The Interfacial Behavior Between Biochar And Soil Minerals And Its Influencing Mechanisms Of Carbon Stability

Biochar has been suggested as a prospective“carbon-sink”material.When added to soil for carbon sequestration,biochar could also improve soil property,increase soil fertility and remediate contaminated soil.These environmental effects of biochar has intimate connection with biochar stability.This dissertation firstly studied the stability of biochar in real soil and explored the influencing fators.We found the contents of soil clay and soil metal ion had a good correlation with biochar statibily,and biochar had two existing states in wet soil:dissolvable biochar and undissolvable biochar.After that,the interfacial interaction between soil minerals and these two biochar fractions was studied,respectively.Besides,the effect of soil minerals on the stabiltiy of dissolvable and undissolvable biochar were detected from the aspects of chemical oxidation,thermal oxidation and microbe degradation.Finally,an estimation was conduct to evaluate the contributions of minerals-associating dissolvable and undissolvable biochars to the entire biochar stability.The main results are as following:?1?The cumulative CO₂ emission of biochar in 5 soils during 28-d incubation period was fitted by double-exponential model,and the fitting parameters were conduct correlation analysis with soil property indexes.Results showed that biochar had the highest stability in Argi-Udic Ferrosols?YT?soil than in other 4 soils:The amount of CO₂ releasing from biochar itself after adding to other 4 soils was 4-6 times than that to YT soil.The stability of biochar in soils had negative correlation with the contents of soil clay and Fe phase,whereas had positive correlation with soil pH and Ca pahse.Besides,the amount of nonresistant C released from soils sharply increased after biochar added,which implied there was nonresistant C in biochar.Meanwhile,under simulated paddy-upland rotation condition,the increase of soil water resulted in one more concentrated emission of CO₂ from biochar.Specifically,the maximum percentage of nonresistant mineralizable C of biochar-added YT soil was 2.4 times than pure YT soil,which provided strong evidence that the nonresistant C of biochar was probablely dissolvable form.Under drying-wetting condition,the dissolvable biochar released from entire biochar particles successively,however,soil minerals,especially CaCl₂ reduced the release of dissolvable biochar from entire biochar particles by declining the pH of ambient solution.?2?Three solutions including CaCl₂,NaCl and NaCl-NaH₂PO₄ were used as background electrolytes to investigate the mian associating mechanisms of dissolvable biochar on kaolinite,bentonite and geothite.Besides,the contributions of 3 mechanisms to the sorption of dissolvable biochar were calculated.Results showed that phyllosilicate clays,like kaolinite or bentonite,could adsorb dissolvable biochar through‘Ca²⁺bridging',‘ligand exchange'and‘van der Waals force';Moreover,the contributions of Ca²⁺bridging and ligand exchange decreased with the increase of dissolvable biochar concentrations,however,the contribution of van der Waals force showed an opposite trend.Specifically,at lower dissolvable biochar concentrations(5-20 mg C L^-1),the Ca²⁺bridging and ligand exchange of kaolinite and bentonite accounted for 56-75%and 97-100%,respectively,while van der Waals occupied less than 5%and 0.4%,respectively;At higher dissolvable biochar concentrations(60-100mg C L^-1),the Ca²⁺bridging and ligand exchange of kaolinite and bentonite accounted for 10-25%and 4-30%,respectively,while van der Waals occupied 70-84%and 69-87%,respectively.Dissolvable biochar was adsorbed on geothite mainly by ligand exchange and van der Waals force,while the presence of Ca²⁺caused an inhibition effect.?3?The chemical oxidation resistance of dissolvable biochar was enhanced after associated with soil clay minerals.Specially,the association with kaolinite reduced the C loss of dissolvable biochar by 18.6-33.3%,compared to the free dissolvable biochar,and the chemical oxidation resistance of 3 mechanisms was in the order:Ca²⁺bridging?ligand exchange>van der Waals.The association with bentonite reduced the C loss of dissolvable biochar by 29.1-33.6%,compared to the free dissolvable biochar,and the chemical oxidation resistance of 3 mechanisms was in the order:Ca²⁺bridging>ligand exchange?van der Waals.The association with geothite reduced the C loss of dissolvable biochar by 23.1-37.0%,compared to the free dissolvable biochar,but the presence of Ca²⁺caused an inhibition effect.As for the thermal