Effects Of Biochar On Aggregate Characteristics And Water And Salt Transport In Saline Soil In Southern Xinjiang

Saline soil is extensively present throughout the country.In the face of limited resources and climate change,developing saline soil is a significant approach to tackle the challenge of meeting the increasing demand for agricultural products brought by the expanding population.The paper uses agricultural waste pig manure,cotton straw,and corn straw as raw materials to produce biochar.The biochar is mixed with0.5%,3%,and 5%of the surface layer（0-10 cm）of the soil column in a saline soil aluminum box.The specific treatments are CK（without adding biochar）,0.5PMB（0.5%pig manure biochar）,0.5CSB（0.5%cotton straw biochar）,0.5MSB（0.5%maize straw biochar）,and 3PMB（3%pig manure biochar）,3CSB（3%cotton straw biochar）,3MSB（3%maize straw biochar）,5PMB（5%cotton straw biochar）,5CSB（5%cotton straw biochar）,and 5MSB（5%maize straw biochar）were used to preliminarily explore the effects of biochar types and additions on the characteristics of saline soil aggregates and water salt transport.The following conclusions were obtained:（1）Through indoor aluminum box test it was found that compared to the control group,the application of biochar can significantly improve the aggregate structure in saline soil in Southern Xinjiang.Biochar application can increase the content of 0.25-1 mm aggregate size and decrease the content of aggregate size less than 0.106 mm,thereby promoting the aggregation of micro-aggregates into macro-aggregates in saline soil.In addition,biochar treatment can also significantly increase the average weight diameter,geometric mean diameter,and content of aggregate size larger than 0.25 mm in soil aggregates,while reducing the fractal dimension of aggregates,thus improving the stability of saline soil aggregate structure.Overall,after 15 days of cultivation,0.5PMB had the best effect;After 30 days of cultivation,3MSB had the best effect;After 45 days of cultivation,5CSB showed the best effect.（2）Through indoor one-dimensional soil column infiltration tests,it was found that except for0.5PMB which reduces soil infiltration time by 2.51%compared to CK,high application rates of CSB and PMB,as well as medium application rate of MSB,can significantly increase soil infiltration time in saline soil of Southern Xinjiang and slow down the movement of the wetting front.Compared to CK,it decreased by 16.67%-38.10%.High application rates of PMB,CSB,and MSB show better effects on reducing infiltration rate.Among them,CSB is superior to other types of biochar in increasing infiltration time,reducing the distance and cumulative infiltration volume of the wetting front,and lowering the infiltration rate.The delaying effect on water infiltration increases with the application rate,which is beneficial for improving the problem of excessive water infiltration and poor water holding capacity in saline soil in Southern Xinjiang.Adding biochar increases soil surface water content.The effect of low and medium application rates of biochar on soil water content is not significant,and high application rate of CSB has the best water-holding capacity.Its average moisture content is higher than CK13.60%.A power function can well fit the relationship between the distance F and time t of the wetting front movement under different types and application rates of biochar.Overall,adding 5CSB is recommended to improve the problem of fast water infiltration and poor water holding capacity in saline soil of Southern Xinjiang.（3）The results indicate that the application of biochar significantly promotes the leaching of Ca²⁺at both P1 and P2 stages.With the exception of 0.5%PMB and 0.5%MSB treatments which suppressed the leaching of Mg²⁺in P1,P2,and P3 stages,all other treatments promoted the leaching of Mg²⁺.Among them,5%CSB exhibited the best promoting effect in P1 stage,with an increase of 86.72%compared to the CK.Except for 0.5%CSB and MSB which suppressed the leaching of K⁺in P1 and P2 stages,all other biochars increased the leaching of Na⁺and K⁺in all stages,and the leaching effect improved with an increase in biochar addition.Regardless of the mass ratio,CSB demonstrated better leaching effects on SO₄^2-than PMB and MSB at P1 and P2 stages,and 5%CSB increased the leaching of SO₄^2-by 27.45%and 23.48%compared to the same level of PMB and MSB,respectively.For HCO₃^-and Cl^-,except for the decrease and subsequent increase in HCO₃^-concentration in 0.5%MSB treatment,the concentration of leached HCO₃^-and Cl^-in all biochar treatments decreased with the decrease in electrical conductivity.Among them,5%MSB exhibited the best leaching effect in all stages,with an increase of 76.40%compared to CK in P1stage.Comparing the residual salt content in soil after leaching,a higher biochar addition led to better desalination effects and lower residual salt content in soil.Overall,the desalination rate of soil increased with the increase in biochar addition,and the optimal treatment for desalination in the topsoil（0-10 cm）was 5%MSB,which could achieve a maximum desalination rate of 67.65%.The optimal treatment for desalination in the subsoil（10-30 cm）was 5%CSB,which not only promoted the leaching of salt ions in the leachate,but also acted as a source of potassium fertilizer in soil due to its rich K⁺content.