Roles of soil amendments in the water and salt transport of coastal saline soils through regulation of microstructure

Increasing scientific knowledge on the improvement of coastal saline soils is critical for spatially expanding coastal development. Biochar and polyacrylamide (PAM) are popular soil amendments, however, it remains unclear how they affect water and salt transport by regulating soil microstructure characteristics. In this study, we conducted a five-year rice barrel trial and investigated the changes in the aggregates and microstructure of saline soils after adding biochar with three different application rates (B1 = 0%, B2 = 2%, and B3 = 5%, mass ratio) and PAM with three different application rates (P1 = 0%, P2 = 0.4 parts per thousand, and P3 = 1.0 parts per thousand, mass ratio), and simulated the water and salt transport. Results showed that at B1 and B2 treatments, soil mu-CT porosity in 2020 increased by 89.8% and 208.0%, respectively, with respect to that in 2016. The development of soil mesopore structure was promoted at B2 treatments, whereas the P2 and P3 treatments promoted the development of the soil macrostructure. Compared with those of the blank control, soil internal mean water flow rate increased by 22.2% at B2 treatments and 69.2% at P2 treatments, respectively. However, their increases were less pronounced at B3 treatments and the water flow rate decreased by 50.5% at P3 treatments. It might be reasonably attributed to the reason that porous biochar helped the formation of soil pore structure while an excessive amount of biochar blocked soil pores. Furthermore, PAM amendment helped to form soil aggregates while an excessive amount of viscous PAM might block soil pores or form a viscous layer. The time corresponding to the maximum salt concentration was negatively correlated with soil mu-CT porosity (R2 = 0.27) and pore connectivity density (R2 = 0.29). Our findings indicate that appropriate amounts of biochar and PAM can help improve saline soil structure in coastal areas, improve their hydraulic properties, and alleviate salt stress.