Effects and microscopic mechanisms of the combined application of three biopolymers for loessial soil improvement

Objective This study aims to evaluate the effects of three biopolymers, chitosan, bacterial cellulose, and sodium alginate, on the improvement of loessial soil in engineering-disturbed areas of the Loess Plateau. The study focused on poor soil structural stability and difficulties in vegetation establishment. Methods A total of 25 treatments was established, including a control, single applications, binary combinations, and ternary orthogonal combinations. Ryegrass growth and soil structural properties were measured. Range analysis and the membership function method were used for comprehensive evaluation. Scanning electron microscopy and other techniques were also employed for microscopic characterization to explore the underlying mechanisms of soil improvement. Results The low-concentration chitosan treatment A1 (0.1%) reduced soil bulk density by 4.20% and increased the ryegrass germination rate by 20.17% compared with the control. The binary combination D2 of bacterial cellulose (0.3%) and chitosan (0.5%) increased the mean weight diameter (MWD) of soil aggregates by 202.50% compared with the control and produced the highest geometric mean diameter (GMD). The membership function analysis showed that treatment M4 had the highest overall score. Its formulation consisted of 0.2% bacterial cellulose, 0.5% chitosan, and 0.1% sodium alginate. Microscopic observations showed that the biopolymers promoted the connection of soil particles and the formation of aggregate structures through fiber bridging, hydrogen bonding, electrostatic adsorption, and surface coating. Conclusions The combined application of the three biopolymers improved soil structure and promoted ryegrass growth in loessial soil. Treatment M4 showed the best overall performance. These findings provide a basis for ecological restoration and soil erosion control in engineering-disturbed areas of the Loess Plateau.