Physical Characteristics Of Farmland Soil And Transport Characteristics Of Snowmelt In Seasonal Frozen Soil Area: The Regulatory Mechanism Of Biochar

Under the condition of freeze-thaw cycle,the change of farmland soil moisture and heat environment in high latitude cold region is complex and special.There is frequent energy exchange between soil active layer and external environment.In seasonal frozen soil area with the hydrological processes such as snow melting,evaporation and infiltration,resulting in the unique spring snowmelt waterlogging leads to frequent root rot of crops at seedling stage,which seriously threatens the improvement of grain production capacity and the sustainable development of agriculture.Therefore,it is urgent to find a soil conditioner that can improve soil physical properties and crop habitat.Based on the farmland soil and indoor simulation test,this paper sets four different biochar application amounts(1%,2% and 3%)and control group(0%)to explore their effects on the soil structure characteristics and soil moisture characteristics under the condition of freeze-thaw cycle,in which the response mechanism between soil freeze-thaw deformation and soil physical characteristics is analyzed.On this basis,the soil state during snow melting in early spring is simulated,and the indoor snowmelt transport simulation test is carried out based on the multi-year average snow melting data,so as to analyze the variation law of characteristic parameters of farmland snowmelt transport during snow melting period and reveal the snowmelt transport mechanism.Combined with the soil moisture demand of sowing in spring and improvement of soil physical properties,the biochar application amount range most suitable for seasonal frozen soil area was selected to make the crop habitat reach the best state.The main research contents and conclusions are as follows:(1)Effect of biochar on soil structure under freeze-thaw cycle.The freezing-thawing increases the proportion of soil voids(>100 μm)and soil pores in the middle section(0.3-100 μm)and reduces the proportion of extremely micro pores(<0.3 μm).The soil average pore size changes from small to large,and the transformation rate changes from fast to slow.With the increase numbers of freeze-thaw cycles,the soil total porosity with low application amount(0%,1%)increased,while the soil total porosity with high application amount(2%,3%)decreased.In addition,the application of biochar increases the proportion of soil pores in the middle section,reduces the proportion of soil voids and extremely micro pores,and significantly increases the soil total porosity.The proportion of soil silt and clay decreased first and then increased with the increase of biochar application amount,which showed the opposite law with medium particle size aggregates,and had little effect on the other two aggregates.The application of biochar also increase the soil aggregate stability.However,the freeze-thaw cycle reduce the soil aggregate stability.With the increase of biochar application amount,the reduction trend will slow down first and then accelerate,and the soil aggregate stability will increase first and then decrease.In addition,freezing-thawing promotes soil expansion,but inhibits soil shrinkage.Proper biochar application amount inhibit soil expansion,while excessive application(3%)promote soil expansion.Soil shrinkage was positively correlated with soil structural indexes.Compared with the 0% biochar application amount soil,STPSD and GSSI also showed the best mechanical structure stability under 1% biochar application amount,while the high application amount treatment caused negative effects and significantly reduced the soil structure stability because the soil was too loose.(2)Effect of biochar on soil moisture characteristics under freeze-thaw cycle.Freezing-thawing make soil moisture migrate upward,and the trend of migration gradually weakened with the increase of biochar application.Freezing-thawing also reduce soil water preserving capability,while the application an appropriate application amount of biochar will reduce its reduction range,significantly increase soil water preserving capability,and gradually increase the slope of soil water characteristic curve.Due to the increase of soil total porosity,the number of water storage pores is increased,and then the soil saturated water content is increased.It was found that when the application amount was 1%,the soil moisture characteristic values such as field moisture capacity and plant available water content were the best,and significantly alleviated the change trend of water characteristic values caused by the freezing-thawing.In addition,the application of biochar will significantly increase the saturated hydraulic conductivity of unfrozen soil and reduce the saturated hydraulic conductivity of frozen soil.With the increase application amount of biochar application,the decrease trend of soil unsaturated hydraulic conductivity increases with the increase of absolute value of suction head.In order to effectively reveal the effect of biochar application on the transport mechanism of snowmelt,Xi is proposed to evaluate the soil infiltration performance during melting period based on the soil saturated hydraulic conductivity and normalization principle.Xi has a good correlation with each snowmelt transport index,which provides a new idea for the later study of transport law during melting period.(3)Analysis of the effect of biochar on soil freezing-thawing deformation.Soil freezing-thawing deformation mainly occurs in the previous freeze-thaw cycles,and then tends to be stable.The essence of freezing-thawing deformation is the process of transformation from small pore size to large pore size.In addition,there is an obvious secondary collapse phenomenon in the monitoring of freezing-thawing deformation process,which will occur in advance with the biochar application.However,after the thaw settlement caused by the first freeze-thaw cycle,there will be an obvious frost heaving phenomenon for the soil column with high biochar application amount(2%,3%).In conclusion,with increasing biochar application amounts,the characteristics of soil freezing-thawing deformation changed from frost heaving to thaw settlement,and the degree of soil freezing-thawing deformation first alleviates down and then enhances.(4)Analysis of the effect of biochar on the transport characteristics of snowmelt.Infiltration,runoff and evaporation also showed a trend of improving first and then producing negative effects with the increase of biochar application amounts.Excessive biochar application amount cause a significant increase in snowmelt runoff and evaporation,thus reducing the snowmelt infiltration,and even forming an impermeable layer on the soil surface to hinder the snowmelt infiltration.From the perspective of soil profile dyeing,the appropriate amount application of biochar will increase the influence depth of snowmelt,improve the soil dyeing concentration,quantitatively calculate the infiltration amount,and provide a new idea for observing the transport characteristics of snowmelt.And it can increase the soil temperature in the snowmelt period and accelerate the soil melting rate.In addition,moisture evaporation in melting period mainly occurred in rapid snow melting period,and the evaporation rate changed from fast to slow,and finally tended to be stable,with little difference among treatments after soil stable.Finally,Applying proper amount of biochar(1%,2%)significantly improve the utilization efficiency of snowmelt in spring,while applying excessive amount of biochar(3%)reduce itTo sum up,the proper biochar application amount will significantly improve soil physical properties,improve the snowmelt utilization rate and realize the efficient utilization of agricultural water and soil resources.Based on the improvement effect of soil physical properties and soil moisture requirement during spring tillage and sowing,it is suggested that the suitable biochar application amount range to seasonal frozen farmland soil is between 1%-2%.The farmland soil can form a good self-regulation mechanism of "drought can preserve moisture,waterlogging can retreat moisture".The research results can provide significant theoretical and technical support for alleviating the impact of waterlogging disasters in spring in cold regions,making rational use of frozen soil moisture-snowmelt resources,and ensuring the health of agricultural water and soil environment and food security.