Effects Of Biochar And Straw Application On The Migration Characteristics Of Soil Freezing Front And Infiltration Process Of Snowmelt Water

The freeze-thaw cycle unique to seasonal tundra destabilizes the physical structure of the soil and affects the snowmelt infiltration process in spring,resulting in soil prone to flooding in early spring,thus affecting crop growth and development in cold regions.Therefore,it is necessary to find an effective measure to improve the physical structure of soil and increase the efficiency of snowmelt water utilization.However,recent studies have mainly focused on the improvement effect of soil by applying a single exogenous biomass material,ignoring the differences in soil physical properties and snowmelt water infiltration processes under different regulation modes.At the same time,studies on freezing fronts have mostly focused on the changes of their migration characteristics under the freeze-thaw effect,without considering the effect of exogenous biomass material application on their migration.Based on this,this study was based on the Songneng Plain in Northeast China,and the mechanism was investigated in an indoor experiment and verified in a field experiment.In the indoor experiment,different water contents（W1:20%,W2:15%,W3:10%）and four treatments（CK:blank control group,B:biochar only,J:straw only,BJ:combined addition of biochar and straw）were used.In the experiment,the differences of soil water stability agglomerate distribution ratio and its water stability,soil total porosity（TP）,soil pore size distribution,soil mechanical structure stability,soil water characteristic curves（SWRCs）,migration characteristics of freezing fronts and melting fronts were explored by continuing the regulation treatment of exogenous biomass materials in the indoor experiment,and the infiltration process of soil was carved by using tension infiltration meter,and the effect of adding biochar and straw on the physical structure and water holding capacity of agricultural soil was analyzed,and the response of soil physical structure characteristics to the infiltration characteristics of snowmelt water was elucidated.The specific findings of the study are as follows:（1）Effect of freeze-thaw application of biochar and straw on soil water and heat distribution.The freeze-thaw action changes the spatial distribution state of soil moisture and temperature.Compared with the pre-freezing period,the thawed soil moisture gradually migrated to the middle layer of the soil and gathered near the middle layer.With the addition of biochar and straw,the water migration increased,and the BJ treatment had the most significant effect.And the water migration capacity decreased with the decrease of the initial soil water content.With the increase of freeze-thaw frequency,the water content of soil in the L1 layer decreased and water migrated to the deeper layer;in the L2 layer,the water content of soil under W1 and W2 showed a decrease and then an increase with the increase of freeze-thaw frequency,while the water content of soil under W3 showed the opposite trend.Soil temperature variation showed the same trend on each stage:at the early stage of freezing,the temperature difference of each soil layer under W1,W2and W3 was 32.9°C,33.3°C and 34.6°C,and the lower the water content,the larger the temperature difference;at the thawing stage the temperature difference of each soil layer was13.0°C,15.2°C and 21.9°C,and the trend was the same as that of freezing.After the soil experienced multiple freeze-thaw cycles,the soil temperature difference decreased and the variation decreased.However,the application of biochar and straw changed the difference of soil temperature difference,and the temperature difference of each soil layer increased by 0.7～1.7°C,0.8～1.7°C,and 0.3～0.5°C of the B,J,and BJ treatments,and the freezing rate increased by 9.68%,4.15%,and 17.07%.（2）Effect of freeze-thaw application of biochar and straw on soil physical structure.In the pre-freezing period,the addition of biochar and straw increased the content of soil large and small agglomerates,and the increase effect of the W3BJ treatment weakened by 50%as the initial water content decreased,while the effect of W3B and W3J,on the contrary,increased by 7%～13%;while in the thawing period,the addition of biochar and straw had the same effect,but the effect of each treatment weakened as the initial water content decreased.Meanwhile,the addition of biochar and straw increased the content of>0.25 mm water-stable aggregates,increased GMD and MWD values,and decreased PAD values,thus increasing the stability of soil aggregates,with the BJ treatment showing the best performance.However,the agglomerate stability was negatively correlated with the initial water content,and for W3,the addition of biochar alone was better than the combined application treatment.Application of biochar and straw changed soil pore distribution and affected pore connectivity.The