oxidation,the resistance of dissolvable biochar adsorbed on kaolinite was in the order:Ca²⁺bridging?van der Waals>ligand exchange,and the order under bentonite treatments was:Ca²⁺bridging>ligand exchange>van der Waals.The thermal oxidation resistance of these 3 binding mechanisms under geothite treatments were similar.As for the biodegradation,compared to the free dissolvable biochar,the maximum percentage of dissolvable biochar mineralizable C(S_max)of the dissolvable biochar adsorbed on kaolinite,bentonite,and geothite decreased by 47.9-85.3%,78.4-85.0%and 72.7-78.8%,respectively.The Ca²⁺enhanced the protective effect of bentonite and geothite on dissolvable biochar,but weakened the protective effect of kaolinite.?4?As for the undissolvable biochar,free Fe³⁺and Al³⁺transfromed into Fe₈O₈?OH?₈Cl_1.35 and AlCl₃·6H₂O crystals on particle surface,respectively.The results of BET-N₂ showed the average pore size of undissolvable biochar decreased from 23.4nm to 10.4 nm.Dual beam focus ion beam?FIB?was used to create a cross-section containing an interface between undissolvable biochar and the mineral particles.After that,SEM was conduct and we found after 3-month incubation,the fine mineral particles attached tightly to undissolvable biochar surface or inserted into undissolvable biochar inner pores.The elemental linescan exhibited the relative mole ratios of both Fe and O elements increased greatly at the interfaces of undissolvable biochar and minerals,meanwhile,the C element still remained at a high level?over 10%?though it decreased compared to that in undissolvable biochar interior,which showed a possible formation of the Fe-O-C organometallic complex.Besides,Fe had diffused into the undissolvable biochar interior,because the concentration of Fe was from high to low along the direction from the interface to the undissolvable biochar interior,and even some Fe appeared at the center of the undissolvable biochar.Compared to the control,kaolinite and CaCl₂ increased the activation energy of the undissolvable biochar by22.1%and 16.3%,respectively.The presence of Ca²⁺seemed to weaken the strengthening effect of kaolinite,and only increased the activation energy by 12.8%.However,clay minerals had antagonism effect with metal ions in raising the activation energy of undissolvable biochar.?5?The association with soil minerals enhanced the resistance of oxidation of undissolvable biochar surface.Specially,compared to the control,the relative amount of C-C/C=C/C-H of mimerals-associating undissolvable biochar surface increased from 63.8%to 72.5-81.8%,while the relative amount of C-O/C=O/COOH decreased from 36.3%to 16.6-26.5%.However,clay minerals had antagonism effect with metal ions,which was in accordance with the results in activation energy.Compared to the control,the resistance of chemical oxidation and S_max of mimerals-associating undissolvable biochar decreased by 13.4-79.6%and 7.4-51.6%,respectively,and the index of thermal oxidation resistance(R_50,bicohar)increased from 44.6%to⁴9.6%,which indicated the association with soil minerals raised the stability of undissolvable biochar.?6?According to the estimating result,as for chemical oxidation,soil minerals reduced the ratio of C loss of entire biochar by 43.0%,due to 34.6%reduction of undissolvable biochar and 8.4%reduction of dissolvable biochar.As for biodegradation,soil minerals reduced the ratio of C loss of entire biochar by 49.3%,due to 48.3%reduction of undissolvable biochar and 1.0%reduction of dissolvable biochar.This result indicated although the protection on entire biochar by minerals was mainly attributed to the stabilization of undissolvable biochar,the stabilization of dissolvable biochar by soil minerals could not be ignored.The stabilization of biochar in soil is mainly due to the the physicochemical and biological influences from the surrounding environment reducing the probability of decomposition,in other words,the stability of biochar in soil is primarily an ecosystem property.In conclusion,the biochar added to wet soil had two existing forms:dissolvable biochar and undissolvable biochar.Soil clays and soil metal ions could raise the stability of entire biochar by associating with dissolvable biochar via Ca²⁺bridging,ligand exchange and van der Waals,meanwhile,providing physical package and block for undissolvable biochar,generating organometallic complexes with biochar and raising the activation energy of undissolvable biochar surface.It implied that the clayey soil like YT soil which is rich in clay minerals and metal ions could be a beneficial environment for biochar in terms of carbon sequestration.