Abstract:

[Objective] Red soils are widely distributed in Yunnan Province, leading to prevalent ecological issues such as soil degradation, runoff erosion, and vegetation deterioration in the region. Under alternate wetting and drying conditions formed by rainfall, high temperature, and evaporation, red soil bodies are prone to structural deformation, shrinkage cracking, surface erosion, and overall instability. This study aims to reveal the mechanism by which glutinous rice gel reconstruction affects the water retention characteristics of red soil under alternate wetting and drying conditions. [Methods] Four gradients of glutinous rice gel concentration levels were designed: 0% (control group), 0.5%, 2.5%, and 5.0%. At each concentration level, two ring cutter samples (separated by filter paper) were prepared, with three replicates for each concentration, resulting in a total of 24 test samples. Ten groups of glutinous rice gel-reconstructed soil samples with different moisture contents were prepared using the gravimetric method, with moisture content gradients evenly distributed from the air-dried state (4.1%) to the saturated state (42.0%). Five equally spaced moisture content gradients were set for each wetting-drying cycle. Two complete cycles of alternate wetting and drying were performed. Matric suction was measured using the filter paper method. By achieving moisture balance exchange between the filter paper and the soil samples, the matric suction of the soil samples was determined based on the standard relationship between the filter paper’s balance moisture content and suction values. Parameter fitting of the soil-water characteristic curve (SWCC) was performed based on the Logistic model. [Results] Alternate wetting and drying significantly influenced soil matric suction. The matric suction of plain soil decreased by 79.43%, while the glutinous rice gel-reconstructed soil exhibited notable protective effects, with the concentration level of 5.0% demonstrating optimal performance and only decreasing by 8.56%. Hysteresis analysis of the SWCC showed that glutinous rice gel effectively suppressed the hysteretic effects caused by alternate wetting and drying. At the concentration level of 5.0%, the hysteresis degrees of the first and second cycles were reduced by 80.76% and 72.42%, respectively, significantly outperforming plain soil (p<0.01). The Logistic model exhibited high fitting accuracy for SWCC (R²>0.99). Parameter analysis indicated that the 2.5% concentration level exhibited optimal water retention performance during the drying phase, while the 5.0% level performed best during the wetting phase. During alternate wetting and drying, the air-entry value and residual value of the glutinous rice gel-reconstructed soil showed regular differences. With increasing cycle numbers, the air-entry value of the sample with 5.0% concentration level decreased by only 36.64% (95% in the control), while the residual value decreased by only 20.24% and 25.43% during drying and wetting, respectively, demonstrating excellent stability. [Conclusions] The incorporation of glutinous rice gel significantly enhances the water retention capacity and matric suction maintenance of red soil, with the 5.0% concentration level demonstrating optimal performance in suppressing suction reduction, followed by 2.5% and 0.5%. Although alternate wetting and drying causes pronounced hysteresis effects in the SWCC of glutinous rice gel-reconstructed soil, higher concentration levels of glutinous rice gel significantly reduce the hysteresis degree and moisture content variation amplitude. The data reveal a significant negative correlation between glutinous rice gel concentration and hysteresis degree (R²=0.92). The 5.0% sample has the maximum hysteresis reduction and is least affected by alternate wetting and drying. The Logistic model can accurately represent the SWCC parameters of glutinous rice gel-reconstructed soil (R²>0.99). Notably, the sample of 2.5% concentration level shows optimal water retention performance during the drying phase, while the 5.0% sample shows the strongest water absorption capacity during the wetting phase, both significantly outperforming the plain soil control group (p<0.01). With increasing numbers of alternate wetting and drying cycles, both the air-entry value and residual value of the soil exhibit decreasing trends. However, the decline in the air-entry value of glutinous rice gel-reconstructed soil is significantly reduced (the 5.0% group decreased by 63% compared to the control group), and the decline rate of residual value tends to stabilize as cycle numbers increase. The residual values of the samples with 5.0% concentration level decrease by 20.24% and 25.43% during drying and wetting phases, respectively, showing optimal water retention stability. Further in-depth research is required on the degradation rate of glutinous rice gel, number of cycles, time variations, and how these affect the properties of red soil and subsequently alter its matric suction.

Key words: water retention characteristics, soil-water characteristic curve (SWCC), Logistic model, alternate wetting and drying, glutinous rice gel reconstruction, red soil