application of biochar and straw increased the proportion of soil voids（>100μm）,which increased and then decreased with the increase of freeze-thaw frequency;increased the proportion of intermediate pore sizes（0.3～100μm）,which gradually increased with the increase of freeze-thaw frequency;and decreased the proportion of very small pore sizes,which gradually decreased with the increase of freeze-thaw frequency.At the same time,the initial water content of the soil decreased,and the magnitude of pore size variation in each section decreased.In addition,the application of biochar and straw improved soil TP and increased soil structural stability.TP increased with the increase of freeze-thaw frequency,but the magnitude of increase was inversely proportional to the magnitude of initial soil water content.In addition,the GSSI under W1BJ and W2BJ treatments could reach 98.06 and 98.11,which were higher than the individual treatments,while the GSSI values under the combined W3BJ treatment were lower than the individual treatments,and the unbalanced triple comparison caused soil destabilization.In addition,the application of biochar and straw increased the soil saturated water content from 4.65%to 15.16%,and the variation increased with the increase of water content.The addition of biochar and straw increased the difference in soil volumetric water content（θ（h））and enhanced soil water holding capacity,but the water holding capacity of W3BJ treatment was weakened.Also increasing the soil field water holding capacity FC reduced the permanent plant wilting coefficient PWP,thus increasing the soil plant available water content PAWC,while freeze-thaw altered the FC,resulting in a substantial increase in PAWC.（3）Effect of freeze-thaw application of biochar and straw on the migration process of freezing front and melting front in agricultural soil.In the pre-freezing period,the temperature variations of the BL,CLS,JLS,and CJLS treatments ranged from-13.81 to 12.31°C,-12.25 to 12.69°C,-11.06to 10.19°C,and-8.25 to 11.00°C.Compared with the BL,CLS,JLS,and CJLS treatments increased the migration depth of freezing front by 5.44%to 7.03%,increased the migration rate by29.63%to 64.44%,and accelerated the downward migration process of freezing front by 14 d,13 d,and 7 d,respectively,but increased the migration time to the maximum depth by 7 d,8 d,and 10 d.The addition of straw and biochar changed the relative change of moisture in the active layer,and the trend of the relative change of the surface soil changed from JLS>CJLS>BL>CLS to BL>JLS>CLS>CJLS.secondly,the relative change of moisture in each layer decreased with the increase of soil depth,and the overall moisture showed migration from deep to shallow layers.During the melting period,the temperature ranges of the BL,CLS,JLS,and CJLS treatments were-2.31～11.81°C,-2.25～13.56°C,-2.13～16.19°C,and-1.88～12.31°C.During the top-down soil melting,the addition of biochar and straw increased the melting front migration depth by3.03%-3.31%,and the CLS,JLS,and CJLS treatments shortened the duration by 25.01%,33.33%,and 54.17%,respectively,and reduced the peak migration velocity size.During the bottom-up melting of the soil,the application of biochar and straw increased the migration depth by 3.53%to6.10%,shortened the duration by 5.79%to 8.44%,and reduced the maximum migration velocity,with small changes in each migration parameter due to stable temperature changes in the deep soil.Soil moisture exhibited shallow to deep migration during the melting period,and the relative change in surface soil moisture under the BL and CLS treatments was reduced,unlike the JLS and CJLS treatments.With the increase of soil depth,the moisture in each layer was in dynamic change,but the amount of change decreased.（4）Effects of biochar and straw application on snowmelt water infiltration processes before and after freeze-thaw.The application of biochar and straw increased the cumulative soil infiltration with the effect of CJLS>CLS>JLS,where the combined application increased the cumulative soil infiltration by 151.10%.In addition,the higher the pressure value of the tension infiltrator,the smaller the trend of cumulative infiltration.Increasing the initial soil infiltration rate increased the CLS,JLS,and CJLS treatments by 19.86%to 27.66%compared to the BL treatment.After the soil experienced repeated freezing and thawing,the effect of biochar and straw weakened,and the maximum change of accumulated infiltration was only 120.39%,and the maximum change of stable infiltration rate could reach 0.06 cm/min,but the change of initial infiltration rate increased and improved by 59.00%～94.25%.The correlation analysis proved that MWD,WR_0.25,intermediate section pore size and soil infiltration characteristics were significantly related,and the fitting results of the typical infiltration model showed that the HORTON model was applicable to the soil with the addition of exogenous media materials.