Micro-CT assessment on the soil structure and hydraulic characteristics of saline/sodic soils subjected to short-term amendment

Salt-affected soils have a poor structure and are prone to swelling and crusting, that negatively affect water and salt migration processes. Biochar and polyacrylamide (PAM) have been widely used to improve this soil structure, but their effects on soil hydraulic properties remain unclear. In this study, field and indoor culture experiments were established with three straw biochar rates (0, 2 and 5%, w/w) and three anionic PAM rates (0, 0.4 and 1‰, w/w) to study their short-term amendment effects on soil structure and soil hydraulic properties. The results based on X-ray micro-CT showed that soil porosity with a 5% biochar amendment was greater than those with 0% and 2% biochar treatments. The saturated hydraulic conductivity (Ks) significantly increased with increasing PAM content, and the Ks was greater with 2% and 5% biochar contents than that with 0% biochar content. The saturated water content (θs) increased with the increase in PAM content, whereas the change in θs was insignificant with the increase in biochar content. With the increase in biochar and PAM contents, the macroaggregate (>0.25 mm) content significantly increased, but the synergistic effects of biochar and PAM on the macroaggregate content were not significant (P > 0.05). θs significantly increased (P < 0.05) with the increasing CT-identified porosity. The reciprocal of air entry value (α) significantly decreased with increase in mean pore diameter and 3D fractal dimension. The 5% biochar + 1‰ PAM addition was the best amendment in our study. This study informs the approach to structural amelioration on saline soil in response to amendment measures, and the changes in behaviors that moderate the hydraulic response.