Effects Of Biochar Application On Albedo And Thermophysical Properties Of Loess Farmland Soil In Northwest China

Loess soils in Northwest China are a special type of soil formed under arid and semi-arid climatic conditions.Loess soils have a loose structure,high porosity,high permeability,poor agglomeration capacity and easy leaching of nutrients,which has become a"bottleneck"limiting the development of agriculture in Northwest China.Therefore,it is advisable to apply biochar to loess soils in order to improve the structure and properties of the soil,increase the water and fertiliser retention capacity of the soil and improve crop yields and the’production potential’of the farmland.However,biochar is a black granular material and its application to the soil will inevitably reduce the albedo of the soil,alter the light absorption characteristics of the soil,increase the absorption of solar radiation at the soil surface and change the thermal status of the soil.On the other hand,the application of biochar changes the composition and structure of the soil,affecting its water-holding properties and thus its thermophysical properties（e.g.thermal conductivity,heat capacity and thermal diffusivity）,which also affects the thermal status of the soil.At present,there are few studies on the effects of biomass sources and pyrolysis conditions on the light absorption and thermophysical properties of biochar,and even fewer studies on the effects of biochar application on soil albedo and thermophysical properties.In addition,the mechanisms by which changes in soil albedo,thermophysical properties and moisture characteristics affect soil temperature as well as vegetation growth patterns due to biochar application are unclear,and systematic and comprehensive studies are lacking.Therefore,in this paper,three biomasses,namely rape straw（RS）,apple tree branches（ATB）and pine wood shavings（PS）,which are commonly found in northwest China,were prepared into biochar by limited oxygen pyrolysis under the conditions of 300,500 and 700℃.The effects of biochar application on the structural characteristics,physicochemical properties and photothermal physical properties of biochar-soil mixtures were investigated,as well as the effects of biochar application on the albedo,bulk density,water content,thermophysical properties and temperature of farmland soils under different seasons,climatic conditions and ground cover conditions.In order to provide a theoretical basis for promoting the sustainable development of loess farmland and improving crop production capacity.The main findings of the study are as follows:（1）Analysis of the physicochemical properties of RS,ATB,and PS biomasses showed significant differences in the elemental composition,industrial analysis,and thermal diffusion properties of the different biomasses,but the rich lignocellulose content,lower moisture,ash,and elemental N content all indicated that the agroforestry waste had good prospects for pyrolysis treatment.Correlation analysis of biomass physicochemical properties with pyrolysis characteristics and kinetic parameters showed that the main process of biomass pyrolysis（devolatilisation process）was jointly determined by the content of cellulose and lignin（P<0.05）;the higher their content,the wider the temperature range at which devolatilisation of biomass occurs and the higher the average activation energy（Ea[mean]）required;the greater the thermal diffusivity of biomass,the higher its The higher the thermal diffusivity of the biomass,the lower the minimum activation energy（Ea[min]）required to start pyrolysis and the faster the maximum rate of weight loss（Prate）during pyrolysis,the easier the pyrolysis will be.The thermodynamic parameters of pyrolysis showed that the pyrolysis of agricultural and forestry waste is a complex process,and the entropy change（ΔS）increases and then decreases with the conversion rate（α）during the pyrolysis process,indicating that the structure of biomass particles undergoes a change from"ordered"to"disordered"to"ordered"during the pyrolysis process.This indicates that the structure of biomass pellets undergoes a process from"order"to"disorder"and then to"order"during pyrolysis.（2）There are significant differences in the physicochemical properties of biochar prepared from different raw materials and pyrolysis temperatures.The higher the pyrolysis temperature,the higher the elemental C content,specific surface area and moisture absorption properties of the biochar,and the lower the volatile content and less surface functional groups.There were significant differences in the thermophysical properties of biochar from different raw materials,with the thermal diffusivity of RS biochar varying in the range of 0.165±0.0014～0.176±00.0021 mm²·s^-1,significantly higher than that of ATB(0.139±0.0025～0.180±0.0000mm²·s^-1)and PS(0.153±0.0054～0.181±0.0014 mm²·s^-1)thermal diffusivity（P<0.05）,but the thermal conductivity and heat capacity of RS biochar were significantly smaller than those of ATB and PS biochar.The correlation analysis of the physicochemical and thermophysical properties of the biochar showed that the thermal conductivity of the biochar was significantly positively correlated with the bulk capacity（P=0.0115,r=0.7887）and negatively correlated with the porosity（P=0.047,r=-0.8840）,mainly because the increase in porosity prolonged the travel of heat through the biochar skeleton,resulting in a decrease in thermal conductivity;and an increase in bulk capacity increases the state of association between biochar particles,increasing the average free range of phonons,so the greater the thermal conductivity.The thermal diffusivity of biochar showed a significant positive correlation with specific surface area（P=0.0386,r=0.6820）and total pore volume（P=0.0199,r=0.7562）,mainly due to the increase in specific surface area and total pore volume,which led to a more ordered biochar structure and a longer phonon free range.