase（AP）and the relationships of stoichiometric were analyzed in the study.These changes and stoichiometry promoted the biotransformation of C,N,P and other elements in biochar and compost remediation of heavy metal contaminated soil.The results showed that the addition of biochar alone promoted the activity of AP and inhibited the activity of BG and LAP,promoting the activity of NAG enzyme on days 0 and 30,while inhibited on days 50 and 80.The enzymes activities could be increased with the applying of compost and biochar-compost.The C:N:P stoichiometry of the three additives,biochar,compost,and biochar-compost,to soil ecological enzymes was as follows:biochar-compost>compost>biochar,and with the increase of culture days,the ratio gradually increased and were close to 1:1:1.Activities of BG,NAG,LAP were negatively correlated with soil C:N（the values of R2 were 0.49,0.50,0.69,P<0.01）,while the activities of BG,NAG,LAP,AP were positively correlated with soil N:P（the values of r2 were 0.60,0.65,0.85,0.40,P<0.01）.The C:P stoichiometric ratio of soil ecological enzymes was positively correlated with the N:P stoichiometric ratio of soil ecological enzymes.The speciation and microbial characteristics of heavy metals in the rhizosphere and non-rhizosphere soils were studied by the combination of biochar and granular organic fertilizer.The results show that the addition of the remediation agent significantly reduces the ratio of the exchangeable and reducible states of Cu and Pb in the soil,while the ratio of the oxidized state and the residual state increased obviously,and the best remediation effect was achieved when 5%was added.The remediation agent can reduce the mobility and toxicity of Zn in soil by converting other forms of Zn into the residue state.Electrical conductivity（EC）showed a positive correlation with the biomass of Gram-positive bacteria,Gram-negative bacteria,bacteria and fungi.Total phosphorus（TP）was positively correlated with the biomass of Gram-negative bacteria,and there was a significant positive correlation with the biomass of Gram-positive bacteria and fungi.After planting Chinese cabbage,the heavy metal activity in both rhizosphere and non-rhizosphere soils were activated.Several remediation agents significantly increased rhizosphere and non-rhizosphere soil fungal biomass,but low doses of remediation agents inhibited bacterial biomass.