The hydraulic effect of plant roots reduces precipitation infiltration and enhances shallow slope stability. However, after root death and decay, soil permeability increases while water-retention capacity decreases. The time-varying mechanisms governing the hydraulic properties of root-soil composites after root decay remain unclear. This study examines the evolution of soil pore structure following root decay. A time-varying soil water retention curve (SWRC) model was developed to characterize changes in water-retention capacity. Additionally, a time-varying saturated infiltration coefficient model and a permeability coefficient prediction model were established to describe variations in hydraulic properties. A one-dimensional soil column infiltration test was conducted on root-soil composites at different stages of root decay to investigate the time-dependent changes in hydraulic properties. The reliability of the proposed models was validated using experimental results. The findings indicate the following: After root death, root biomass, diameter, length, and number decreased with increasing decay time, stabilizing after four months. Root decay led to a reduction in root volume ratio, which altered soil structure and enhanced the permeability of root-soil composites. Longer decay periods increased soil porosity, modifying the soil water characteristic curve and reducing water-retention capacity. Creeping roots decayed more significantly than fibrous roots due to their distinct morphological traits, making changes in hydraulic properties more pronounced in the topsoil. Therefore, plant root decay negatively affects soil hydraulic properties by continuously altering soil pore structure. These findings provide a crucial foundation for understanding the time-dependent mechanisms of hydraulic property variations in root-soil composites during plant root decay.

Background and aims The changes in soil physical properties caused by root exudates depend largely on the chemical composition of root exudates. Our aim was to explore the effects of non-specific root exudates on the physical properties of soil change. Methods Five sugar compounds, five amino acid compounds, and five organic acid compounds were selected and added to loess as three single addition treatments (amino acids, organic acids, and sugars) and four combined addition treatments (amino acids + organic acids, amino acids + sugars, organic acids + sugars, and amino acids + organic acids + sugars). Soil water repellency, aggregate stability, and shear resistance tests were performed on the loess. Results The treatments sugars, amino acids, and amino acids + sugars significantly increased soil water repellency. In addition, organic acids + sugars maximised mean weight diameter (MWD), geometric mean diameter (GMD) and the content of > 0.25 mm water-stable aggregates (R0.25), and minimised the percentage of aggregates destroyed (PAD) in the addition treatments. All treatments except for amino acids significantly increased soil shear strength and cohesion of the loess. Amino acids, amino acids + sugars, and amino acids + organic acids + sugars significantly increased the internal friction angle. Conclusion The single addition treatments had a higher effect on soil hydraulic properties, while the combined addition treatments had a higher effect on soil mechanical properties. Sugars and amino acids substantially increased soil hydraulic stability. Sugars combined with other compounds, especially with organic acids, significantly improved soil mechanical stability.

Agricultural soils are often affected by compaction due to machinery loads, which alters pore-size distribution and thus hydraulic properties. Up to date most studies on traffic and its impact on soil functions lack a detailed analysis of the effect on pore-size distribution (PSD). Our study aimed to understand how different machinery types, load levels, and moisture conditions impact the water retention curve (WRC) and PSD at various soil depths and field areas (headland or inner field). Eight field campaigns were conducted between 2016 and 2019 on a variety of sub-fields within one agricultural farm site with a clayey-silty soil. Undisturbed soil samples were collected before and after the harvest of winter wheat, silage maize, and sugar beet, and before and after digestate application. The van Genuchten model was fitted to the laboratory data, and parameters were interpreted to deduce WRC features. Additionally, the pore water pressure head at the pore-size density maximum (PSDmax) was determined and interpreted. The parameter alpha responded to all types of field traffic and decreased with increased load, indicating a shift from coarser to finer pores. The parameter n generally increased due to field traffic, suggesting a narrowed pore-size distribution. The theta s parameter, associated with porosity, decreased in all trials, with the tendency of lowest values occurring after wheeling under moist conditions. Load-induced shifts in the PSDmax towards finer pores were obvious down to 50 cm depth, even with relatively low loads. Our findings indicate that the majority of vehicles utilized in conventional agricultural operations can lead to severe soil compaction.

