Effects Of Corn Straw Biochars On Thermal Physics Property Of Brown Earth And Its Mechanisms

Soil thermal property is one of the important physical properties of soil.Soil thermal properties affect the storage and transfer of heat in the soil,as well as the growth and development of crops and soil biochemical processes.The application of black and porous biochar to the soil will inevitably affect the soil color and bulk density,and affect the soil temperature by changing the soil porosity and reflectance.Soil thermal conductivity,thermal capacity,thermal diffusivity and surface reflectance are the intrinsic determinants of soil temperature.However,there are few in-depth studies on the effects of biochar application on such photothermal properties as thermal property and surface reflectance of soil.In this thesis,corn stalk biochar（CS300,CS500 and CS700）was prepared at 300,500and 700℃,respectively.The structure and properties of the obtained biochar were characterized and determined,and the effects of pyrolysis temperature on the structure and properties of corn stalk biochar were analyzed.Through indoor soil column simulation experiment to analyce the effects of corn straw biochar on thermal conductivity,thermal capacity,thermal diffusivity,in order to explore the influence mechanism.The filling method of test soil column was CS300,CS500 and CS700 were mixed with brown earth in proportion of 0,1,3 and 5%（mass content）,and the soil water content levels were set as 0,10,20,30and 40%.In addition,the measured value of soil thermal conductivity was compared with the modified model fitting value to analyze the fitting effect.The main conclusions are as follows:（1）pyrolysis temperature had a decisive influence on the structure and properties of biochar.With the increase of pyrolysis temperature,the yield decreases,the ash content and the pH value increased.In addition,with the increase of pyrolysis temperature,the C content of biochar increased,the N,H and O content decreased,and the atomic ratio of H/C,O/C,（O+N）/C decreased.Biochar has abundant pore structure.As the pyrolysis temperature increases,biochar’s pore number and specific surface area were increased,but the average pore size decreases.According to FTIR spectra and Boehm titration analysis,the unstable functional groups in corn straw biochar gradually weaken and disappeared with the increase of pyrolysis temperature,while the aromatic compounds increase.When the density of biochar is 0.098g/cm³,the thermal conductivity of CS300,CS500 and CS700 were measured as 0.060,0.057and 0.058 W m^-11 k^-1 respectively,the thermal capacity were 0.391,0.366 and 0.332 MJ m^-33 k^-1respectively,and the thermal diffusivity were 0.157,0.165 and 0.205 mm² s^-1,which were all smaller than the thermal conductivity,thermal capacity and thermal diffusivity of the solid and liquid phases of the soil.（2）The application of corn straw biochar in brown earth can significantly reduce soil bulk density,increase porosity and increase soil organic matter content.Under the condition of constant volume,the thermal conductivity and thermal diffusivity of soil decreased with the increase of biochar,but putted small effect on soil thermal capacity.Because of the different treatments of biochar,mineral and soil organic matter particles must be replaced by porous biochar particles,thus changing the soil composition.Therefore,biochar can affect soil thermal characteristics by changing the solid phase composition of soil,mainly because the application of biochar increases the content of organic matter in soil.From the experimental results of different bulk density treatments,it was found that the bulk density had a significant effect on the thermal properties of soil.It can be seen that the application of biochar can indirectly affect the thermal properties of soil by changing the soil structure.Soil volumetric water content had the most significant effect on soil thermal characteristics.When the volumetric water content was less than 30%,the thermal conductivity and thermal diffusivity of soil increase rapidly with the increase of water content.However,with the further increase of water content,the increasing trend of soil thermal conductivity slows down and the thermal diffusivity decreased.The thermal capacity was linearly related to the soil moisture content.（3）The biochar pyrolysis temperature had a certain effect on the water absorption of corn straw biochar.The results of biochar contact angle and O/C show that the higher the pyrolysis temperature,the more hydrophobic the biochar.However CS700 has a larger specific surface area,and CS300 has more functional groups and stronger hydrophilicity,making the water absorption capacity of biochar and carbonous soil CS700>CS300>CS500.The environmental temperature and relative humidity had a direct influence on the water absorption of biochar and carbonous soil.The higher the environmental humidity and the lower the temperature,the stronger the water absorption capacity and the higher the equilibrium water content.Biochar can indirectly affect soil water absorption capacity by affecting soil bulk density and porosity,the greater the amount applied,the stronger the water absorption capacity and the higher the equilibrium water content.By analyzing the relationship of physicochemical properties of brown earth applied to the equilibrium water content,it was found that the correlation between pH value and equilibrium water content was not significant.Porosity was positively correlated with equilibrium water content（P<0.01）,the bulk density and true density were negatively correlated with the equilibrium water content（P<0.05 at lower humidity and P<0.01 at higher humidity）,and there was a significant correlation between organic matter content and equilibrium water content（P<0.01,R²>0.76）.（4）The corn straw biochar in brown earth significantly reduced soil reflectivity compared with that in the control group.The reduction of reflectivity of biochar prepared at different pyrolysis temperature was CS300<CS500<CS700.In addition,the reflectivity of carbonaous soil decreases gradually with the increase of the amount of corn straw biochar applied.Water content is sensitive to the reflectivity of soil,when SWC was lower than 20%,the increase of water content can significantly reduce the reflectivity of soil,but when SWC was higher than 20%,the increase of water content has no obvious effect on the reflectivity.The result shows that the effects of biochar on soil thermal properties were mainly through reducing soil bulk density,changing soil three phase composition,affecting soil water content and reducing soil reflectance,and thus affecting soil photothermophysical properties.In addition,the low thermal properties of biochar also directly affect the thermal characteristics of soil.