Abstract:Biochar can mitigate the risk of soil structure degradation under brackish water irrigation, but its mechanisms of action at the micro- and mesoscale remain unclear. Selecting typical coastal saline soils in Jiangsu Province, totally four biochar application treatments (0t/hm2, 10t/hm2, 20t/hm2, 30t/hm2) were established, and brackish water infiltration experiments under 16 different electrical conductivity (EC, 0.5dS/m, 1dS/m, 3dS/m, 5dS/m) and sodium adsorption ratios (SAR, 0(mol/L)1/2, 15(mol/L)1/2, 30(mol/L)1/2, ∞ (mol/L)1/2) were conducted. The results showed that with the application of biochar, soil pH value and organic carbon content significantly increased by 0.7%~2.7% and 130.7%~675.7%, respectively, and significantly affected particle surface properties (p<0.05). The cation exchange capacity, specific surface area, surface charge density, and Hamaker constant of the soil was increased by 20.0%~88.2%, 10.7%~30.0%, 8.4%~44.7%, and 35.6%~62.8%, respectively. Reduced EC and increased SAR of the infiltration solution both enhanced surface potential and electrostatic repulsion. Additionally, with biochar application, particle surface potential and interparticle electrostatic repulsion was increased by 0.47%~17.65% and 0.16%~14.45%, respectively, under different EC and SAR solution interfaces. However, van der Waals forces were also increased by 35.61%~62.81%, resulting in a decrease followed by an increase in net repulsive force with biochar application. Compared with the 0t/hm2 treatment, the 10t/hm2, 20t/hm2, and 30t/hm2 treatments showed decreases of 9.39%~147.43%, 15.31%~219.55%, and 9.69%~217.13%, respectively. Soil porosity, connected pore fraction, and large pore (equivalent diameter greater than or equal to 1mm) fraction showed a significant negative correlation with net repulsive force (p<0.001), increasing by 7.60%~40.90%, 0.15%~34.37% and 3.78%~43.09%, respectively, as net repulsive force decreased, which increased the saturated hydraulic conductivity by 7.77%~24.70%, 15.15%~31.60%, 10.81%~28.22% under different brackish water irrigation solutions for the 10t/hm2, 20t/hm2 and 30t/hm2 treatments respectively. In summary, the interactive effects of surface properties, internal forces, pore structure, and water movement in saline soils under biochar application were investigated. The 20t/hm2 biochar treatment exhibited the optimal regulatory effect on soil structure degradation, providing scientific basis for the safe utilization of brackish water.
