Experimental Study On The Effect Of Soil Mixing Medium On Brackish Water Evaporation And Salinity Distribution

In order to explore the influence of soil mixing medium on brackish water evaporation and hydrothermal salt migration,centrifuge test and laboratory soil column simulation test were used,and biochar（wheat straw）and water retaining agent（PAM）were selected as mixing medium.Two kinds of mixing thickness（10 cm,20 cm）,three biochar mass addition ratios（B₀,B₂,B₄）,three water-retaining agent mass addition ratios(S₀,S_0.08,S_0.1)and four brackish water salinity degrees（W₀,W₁,W₃,W₅）were set.To explore the effects of brackish water irrigation with two soil mixing media on soil water holding capacity,evaporation and water,heat and salt transport.The results show that:（1）Soil water holding capacity and pore size distribution are affected by mixing medium and brackish water salinity.When biochar was applied with brackish water,soil water holding capacity first increased and then decreased with the increase of brackish water salinity,and the proportion of larger and smaller pores in soil increased.When applying brackish water with water retaining agent,soil water holding capacity decreased with the increase of brackish water salinity,and the proportion of large pores decreased while that of small and medium pores increased.Van-Genuchten model can accurately describe soil water characteristic curves of different treatments,and water-retaining agent is superior to biochar in improving soil water holding capacity.（2）Soil cumulative evaporation is affected by mixing medium,mixing amount,mixing thickness and brackish water salinity.Mixing medium can inhibit brackish water evaporation to a certain extent,while salinity can promote soil water evaporation.When biochar is mixed with soil,brackish water evaporation can be inhibited by different treatments.The brackish water evaporation can be better inhibited with the increase of biochar mixing thickness or the decrease of biochar mixing thickness or the increase of biochar mixing amount.When biochar and brackish water act synergically,the treatment of 2%biochar with a thickness of 20 cm and brackish water with a salinity of 3 g/L has the best effect on inhibiting evaporation.When water retaining agent is mixed with soil,brackish water evaporation can be better inhibited with the increase of the thickness and the decrease of the mixing amount of water retaining agent or the decrease of the thickness and the increase of the mixing amount of water retaining agent.When the mixing amount of water retaining agent and the mixing thickness increased at the same time（mixing amount 0.1%,mixing thickness 20 cm）,the soil water evaporation was promoted.When the water-retaining agent and brackish water act synergically,the water-retaining agent with mixing amount of 0.08%,mixing thickness of 20cm and brackish water with salinity of 1 g/L have the best effect on inhibiting evaporation.The fitting result of Rose model is better,which is more suitable to simulate the evaporation process of soil water with brackish water mixed medium.（3）The effect of soil biochar mixing on soil average mass moisture content was better than that of water retaining agent.When the soil mixing medium was water retaining agent,the water holding capacity of brackish water treatment with salinity of 1 g/L was better than other treatments.When the soil mixing medium was biochar,the water holding capacity of brackish water treatment with salinity of 3 g/L was better than other treatments.Biochar and water-retaining agent can reduce soil temperature fluctuation and maximum temperature under a certain proportion of soil mixture,and the daily variation of topsoil temperature is positively correlated with cumulative evaporation.Compared with mixed water-retaining agent,mixed biochar can better reduce the fluctuation of soil temperature and make the daily variation of soil temperature reach the peak earlier.The peak value of soil temperature fluctuation under the mixing of water-retaining agent is basically similar to that of brackish water alone,but excessive application of water-retaining agent（mixing amount 0.1%,mixing thickness 20 cm）will lead to the rise of topsoil temperature.Soil salinity is affected by mixing medium and brackish water salinity.Soil salinity increases with brackish water salinity increasing.Both biochar and water-retaining agent can inhibit salt surface accumulation well,but the effect of biochar on salt migration to soil surface is better than that of water-retaining agent,and the effect of biochar with a mixing amount of 2%and a mixing thickness of 20 cm is the best.According to the comprehensive comparison of various indexes of different treatments,under the conditions of biochar mixing thickness of 20 cm,mixing amount of 2%and brackish water salinity of 3 g/L,brackish water evaporation can be effectively inhibited,which plays the role of water conservation and salt inhibition.It is more suitable for farmland application,and provides a theoretical basis for better utilization of brackish water irrigation in arid areas of Northwest China.