（3）Application of biochar with different raw materials,pyrolysis temperatures and proportions significantly altered the thermophysical properties of the soil.In the absence of moisture in the soil:the material and application rate of biochar（0,1%,3%and 5%）significantly changed the thermal conductivity（P<0.05）,heat capacity（P<0.05）and thermal resistance（P<0.05）of the soil;the application rate did not significantly affect the thermal diffusivity of the soil（P>0.05）,but the application of biochar prepared from different raw materials showed significant differences in the thermal diffusivity of the soil（P<0.05）.There was no significant effect of biochar at different pyrolysis temperatures on the thermophysical properties of the soil（P>0.05）.At lower soil volumetric moisture content（10%）:the proportion of biochar applied had a significant effect on soil thermal conductivity and thermal diffusivity（P<0.05）,biochar raw material had a significant effect on thermal conductivity and thermal resistance（P<0.05）and pyrolysis temperature had a significant effect on thermal diffusivity and thermal resistance（P<0.05）;at higher soil volumetric moisture content（30%）:only the proportion of biochar applied had a significant effect on soil thermal diffusivity（P<0.05）.proportion had a significant effect on the thermal diffusivity of the soil.Thus,the thermophysical properties of the soil were influenced by a combination of material,pyrolysis temperature and applied ratio under no moisture or lower moisture conditions;however,under higher moisture conditions,the thermophysical properties of the soil were only influenced by the applied ratio,mainly by changing the thermal diffusivity.（4）One-dimensional wet heat transport characteristics of the soil showed that biochar application had a significant effect on the wet heat transport characteristics of the soil.At lower moisture contents（10%）,the rates of warming and stabilisation at the P1（3cm）monitoring point closer to the heat source were significantly increased by 1,3 and 5%biochar application rates at 30°C.The rates of warming at the P1 monitoring point in the soil column for the first 3hours were 3.90,3.43 and 3.63°C·h^-1 at 1,3 and 5%biochar application rates respectively,which were approximately 2.85,2.50 and 2.65 times that of the control（CK）.When the temperature at the P1 monitoring site reached stability,the temperatures in soil column P1 were30.4,29.3 and 30°C at 1,3 and 5%biochar application rates,respectively,7.0,5.9 and 6.3°C higher than CK.The range of soil moisture migration extended with increasing biochar application rates.Soil columns with different biochar application ratios had the same moisture heat migration pattern at a heat source temperature of 50°C.At higher moisture content conditions（25%）and a heat source temperature of 30°C,the rate of warming at point P1 and the temperature at stabilisation of the soil column did not differ significantly from CK for different biochar application ratios.However,when the heat source temperature was 50°C,the rate of warming at point P1 of the soil column for the first 3 hours was 8.47,8.59,and 8.90°C·h^-1 at 1,3 and 5%biochar application ratios,respectively,which was approximately 2.27,2.30and 2.39 times higher than that of CK.When the temperature at the P1 monitoring point reached stability,the temperatures in soil column P1 were 49.6,49.3 and 48.7°C at 1,3 and 5%biochar application rates,respectively,13.9,13.6 and 13°C higher than CK.Soil moisture also migrated in the direction away from the heat source location and the range of migration shortened with increasing biochar application rates.Thus,the more significant the effect of biochar application on water and heat migration in low moisture content,high temperature soils.（5）Studies on the absorbance characteristics of biochar itself showed that it has a high absorption of both UV and visible light,varying between 87.12 and 88.62%and 84.27 and86.53%respectively,and is less affected by the material and pyrolysis temperature;the absorption of the infrared and full band spectra increases with the increase in pyrolysis temperature.Biochar application with different raw materials,pyrolysis temperatures and application ratios had a significant effect on the albedo of the soil.In air-dried soils,the mean values of soil albedo were 36.28,29.41 and 25.82%at 1,3 and 5%biochar application rates,respectively,which were 11.17,28.02 and 36.81%lower than those of the low CK group.Biochar application of different materials had different effects on soil albedo,with RS,ATB and PS biochar applications corresponding to mean soil albedo values of 25.80,32.43 and32.48%.The pyrolysis temperature of the biochar also had an effect on the albedo of the applied soils.300,500,and 700°C biochar applied to the soils reduced the albedo by 4.7,12.1,and14.2%,respectively,compared to the CK group,showing a trend that the higher the pyrolysis temperature of the biochar,the more significant the reduction in soil albedo.The albedo of biochar varied from 22.97 to 32.27%%,13.33 to 19.68%and 12.11 to 20.35%under 10,20 and30%volumetric water content conditions,respectively,showing a trend that the higher the biochar application rate,the lower the soil albedo.At 30%volumetric water content,the application of 5%biochar resulted in a reduction in soil albedo of up to 40.49%compared to the CK group.Biochar application on soil light absorption and heat transfer simulation experiments showed that biochar was able to significantly increase the maximum soil temperature at the surface of the soil column by changing the soil albedo（P<0.05）.The maximum soil surface temperature was 47.6,48.4 and 49.2°C at 1,3 and 5%biochar application rates,respectively,which were 1.1,2.9 and 3.7°C higher than the maximum temperature at the surface of CK.The heat flux analysis showed that biochar application increased the fluctuation of heat flux in the surface soil,which ranged from-41.17 to 87.43 W·m^-2,-28.42 to 67.47 W·m^-2,-51.77 to 144.33 W·m^-2 and-78.09 to 230.8 W·m^-2 at CK,1,3 and 5%biochar application rates,respectively.（6）The results of the field experiment showed that:during the fallow period,the soil albedo decreased significantly（P<0.05）with the increase of biochar application rate,while the soil temperature increased with the increase of application rate;during the planting period,the surface albedo showed a trend of decreasing and then increasing due to the growth stage of maize,and the difference between different application rates gradually disappeared with the growth of maize.After the maize was harvested,the albedo of the soils showed some variability again at different rates of biochar application.The correlation coefficients between surface temperature and albedo,heat capacity,thermal conductivity,thermal diffusivity,bulk weight and moisture were analysed during the fallow and planting periods.The lower the soil albedo,the higher the thermal conductivity and the bulk weight,the higher the temperature of the soil.