Impact Of Straw Biochar On Hydraulic Characteristics Of Northeast Meadow Black Soil

Because of the serious loss of black soil in recent years,the soil fertility and water holding capacity continue to decline,threatening China's food security and sustainable development.Biochar produced by agricultural waste such as agricultural straw can be added to the soil to repair the soil,improve utilization rate and effective of soil water,and increase crop yield.This paper mainly analyzes the effect of biochar on the hydraulic characteristics of black soil in Northeastern meadow.The study included the changes of hydraulic characteristics of the soil that had been added to biochar for four years,and the effects of continuous add to biochar on the soil hydraulic characteristics for four years.The test site was selected in Hongxing Farm,Bei'an City.The combination of field experiment and laboratory test,basic theory and numerical simulation were combined to study the hydraulic characteristics of black soil with different bioch ar addition.the main findings are follows:?1?Biochar and soil have different hydraulic characteristics,The added biochar soil is composed of biochar particles and soil,and its hydraulic characteristics are different from those of soil.This paper assumes that soil and biochar fit in the single porosity model of van Genuchten?VG?and derived the hydraulic model for added biochar soil.The derivation results shown that the characteristics of added biochar soil are different from those of VG soil model,and are more fit in the porous soil model.?2?The experimental results shown that the hydraulic characteristics of the black soil of Northeastern meadow that added biochar do not satisfy the theoretically derived soil model.Under low suction,the experimental value of soil water content is significantly higher than the predicted value of soil model,it indicated that the added biochar soil had more porosity than the control soil sample.In this paper,the deduced soil model is modified based on the experimental results.When the amount of biochar was no more than 100t?hm-2,the predicted value of the modified model agrees well with the experimental measured value,and the maximum error between the two is less than 5%.The modified soil model is very close to the VG model,a single pore VG model can be used to approximate the water characteristics of the added biochar soil,and combined with the Mualem theory,the unsaturated hydraulic conductivity of the added biochar soil is given.?3?This study assumes that the different water contents of biochar and soil,under the same matrix suction are caused by capillaries of different numbers,and same pore size.By testing the biochar and soil contents curves separately,the capillary model can be used to give the total number of capillaries in added biochar soil.With the pore size and total number of capillaries,the unsaturated hydraulic conductivity of the added biochar soil is derived.Based on the capillary model method,the calculation of the unsaturated hydraulic conductivity of the a dded biochar soil can be simplified.The numerical calculation of water penetration using the capillary model shows,that the deviation between the calculated results and the tested data is less than 15%,which indicates that the capillary model can be used to calculate the water movement in the soil.?4?In the field,it was found that added biochar in the black soil of the northeastern meadow,with the increase of the amount of biochar added,the soil bulk density gradually decreased,the total soil porosity increased gradually;the soil water stable aggregate content and total organic carbon gradually increased;organic matter,available potassium,and available phosphorus also increased gradually.The test results of soil porosity show that added biochar to the soil will improve the soil structure,facilitate the transport and retention of water,and improve the water conductivity and saturated water content of the black soil in northeast meadow.Therefore,in the agricultural production,the application of biochar can improve the hydraulic characteristics of black soil in northeastern meadows.?5?From 2015 to 2018,measured many parameters of soil that added biochar in 2015,to study the long-term effects of aging biochar on meadow black soil under natural conditions.It shown that after one year,the soil properties changed,the soil water content has decreased slightly,the bulk density has increased,and the porosity has decreased.The next years,soil water holding capacity continues to decrease and the coefficient of wilting are slightly increased,which makes the plant available water reduced,and the ability of the soil to maintain and release water is gradually reduced.Aging biochar makes the combination of biochar and soil agglomerates more compact,and the pores in the soil will gradually decrease,resulting in the soil water content and water conductivity decreased with time.Although the properties of biochar are stable,the characteristics of the soil still changed over four years.?6?From 2015 to 2018,this study measured the soil parameters that added biochar sustained for four years,and studied the effect of period and rate of biochar on soil hydraulic characteristics.The results showed that after two years of added biochar,with added volume of 75 and 100t?hm-2 had a small increased in water content under the each suction,and the field water holding capacity and wilting coefficient both increased slowly.After three years of continuous added biochar,the hydraulic properties of soils with added volume of 50,75,and 100 t?hm-2 were increased small.The saturated water content,field water holding capacity and wilting coefficient of added volume of 100t?hm^-2 for four years will still show a very slow increase trend.?7?In this paper,the water movement and distribution in the soil after rainfall infiltration were numerically calculated,and explored the mechanism of water conservation for added biochar of meadow black soil.The compare results of calculated and tested value showed that the numerical calculation can give the water movement in the black soil of northeastern meadow more accurately.The calculation results can provide guidance for the application of biochar in the black soil of Northeast China.The results also show that the soil water content of the added biochar layer after rainfall increases with the increase of biochar added,but with the increase of soil depth,the added biochar has no effect on the soil water content of the bottom soil.?8?The van Genuchten/Mualem?VG-M?model is used to calculate the water movement of the aging added biochar soil.The rainfall infiltration study on the soil of northeastern meadow of indicates that sometimes the mistake occur,and the VG-M model cannot accurately describe the hydraulic characteristics of the added biochar soil;in addition,the rainfall infiltration study of four-year continuous added biochar soil shown that the effect of continuous added biochar on the hydraulic characteristics of black soil in Northeastern meadow is nonlinear,after the biochar has accumulated to a certain amount,its effect on the hydraulic characteristics of the soil is gradually weakened.