Effect Of Biochar On Water And Solute Transport In Coastal Saline Soils

Coastal-saline land is one of the obstacle soils that urgently need to be improved and regulated in the eastern area of China.The key in improving coastal saline land is to suppress salt and alkali and enhance the ability of saline soil to conduct water and wash salt.Because of its rich pore space,large specific surface area and stable physical and chemical properties,biochar is often applied to soil as soil conditioner material to optimize soil structure and improve soil microcosm environment.Therefore,the use of biochar to improve coastal saline soils is receiving more and more attention.However,the application of alkaline biochar to coastal saline soils is controversial,especially the effect of biochar application on water and salt transport in coastal saline soils is not well understood.In this study,we selected a typical coastal saline soil in Daishan County,Zhoushan City,Zhejiang Province,and analyzed the effects of different additions of straw char（0%,0.3%,0.5%,0.8%,1%、3%,5%,and 8%by mass fraction,respectively）on the hydraulic conductivity of a typical coastal saline soil through an indoor soil column simulation infiltration experiment.The migration of ions and nutrient loss in coastal saline soils under different charcoal additions were followed up.The results showed that:（1）biochar can improve the water-holding and hydraulic conductivity of the soil.The water permeability of coastal saline soils showed a trend of increasing and then decreasing with the increase of biocharcoal application.When the application amount of biocharcoal<1.0%,the application of biocharcoal helps to improve soil water hydraulic conductivity;when the application amount of biocharcoal>1.0%,the application of biocharcoal causes the reduction of water infiltration rate of the test soil and hinders the soil hydraulic conductivity,but the trend of improvement is shown again with the increase of charcoal application amount within this stage.The application rate of 1.0%is an important reference threshold for biocharcoal to affect water holding and hydraulic conductivity of coastal saline soils.The simulation results using the Philip soil water infiltration model indicated that the change in soil moisture absorption was a key factor in the hydraulic conductivity of the coastal saline soil by biocharcoal.The effect of biocharcoal addition on the saturated hydraulic conductivity of coastal saline soil also showed an increase followed by a rapid decrease,similar to the effect on infiltration process.The addition of biocharcoal can significantly improve the water holding capacity of coastal saline soils.The water holding capacity of the soil increased with the increase of biocharcoal application,but the excessive addition of biocharcoal reduced the water release efficiency of soil pore space and promoted the tendency of soil pore space distribution to be concentrated in small scale,which was not conducive to improving the hydraulic conductivity of coastal soils.（2）The measurement of alkaline cations at different depths after infiltration showed that biocharcoal could promote the migration of salt ions downstream in the soil.Biocharcoal was able to reduce soil salinity in the middle and upper soil layers,but intensified the salt accumulation in the lower soil layer Adding biocharcoal was able to reduce p H and conductivity in the middle and upper soil layers,but intensified the p H and conductivity in the lower soil layer.The average Na⁺content of coastal saline soils was reduced to different degrees after the addition of biocharcoal,and the soil Na⁺content increased with the depth of soil column.The results of Ca²⁺and Mg²⁺contents after infiltration showed that the content of biochar itself cations would increase the content of salt ions in the soil under the condition of high addition of biochar.（3）Biochar can increase the nutrient content in the soil,but the excessive addition pushed the nutrient loss.Except for the B_0.8 treatment,the TN content tended to increase with increasing soil depth;however,the mean values of TN content at different depths in the soil column were slightly lower in the B_0.8 and B₃ treatments;the soil TP content increased to different degrees compared with B₀,and the TP content was higher in the middle soil layer;the mean values of total soil potassium tended to decrease under the low amount of biocharcoal treatment,while it tended to increase under the high amount of adult charcoal treatment;With the increase of biocharcoal addition,the overall SOM content of different treatments showed a continuous trend of increase.All the treatments with biomass carbon addition reduced the leaching of TN.In the early stage of leaching,the amount of TN leaching in the low biochar addition treatment was much larger than that in the high biochar addition treatment,so the leaching effect of the low biochar addition treatment was more significant in the early stage of leaching.