| In seasonally frozen soil areas,frequent freeze-thaw cycles are often accompanied by complex processes such as soil water and heat cycles and energy transfer,which affect soil physicochemical properties and farmland ecological environment.The soil water and heat parameters directly affect the water and energy exchange process between the soil and the atmospheric environment.The large-scale cultivation and unreasonable farming patterns in farmland have led to gradual soil degradation,and regulating the soil and water environment of frozen and thawed farmland has become a hot topic in soil habitat health research.In this study,different straw application amounts S1,S2(1%,2%),different biochar application amounts B1,B2(1%,2%)and untreated control group CK were set,and different initial water contents W1,W2,W3(15%,20%,25%)were set to explore the mechanism of different control measures on the characteristics of farmland freeze-thaw soil structure structure and hydrothermal characteristics.The main research content and conclusions are as follows:(1)Analysis of the impact of applying straw and biochar on the water and thermal status and structural characteristics of farmland freeze-thaw soil.During the freeze-thaw cycle,soil moisture gradually migrates upwards,and the phase change process of soil moisture also causes energy loss within the soil,leading to a decrease in soil temperature.The application of straw and biochar can adjust the soil water and heat status,reduce the amount of soil water migration and temperature decline,and thus reduce the damage of freeze-thaw cycles to soil structure.The freeze-thaw cycle can expand soil pores and improve soil pore connectivity.The freeze-thaw effect causes soil aggregates to break.After 20 freeze-thaw cycles,the MWD of CK,S1,S2,B1,B2 treatments at different moisture contents decreased by an average of 30.67%,23.43%,23.28%,21.11%,and 21.92%compared to the initial state.The application of straw and biochar increases the total porosity of the soil,enhances pore connectivity,reduces the proportion of small to large pores in the soil,and increases the proportion of pores in the soil.Moreover,with the increase of the amount of amendments applied,the improvement effect will gradually increase.The application of biochar has a better effect on improving pore conditions than the application of straw.Straw and biochar can also promote the formation of soil aggregates,thereby improving the stability of soil aggregates.Compared with CK,the MWD of S1,S2,B1,and B2 treatments increased by an average of 19.61%,54.97%,18.89%,and 3.97% under different moisture contents.(2)Analysis of the impact of applying straw and biochar on the hydraulic characteristics of farmland freeze-thaw soil.The application of straw and biochar will increase the slope of the soil moisture characteristic curve and improve the soil’s water retention performance.The application of straw and biochar has a promoting effect on the saturated hydraulic conductivity of farmland freeze-thaw soil.After 20 freeze-thaw cycles,compared with the control group,the saturated hydraulic conductivity of S1 and B1 increased by an average of 12.49% and 25.49% at different moisture contents.However,excessive small straw and biochar particles blocked the soil pores,causing a decrease in the saturated hydraulic conductivity of S2 and B2.After the freeze-thaw process,S2 and B2 decreased by an average of 3.65% and 25.58% compared to CK at different moisture contents.Reasonable application of straw and biochar can also increase the unsaturated hydraulic conductivity of soil,but excessive application can also reduce the unsaturated hydraulic conductivity.The appearance of soil potential energy difference caused by freeze-thaw action and the gravity effect of soil moisture itself make the soil evaporation rate show a trend of first increasing,then stabilizing,and then decreasing.Straw and biochar can reduce soil evaporation by inhibiting the upward movement of soil moisture.For the improvement of soil hydraulic characteristics,the application of biochar has a better effect on improving various hydraulic parameters than straw.(3)Analysis of the effect of applying straw and biochar on the thermal characteristics of frozen thawed soil in farmland.During the freeze-thaw cycle,the soil thermal capacity under different control modes shows a trend of first slight increase and then significant decrease.Applying straw and biochar will reduce soil thermal capacity and also slow down the downward trend of soil thermal capacity caused by freeze-thaw action.The soil thermal capacity also increases with the increase of initial moisture content.Compared with W1,the thermal capacity of W2 and W3 at different moisture contents increased by an average of 9.65% and 18.25%,respectively.During the freezethaw cycle,the soil thermal conductivity of each treatment showed a trend of first stabilizing and then slowly decreasing.However,an increase in initial moisture content would significantly increase the soil thermal conductivity.Applying straw and biochar can reduce soil thermal conductivity,and the reduction is proportional to the amount applied.In the process of soil freezing and thawing,the thermal diffusivity of soil showed a slow decline trend,and the application of straw and biochar would reduce the thermal diffusivity of soil,thus improving the soil thermal insulation performance.After 20 freezing and thawing cycles,compared with the initial state,the thermal diffusivity of CK,S1,S2,B1,B2 treatments decreased by 10.77%,9.25%,8.45%,8.07%,and-5.86%.The increase of soil moisture content will not significantly change the soil thermal diffusivity,that is,soil moisture content is not an important driving factor affecting the change of thermal diffusivity.For the improvement of soil thermal characteristics,the application of biochar has a better effect on improving various thermal characteristics parameters than straw.(4)Study on the response relationship between soil structure and soil hydrothermal parameters and the mechanism of physical calculation model of hydrothermal parameters.The relationship between soil structure characteristics and soil saturated hydraulic conductivity is linear,while the relationship between soil structure characteristics and soil thermal conductivity is Gaussian function,in which soil hydraulic conductivity and thermal conductivity are more sensitive to the change of soil pore status.The Kozeny Carman model and the method of calculating inter particle pores were used to predict soil saturated hydraulic conductivity.The RMSE values of CK,B1,and B2 treatment simulation results were 0.0689,0.1210,and 0.1015,respectively.The method based on the amount of biochar applied was used to predict soil heat capacity,with RMSE values of 0.0514,0.0464,and0.0403 for W1,W2,and W3 treatments,respectively.Using an improved de Vries thermal conductivity model to predict soil thermal conductivity,and considering the influence of soil organic matter content on the original model.The RMSE values of W1,W2,and W3 processing simulation results are 0.0272,0.0252,and 0.0259,respectively.The above three water and heat parameter models can effectively simulate the water and heat parameters of farmland freeze-thaw soil under different control modes,and can be used for predicting and calculating the soil water and heat parameters of typical farmland in the Songnen Plain.In conclusion,both straw and biochar can improve the soil hydrothermal environment,regulate the soil structure,and affect the soil hydrothermal cycle and energy transmission by changing the soil microstructure and physical characteristics.This study shows that B1 treatment has better comprehensive control effect than other treatments in improving soil structure,enhancing soil permeability and enhancing soil thermal insulation capacity.It is suggested that 1% biochar application amount be the best treatment method for soil.The research results can provide a theoretical basis for the rational use of straw and biochar in seasonally frozen soil areas,and are of great significance for improving soil structure,reducing black soil degradation,and realizing the comprehensive use of agricultural straw. |
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