Objective To address the prominent problems of loose structure, weak water-retention capacity, and high susceptibility to hydraulic erosion in aeolian sandy soils of arid regions, aeolian sandy soil from a typical sandy area in China was selected as the research object. This study investigates the enhancing effects of combined application of bentonite and cotton straw fiber on the physical properties of aeolian sandy soil and its significance for soil and water conservation.

Methods Aeolian sandy soil was mixed with different proportions of bentonite and cotton straw fiber. Soil bulk density, capillary water-holding capacity, total porosity, and saturated water content were determined using the cutting ring method. Soil water-retention performance and structural stability were evaluated through dry–wet cycle tests, during which water-retention dynamics were monitored by daily weighing in the drying process, and soil hardness was measured after the first and second drying stages. In addition, the microstructure of the amended soil was examined by scanning electron microscopy (SEM).

Results 1) Under the same bentonite application rate, soil bulk density decreased by 14.66%–17.62% with increasing cotton straw fiber content, while the capillary water-holding capacity, total porosity, and saturated water content increased by 20.96%–24.59%, 5.00%–8.01%, and 25.72%–27.50%, respectively. 2) Cotton straw fiber significantly enhanced the water-retention capacity of aeolian sandy soil, with fine cotton straw fiber exhibiting a stronger enhancing effect than coarse ones. The optimal water-retention performance was achieved at addition rates of 1.2% and 1.5%. 3) Bentonite improved soil structural stability by enhancing interparticle bonding, resulting in an increase in soil hardness of 476.13%. However, after the second dry–wet cycle, soil hardness decreased significantly, with a maximum reduction of 84.16%. 4) SEM observations revealed that bentonite enhanced soil compactness by coating aeolian sandy soil particles and filling interparticle pores, while the addition of cotton straw fiber further optimized the soil structure through a bridging effect.

Conclusions The combined application of bentonite and cotton straw fiber effectively improves the physical structure of aeolian sandy soil and significantly enhances its water-retention and erosion-resistance capacities. This composite amendment demonstrates good ecological applicability and engineering potential, providing technical support and theoretical basis for slope stabilization, soil and water conservation, and ecological restoration projects in arid regions.