Effects And Mechanism Of Biochar Amendment On Soil Thermal Properties

Heat storage and transfer,heat and water movement and soil surface energy balance were affected by soil thermal properties,and these variations affected crops growth and bio-chemical process.Biochar amendment can reduce surface albedo and affect soil temperature.Soil thermal properties were the intrinsic reason to determine the soil thermal condition,but few study focused attention on soil heat capacity?C?,soil thermal conductivity???and soil thermal diffusivity???.A laboratory experiment was carried out to investigate the effects of biochar on soil thermal properties of clay,sandy loam and sandy soil with four biochar application rates,viz.,0?B0?,0.01 g g^-1?B1?,0.03 g g^-1?B3?and 0.05 g g^-1?B5?.Soil water content covers dried soil to the saturated soil.A7-year field experiment was also carried out in this study to investigate the effects of biochar amendment on soil thermal properties with three biochar application rates,viz.,0?control?,4.5 t ha^-1yr^-1?B4.5?,and 9.0 t ha^-1 yr^-1?B9.0?.The experimentalfield located in Huantai Agroecosystem Experimental Station of China Agricultural University,Shandong province,where the winter wheat and summer maize rotations is the typical cropping system.The soil was sandy loam.Soil heat capacity?C?,thermal conductivity???,and thermal diffusivity???of the 0–5 cm depth topsoil were determined throughout corn/wheat growing period using heat pulse method.The relationship between soil thermal properties and biochar amount,soil bulk density,and water content were studied,respectively.Soil temperature of 5-cm depth and soil heat flux of 10-cm depth were also determined during the whole experimental period.The main conclusions in our study are as follows:?1?The soil heat capacity increased in a straight line with the increasing of water content at a certain bulk density in laboratory experiment.The effects of biochar treatment on soil heat capacity of three different soils at certain water content were not significant.The variation of soil heat capacity at certain bulk density and water content may be caused by the smaller proportion of biochar.Significant difference was observed at different biochar treatment?P<0.0001?,and the soil thermal conductivity gradually reduced with the increase of amount of biochar.The soil thermal conductivity of B1,B3 and B5 treatments decreased by an average of 3.6%,6.6%and 9.5%for clay soil,an average of 5.0%,8.5%and 14.5%for sandy loam,and an average of 5.3%,10.2%,14.7%for sandy soil at the whole water content,when compared to control.The smaller thermal conductivity of biochar,the surface structure and the interaction between biochar and water might cause these variations.Effects of biochar on thermal conductivity of dried soil were not significant.Water films were not formed because of the lower water content,which lead to no significant difference affected by biochar amendment.The soil thermal diffusivity reached to a peak with the increase of water content and then decreased with the increasing water content.Overall,biochar significantly reduced the soil thermal diffusivity of three soils?P<0.0001?.The soil thermal diffusivity of B1,B3 and B5 treatments decreased by an average of 3.6%,6.6%and 9.5%for clay soil,an average of 4.2%,7.8%and 14.1%for sandy loam,and an average of5.3%,10.2%,16.3%for sandy soil at the whole water content range,when compared to control.The increase of soil insulation caused by lower thermal diffusivity and larger porosity of biochar might lead to the decrease of thermal diffusivity.?2?Under the condition of field experiment,biochar amendment had no significant effect on soil thermal capacity at 0-5 cm depth.Biochar treatments significant reduced the soil thermal conductivity at0-5 cm depth.When compared with the control,soil thermal conductivity in treatments B4.5 and B9.0was decreased averagely by 8.0%and 14.6%,respectively.Biochar treatments also significant reduced the soil thermal diffusivity at 0-5 cm depth.When compared to the control,soil thermal diffusivity in treatments B4.5 and B9.0 was decreased averagely by 6.0%and 11.0%,respectively.These findings appear to be attributed primarily to the characteristics of biochar and variation in soil physical properties?e.g.bulk density and water content?caused by biochar.On the one hand,biochar had relatively lower thermal conductivity and diffusivity.Specific heat capacity of biochar is 2.14 times higher than that of organic matter and minerals in soil.On the other hand,soil bulk density was decreased significantly by5.5%–10.6%after biochar amendment,while there was no correlation between soil water content and biochar.?3?Soil temperature daily variation at 5-cm depth was affected by biochar.The daily mean soil heat flux at 10-cm depth of April,May and August were 21.79 W m^-2,14.14 W m^-2 and 4.85 W m^-2,respectively.The heat was absorbed by soil and then transferred downward.However,the daily value of October and November was-25.75 W m^-2 and-22.99 W m^-2,respectively.The heat released from the underling soil.The soil heat flux under B4.5 treatment was decreased in the daytime and increased at night,however,a greater difference was observed under B9.0 treatment.The maximum value of soil heat flux under biochar treatment was an hour later than that of control and decrease of thermal conductivity might cause this variation.Biochar treatment could affect the variation of soil temperature and soil heat flux,significantly decreased the soil thermal conductivity and thermal diffusivity,which may further affect the soil biogeochemical cycle and plant growth.