Effects Of Biochar Addition On Hydrothermal Changes Of Loessial Soil

The Loess Plateau soil is poor,water resources are scarce,and soil erosion is serious.Biochar is a kind of special organic matter.As a soil additive,it plays an important role in improving soil quality and improving soil quality on the Loess Plateau.At present,the researches on the effects of biochar on soil properties are relatively concentrated,but the mechanism of biochar on the changes of water and heat of yellow spongy soil under the condition of long-term field fixed point observation remains unclear.This study takes yellow spongy soil as the research object.Based on the long-term continuous monitoring data obtained from field fixed point observation,the daily average soil moisture content and daily average soil temperature of yellow spongy soil under different biochar addition amounts are taken as the entry point,and combined with the hydrothermal characteristic parameters of yellow spongy soil,Considering the dynamic change characteristics of soil moisture and temperature under the treatment of different amounts of biochar after typical rainfall and drought events,the leading factors of the change of water and heat of yellow caviar soil after biochar addition were identified,and the influence mechanism of biochar regulating the water and heat characteristics of yellow caviar soil was clarified.It provides a scientific basis for the rational utilization of biochar to improve degraded soil and enhance soil productivity on the Loess Plateau,and has important significance for the high-quality sustainable development of agriculture on the Loess Plateau and dryland agriculture.The main research results are as follows:1.The effect of biochar addition on soil moisture was clarified.(1)The addition of biochar increased the soil volume water content in each soil layer,and the water content in the surface soil increased more than that in the deep soil.Biochar improved the water holding capacity of soil,and the water storage in 0-40 cm soil layer increased after the addition of biochar.(2)After adding biochar into the soil,the soil infiltration rate is accelerated,and the cumulative infiltration rate is increased.With the increase of biochar addition,the initial infiltration rate of soil decreased first and then increased,the stable infiltration rate decreased,and the average permeability increased.(3)In this study,van Genuchten model showed a good fit on the soil water characteristic curve,and water movement parameters α,θs and θr showed an obvious positive correlation with the amount of biochar added.(4)Biochar enhances soil water supply capacity and drought resistance,and can quickly infiltrate 0-40 cm soil layer for water supply after rainfall,which is not easy to form runoff on the soil surface,and can effectively prevent soil erosion and other problems.Under drought stress,the water retention and water retention characteristics of biochar can make it retain more water in the topsoil.2.The effects of biochar addition on soil temperature and thermal properties were clarified.(1)The variation trend of soil temperature in each layer is consistent with that of atmospheric temperature,but the temperature change of deep soil has a certain lag.Biochar can increase the average daily temperature of soil,the maximum temperature of surface soil and the minimum temperature of deep soil,which is conducive to the growth and development of crops.(2)The application of biochar can maintain the stability of soil temperature at night,avoid the rapid cooling of soil,has the effect of soil insulation,and can prevent crops from freezing damage.(3)Rainfall can significantly reduce soil temperature,especially the soil temperature on the surface.Biochar can play a role in soil heat preservation under light rain and moderate rain,reducing the temperature range of soil.Soil thermal conductivity and soil thermal diffusivity decreased with the increase of biochar content.(4)When soil water content is high,soil temperature is difficult to rise under the influence of specific heat capacity;when soil temperature is low,the evaporation rate of soil water decreases.Therefore,biochar reduces the temperature rise rate of yellow soil by affecting soil water content.3.The main controlling environmental factors of soil water and heat after adding biochar were identified(1)The main controlling factors affecting soil moisture change are rainfall,atmospheric humidity,air temperature and air pressure.Among them,rainfall,atmospheric humidity and air pressure can promote the increase of soil moisture,and the increase of rainfall and atmospheric humidity will directly increase the soil moisture content of surface soil,and then increase the water content of underlying soil with the infiltration of water.The increase of air pressure will restrain the evaporation and diffusion of water and thus increase the soil water content,while the increase of air temperature will promote the evaporation of soil water and decrease the soil water content.After addition of biochar,the sensitivity of soil moisture to rainfall and atmospheric moisture decreased,but that to air temperature and air pressure increased.(2)The main controlling factors affecting soil temperature change are wind speed,air temperature and air pressure.The increase of wind speed and air pressure will reduce soil temperature,while the increase of wind speed will increase ground turbulence and promote evaporation heat consumption,thus reducing surface temperature.The increase of air pressure will increase soil moisture content,so that soil temperature needs to absorb more heat,and the increase of air temperature will increase soil temperature.After the addition of biochar,the sensitivity of soil temperature to rainfall and atmospheric humidity was weakened,and that to wind speed was enhanced.The sensitivity of upper soil temperature to air temperature was enhanced,and that of lower soil temperature to air temperature was weakened.4.A one-dimensional numerical model of vertical soil water and heat transport was constructedA one-dimensional vertical soil water and heat transport model was established based on the Hydraus-1D software to simulate the water infiltration rule of yellow soil under rainfall infiltration and soil temperature variation during the growth period.The model was verified by real-time monitoring of 0-40 cm soil layer water content and temperature data.The results showed that: The predicted results were in good agreement with the actual monitoring results.The RMSE of measured and simulated soil water content ranged from0.07-0.60,NSE was above 0.88,and R2 was above 0.78.The RMSE of measured soil temperature and simulated soil temperature ranged from 0.06 to 2.21,NSE was above 0.7,R2 was above 0.88,and the evaluation indexes were all within the reasonable range,with good coincidence effect and high simulation accuracy.Hydraus-1D software can simulate the migration law of soil water and heat under the condition of biochar.For the model of soil water transport,the increase of soil depth will make the model more accurate,but the increase of biochar will lead to the decrease of the model accuracy.For the soil heat transport model,the accuracy of the model simulation increased with the increase of soil depth,and the influence of biochar on the simulation results of the heat transport model was small.