Effect Of Wheat Straw Biochar On Thermophysical Properties Of Loessial Soil And Its Mechanism

Biochar is a black particulate material obtained by pyrolysis of biomass such as straw and sawdust in an anoxic environment at high temperature.It has the characteristics of high carbon content,rich pore structure and functional groups,and high specific surface area.Loessial soil is widely distributed in the northwestern region.The application of biochar to loessial soil can effectively alleviate the characteristics of looseness,barrenness,poor cohesion and poor organic matter.Soil thermophysical properties are the key factor affecting the internal heat balance of the soil.Soil temperature affects many physical and biochemical processes such as soil microbial activity,nutrient supply,water and air movement.Studying the change laws of thermophysical properties is of great significance for regulating soil temperature and improving soil fertility.The application of biochar to loessial soil will affect the bulk density,organic matter content and water holding capability of loessial soil,which indirectly affect the thermophysical properties such as thermal conductivity,heat capacity and thermal diffusivity of loessial soil.In this paper,wheat straw was selected as biomass,and biochar was produced by limiting oxygen at pyrolysis temperatures of 300,500,and 700 ℃,respectively(BC300,BC500 and BC700).The physicochemical properties of biochar such as its surface structure,pore size,and surface functional groups were characterized by measuring the elemental composition,specific surface,and infrared spectroscopy of biochar.The biochar was added to loessial soil at the addition amounts of 0%,1%,3%,and 5%.After one week of incubation in a 25 ℃incubator,the bulk density,organic matter content,and water holding capability of biochar on physicochemical properties of loessial soil were investigated.The KD2 Pro thermal characteristic analyzer was used to determine the thermophysical properties of loessial soil.The effects of different addition amounts(BC300-1,BC300-3,BC300-5,BC500-1,BC500-3,BC500-5,BC700-1,BC700-3 and BC700-5;y-BC300-1,y-BC300-3,y-BC300-5,y-BC500-1,y-BC500-3,y-BC500-5,y-BC700-1,y-BC700-3 and y-BC700-5)on the thermal conductivity,heat capacity,thermal diffusivity and thermal resistance of loessial soil and their relationship with bulk density and moisture content were investigated.The main findings are as follows:(1)The structure of wheat straw biochar was gradually developing towards aromatization,and the polarity and hydrophilicity gradually decreased with the increasing pyrolysis temperature.When the pyrolysis temperature rose,the BET specific surface area and the micropore volume started to rise rapidly,and the average pore size gradually decreased.The FTIR characterization showed that aliphatic carbon and weak oxygen-containing functional groups were gradually decomposed,and biochar was gradually aromatized as the pyrolysis temperature increased.BC300,BC500 and BC700 were all hydrophobic,and high temperature pyrolysis was conducive to the enhancement of hydrophobicity.Biocharproduced by low-temperature and high-temperature pyrolysis had stronger hygroscopicity.Biochar mainly absorbed infrared light in the spectrum and the higher the pyrolysis temperature,the greater the absorption rate.The tar attached to biochar would increase the absorption of infrared light.(2)The organic matter content and moisture absorption of loessial soil were increased affter the application of wheat straw biochar.But the bulk density of loessial soil was decreased,and the larger the addition amount,the smaller the bulk density.Compared to the control,the organic matter contents of BC500-1,BC500-3,and BC500-5 increased by67.1%、169.6% and 225.3%,respectively;the moisture absorption increased by 19.6%,16.6%and 24.9%;the bulk density decreased by 6.4%,16.2% and 24.9%,respectively.The bulk density of loessial soil decreased with the increase of pyrolysis temperature of biochar.High addition amount or medium and high pyrolysis temperature of biochar were beneficial to increase the porosity of loessial soil.The texture of loessial soil was sandy loam.Only BC300-1 changed the texture of loessial soil in different treatments.Low addition amount of biochar could increase the medium particle size of loessial soil,and high addition amount of biochar would decrease the medium particle size.BC300 and BC500 were beneficial to improve the water holding capability of loessial soil in the later stage while the water holding capability was decreased by BC700-5.Thus,from the perspective of increasing the moisture content of loessial soil,we can consider selecting medium temperature pyrolysis,low addition or high temperature pyrolysis,medium addition amount of biochar to improve the loessial soil moisture condition.Loessial soil with biochar mainly reflected visible and infrared light,and the reflectance gradually increased with increasing wavelength.Before the wavelength of1750 nm,the absorption of light by loessial soil increased with the increase of pyrolysis temperature and the addition amount of biochar.Removal of tar and other substances attached to the surface of biochar was beneficial to increase the light reflection of loessial soil.(3)When the influence of moisture was not considered,the correlation analysis showed that there was a significant correlation between the thermophysical properties and the bulk density and the addition amount of loessial soil added with biochar,and no significant correlation with the pyrolysis temperature and whether or not to wash.The thermal conductivity of loessial soil was increased by the addition of 1% biochar,and the thermal conductivity of loessial soil was decreased by the additions of 3% and 5%.The larger the amount of addition,the greater the decrease.Compared with the control,BC500-1 increased by 3.8%,and BC500-3 and BC500-5 decreased by 3.4% and 7.1%,respectively.The thermal conductivity of loessial soil gradually decreased with the increase of the addition amount,and the thermal resistance gradually increased with the increase of the addition amount.There was no obvious rule of influence between the addition amount and the thermal capacity and thermal diffusivity of loessial soil.The thermal conductivity of loessial soil was proportional to the pyrolysis temperature,but the effect of pyrolysis temperature on the thermal capacity,thermal diffusivity and thermal resistance was not obvious.The thermal conductivity,heat capacity,and thermal diffusivity of loessial soil added with washed biochar were higher than those with unwashed biochar,and the effect of washing with biochar on the thermal resistance was just the opposite.The effect of bulk density on the thermal properties of loessial soil was different under different addition amounts.The thermal conductivity and thermal diffusivity with the additions of 0%,1%,and 3% increased with the increase of the bulk density.Increase and decrease.On the contrary,the thermal conductivity and thermal diffusivity of the loessial soil with the addition of 5% decreased with the increase of the bulk density,and the thermal resistance increased with the increase of the bulk density.The heat capacity was proportional to the bulk density at each addition amount.When moisture was added to loessial soil,the thermal conductivity,heat capacity,thermal diffusivity,and thermal resistance of loessial soil were only significantly related to the moisture content.Under a certain water content,biochar reduced the thermal conductivity,heat capacity and thermal diffusivity of loessial soil,and the larger the addition amount,the greater the decline.On the contrary,the biochar under a certain water content increased the thermal resistance of loessial soil,and the larger the addition amount,the higher the thermal resistance.Thermal conductivity and heat capacity of loessial soil added with biochar was proportional to water content.The thermal diffusivity first increased and then decreased with the increase of water content,and the thermal resistance decreased rapidly and then decreased gradually with the increase of water content.The mechanism of biochar affecting the thermophysical properties of loessial soil was mainly affecting the bulk density,porosity,organic matter and moisture content of loessial soil.Among them,the effect of moisture content was the most significant.