| Heavy metal pollution of soil is becoming increasingly serious and has become a hot and difficult issue to be solved at home and abroad.Biochar and compost have been proven to be effective remediation agents for heavy metals in soil.The interaction between biochar and compost induces possible changes in each other's properties,which may subsequently affect their remediation effectiveness in soils contaminated by heavy metal.Comparing the fixation effect of biochar and compost added separately and in combination on heavy metals in soil is conducive to selecting efficient remediation agents,so as to improve the remediation efficiency of soil heavy metal pollution.The effect of additives on soil is the result of the combination of physicochemical and biological properties.The ideal remediation should not only reduce the availability of potentially toxic heavy metals in soil,but also improve soil fertility and biological properties.Therefore,this study took As,Cu,Cd,Zn-contaminated soil as the research object,and used biochar prepared from straw and compost made from agricultural waste as remediation agents to explore the effects of separate and combined additions of biochar and compost on the physicochemical properties,availability of heavy metals,and soil enzyme activities of heavy metal contaminated soils.The reason and mechanism of their change were also discussed.This paper will provide theoretical support for the selection of suitable restorers for practical application.The main findings are as follows:1.Effects of additives on soil physicochemical properties(1)Biochar,compost,biochar-compost added to heavy metal contaminated soils significantly changed the physicochemical properties of the soil,including pH,electrical conductivity(EC),organic matter(OM),organic carbon(TOC),ammonium(NH4+-N),nitrate(NO3–-N),available phosphorus(AP),and available potassium(AK).Compared to the separate application of biochar or compost,their combined addition was more effective to improve soil pH,OM,TOC,AP and AK.(2)The changes of soil physicochemical properties induced by additives were related to the close relationship between physicochemical factors.Pearson correlation analysis results showed that soil pH had a negative/positive correlation with NH4+-N/NO3–-N;Soil NH4+-N had a significant negative correlation with NO3–-N;Soil EC had a positive correlation with NO3–-N and AK;AP had a significant positive correlation with EC,OM and AK.2.Effects of additives on the availability of heavy metals in soil(1)Adding biochar,compost,and biochar-compost to heavy metal polluted soil significantly reduced the availability of Cd and Zn,but increased the availability of As and Cu(except that Cu decreased slightly under biochar treatment).Compared with the separate addition of biochar and compost,their combined addition was more effective to passivate Cd in soil.And all the remediation agents completely passivate Zn in the soil.On the contrary,the activation of additives to As in soil was shown as:biochar-compost>compost>biochar,while the activation of Cu in soil was higher in compost than that of biochar-compost.(2)Soil physicochemical properties induced by additives affect the availability of heavy metals(As,Cu,Cd,Zn)in soil.The correlation analysis within 80 days showed that soil EC,AP,AK and As,Cu/Cd,Zn showed positive/negative correlation;Soil OM and NO3-N were positively correlated with As;Soil OM was negatively correlated with Cd.3.Effects of additives on soil enzyme activity(1)Biochar,compost,biochar-compost applied to heavy metal contaminated soil affected oxidoreductase and hydrolase related to the carbon/nitrogen/phosphorus/sulfur cycle.All selected soil enzymes activity including dehydrogenase,catalase,?-glucosidase,urease,invertase,protease,acid phosphatase,alkaline phosphatase and arylsulfatase was inhibited by biochar(with the exception of urease)and activated by compost,but exhibited highly variable responses to their combinations.Soil enzymes activity under biochar-compost treatment was either inhibited or activated,in which the activity of?-glucosidase,acid phosphatase,alkaline phosphatase was inhibited on day 30.(2)The change of soil enzyme activity was related to the changes of soil physicochemical factors and the availability of heavy metals induced by additives.Soil EC and AK were the most important environmental factors affecting the activity of all selected enzymes except for protease and invertase.AP had a significant effect on the activity of dehydrogenase,catalase,urease and arylsulfatase.The availability of As and Cu in soil affected the activity of soil dehydrogenase,catalase,?-glucosidase,urease,alkaline phosphatase and arylsulfatase,while the availability of Cd and Zn in soil affect the activity of soil dehydrogenase,catalase and urease. |
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