Effects of different soil amendments on saline–alkali soils by improving distribution and stability of soil aggregation

Context Saline–alkali soils in Huanghua City in China are characterized by severe compaction and poor aggregate stability, which hinder soil quality improvement and the sustainable development of agriculture in coastal areas. Aims We evaluated different soil amendments and application rates in restoring soil aggregate structure and to identify the optimal amendment formulation for ameliorating the effects of saline–alkali soil. Methods A pot experiment was conducted with 25 treatments of soil amendments that included bentonite (BT), zeolite powder (ZP), biochar (BC), a mixture of bentonite and biochemical fulvic acid (1:1, BF), and a mixture of bentonite, biochar, and biochemical fulvic acid (1:1:1, BBF). Each amendment was applied at five rates (0.25%, 0.5%, 1%, 2%, and 4% relative to soil dry mass). Soil aggregate distribution and stability were determined using dry- and wet-sieving methods, along with the calculation of relevant stability indices. Key results All amendment treatments significantly improved soil aggregate structure compared to the control (CK). Among these, BBF-3 (BBF applied at 1%) exerted the most prominent effect; it increased the proportion of dry-stable large aggregates (DR0.25) from 79.45 to 93.08% and water-stable large aggregates (WR0.25) from 0.52 to 9.59% (P < 0.05). Additionally, the mean weight diameter (MWD) increased significantly, while the percentage of aggregate destruction (PAD) and unstable aggregate index (ELT) decreased markedly. Conclusions The combined application of bentonite, biochar, and biochemical fulvic acid at an appropriate rate is more effective than single amendments in enhancing aggregate stability and resistance to structural degradation. Implications The BBF amendment offers a feasible technical approach for restoring soil structure in coastal saline–alkali soils.