Mechanisms Of Biochar On Soil Organic Carbon Turnover And Fixation Of Heavy Metals

Greenhouse effect,energy crisis and environmental pollution are severe issues confronting the world today,influencing human life and development.Soil is the largest terrestrial carbon pool,and small changes in soil carbon stocks can significantly affect atmospheric CO₂ concentrations.Frequent human activities resulted in the massive loss of soil organic carbon（SOC）and the intensification of soil heavy metal pollution.Numerous studies have shown that biochar has high stability,large specific surface area（SSA）,abundant oxygen-containing functional groups and other unique properties,showing great application potential in soil carbon sequestration and heavy metal remediation.The carbon sequestration potential of biochar depends not only on its own stability,but also on the interaction of soil components.Clarifying the interaction between biochar and soil can help to elucidate the carbon sequestration mechanism of biochar and deepen the understanding of soil carbon turnover.The current difficulty is to separate biochar from the soil after it enters into soil environment,which hinders the dynamic description of biochar properties and its interaction with soil components.In heavy metal remediation,the aging process of biochar in the soil environment affects its fixation of heavy metals.Understanding the changes of heavy metal adsorption over time by the interaction between biochar and soil can provide a reference for selecting suitable biochar for heavy metal contaminated soil remediation.In this study,the chemical oxidation,biochar-soil mixed incubation and batch adsorption experiments,combined with particle density fractionation,benzene polycarboxylic acids（BPCAs）molecular biomarkers and a series of characterization methods,analyzed the property changes of biochar before and after aging.The interaction between biochar and soil minerals and its effect on soil carbon turnover were also revealed.In addition,the potential mechanisms of heavy metal fixation by biochar-soil interactions were explored.The results of this study mainly include the following points:（1）The stability of biochar was evaluated by molecular biomarkers and the effect of biochar aging on the fixation of heavy metals was discussed.Lignin-phenol markers revealed that pine wood had more vanillyl unit,indicating it was greater stability than corn straw.The total BPCAs content and mellitic acid（B6CA）content of all biochars increased with increasing of pyrolysis temperature,suggesting an increase in aromaticity/aromatic condensation of biochar.HNO₃/H₂SO₄ oxidation significantly reduced the lignin fractions of corn straw biochar and removed more vanillyl and cinnamyl units than pine wood biochar.In addition,the BPCAs content of corn straw biochar decreased by 41-60 mg/g,while that of pine biochar increased by 86-133mg/g,which was attributed to the release of condensed aromatic structures in corn straw biochar and enrichment of condensed aromatic structures in pine wood biochar by aging process.The results suggest that pine wood biochar is more stable than corn straw biochar.It was also found that HNO₃/H₂SO₄ treatment increased the oxygen-containing functional groups and nitrogen-containing functional groups on biochar and thus improved their adsorption capacity for Cu²⁺and Cd²⁺.（2）The combination of BPCAs molecular biomarkers and soil particle density fractionation could accurately quantify the distribution of biochar in different soil fractions.The results of soil-biochar mixed incubation for one month showed that BPCAs were mainly distributed in free light fraction（f LF）,accounting for 87%of the total BPCAs contents.The BPCAs were detected in the aggregate-occluded light fraction（o LF）and mineral bound heavy fraction（HF）,suggesting a significant interaction between biochar and soil mineral particles occurred after one month incubation.In addition,the percentage of B6CA is comparable or even higher in HF than in f LF or o LF,indicating that biochar-mineral interactions play a key role in biochar stabilization.（3）The effect of biochar addition on soil carbon turnover was explored.Compared with biochar-soil mixed incubation for one month,the BPCAs content in f LF decreased after one year of incubation,while the content of BPCAs in o LF and HF increased,indicating that the interaction between biochar and minerals was enhanced with the extension of incubation time.The carbon content of biochar was calculated according to BPCAs content.After deducting the carbon contributed by biochar,it was found that the input of biochar could cause both positive and negative priming effects of n SOC.The different priming effects were related to soil properties,biochar addition ratio and incubation time.All fractions of Muding and Shilin soils exhibited negative priming during the incubation period,but the negative priming effect was significantly enhanced with increasing incubation time at 5%biochar addition ratio.Meanwhile,we noted that positive priming was dominant in the HF of Lijiang soil at one year incubation,but the positive priming effect was significantly weaker compared with the one-month incubation.The variation of stable carbon isotopeδ¹³C values among the fractions demonstrated that isotopic fractionation occurred during incubation.Evaluation of the differences of?¹³C values in the density fractions suggested that the carbon flow direction was f LF-o LF-HF for the control soil and o LF-f LF-HF for the soil with biochar addition.（4）The fixation mechanism of heavy metal by biochar-soil interaction was clarified.The changes of heavy metal speciation in soil mixed just with biochar and after one year of mixed incubation were determined.The results showed that biochar newly added to the soil had almost no effect on heavy metal speciation transformation.After one year of incubation,the transformation of heavy metal speciation by biochar was related to the residue fraction content of heavy metals themselves.When the heavy metal residue fraction is high,biochar addition will not change the heavy metal speciation.When the heavy metal has a low residue fraction,the addition of high concentration biochar can promote the transformation of heavy metal to residue fraction,but the effect is not significant.The adsorption of Cu²⁺on biochar and biochar-added soil was studied.It was found that the adsorption capacity of biochar itself was lower than control soil,and the soil could adsorb and immobilize Cu²⁺by surface complexation,pore filling,ion exchange and other mechanisms.Biochar addition to soil can weaken the adsorption of Cu²⁺by the soil.Comparing the theoretical K_d value and the real K_d value of the biochar soil mixture,it was found that the adsorption capacity of the mixed soil for Cu²⁺was greater than the superposition of the adsorption capacity of the two alone,indicating that the interaction between biochar and soil promoted the adsorption of the mixed soil for Cu²⁺.In this study,the dynamic change process of biochar-soil mineral interaction and biochar-induced native SOC priming effect over time was investigated by combining density fractionation and BPCAs molecular biomarker methods,which provided a new research perspective to comprehensively understand the effect of biochar addition on soil carbon turnover.In addition,the adsorption efficacy of aging biochar and biochar-soil particle interaction on heavy metals was investigated which provide a reference for the selection of suitable biochar for remediation of heavy metal contaminated soil.