This study was designed to assess the potential impact of microplastic (MP) pollution on soil hydrology, specifically in retaining and releasing moisture. Herein, High-Density Polyethylene (HDPE) MP of different sizes (i.e., 0.5-1, 1-3, and 3-5 mm) and shapes (i.e., fiber, film, and fragment) were evaluated for their effects on water retention curve (WRC) of sandy loam soil, chosen for its agricultural relevance and widespread environmental presence of HDPE. Nine contamination scenarios were simulated with a low MP pollution rate, 0.01% w/w. Van Genuchten models were used to assess plant available water (PAW), wilting point (WP), and water holding capacity (WHC). Results showed that studied MP could significantly affect WRC and PAW mainly by changing WHC rather than WP and that this effect varied with MP shape and size. According to the results, fragment MP had the greatest impact on soil WHC by increasing 36.3%, followed by fibers and films by 19.8% and 15.7%. MP particles significantly increased WHC, while WP remained relatively unchanged. An observed trend indicated that the impact on WHC increased with the size of the MP particles. These findings emphasize the need to manage soil MP pollution to protect plant growth, agriculture, and water dynamics.

Understanding the reactivation causes of ancient landslides is imperative for the prevention of landslides. However, the reasons for the reactivation of thick loess-mudstone ancient landslides and evolutionary mechanisms are unclear. This paper investigates the Gaojiawan thick loess-mudstone ancient landslide as an example using field investigation, InSAR time series analysis, and laboratory testing methods to analyze the reactivation deformation characteristics and reactivation causes of the thick loess mudstone ancient landslides, which were and verified by numerical simulation. The results show that fault fracture zones and groundwater primarily control the reactivation of Gaojiawan's thick loess-mudstone ancient landslide. Due to the fragmentation of rock mass and the development of structural planes in the fault fracture zones, as well as the excavation and unloading zone formed by the surrounding rock of the tunnel, it is beneficial to the enrichment of groundwater. It intensifies the interaction of groundwater-rock-fault fracture zones, especially for the red mudstone with more clay mineral content. The strength degradation is significant after encountering water, resulting in an imbalance in the stress state in deep strata and the reactivation of the landslide.

Hydraulic conductivity (K) is a crucial parameter in hydrogeology but is highly heterogeneous and anisotropic due to variations in sediment texture, making its large-scale estimation challenging. Traditional laboratory and empirical methods based on grain-size distribution (GSD) analysis from limited data provide local K measurements, resulting in a poor representation of aquifer heterogeneity. In contrast, pumping tests estimate an integrated K value over a of the aquifer within the cone of depression but still lack the spatial resolution needed to reveal detailed variations in K across larger aquifer extents. In this study, the Di models method was used to simulate local GSD in three-dimensional (3-D) detrital systems. The focus was to explore the potential to estimate K through simulated particle-size fractions derived from a 3-D geological model of the City of Munich. By employing log-cubic interpolation, a complete and accurate representation of the fictive GSD enabled the application of multiple empirical relationships for K estimation. The resulting 3-D K fields preserved the variability in K within each aquifer system. When averaged for each separate aquifer system across different lateral extents, i.e., 50-150 and 550 m, the predicted K values showed success rates of 44-47% with deviations of at least one order of magnitude in 15-19% of cases when compared to 364 K values derived from pumping-test data. The results highlight the ability of the approach to successfully estimate K while accounting for spatial heterogeneity, suggesting its potential for groundwater modeling, aquifer yield assessments and groundwater heat pump system design.

The low liquid limit silty soil in the North China plain area is generally unsuitable for direct use as roadbed and slope soil. In order to improve the performance of low liquid limit silty soil, xanthan gum was used as an improver. Through a series of tests, the improvement effect of xanthan gum on low liquid limit silty soil was studied. The test results showed that Xanthan gum as an improver could significantly improve the unconfined compressive strength of silty soil. With the increase in dosage and curing age, the unconfined compressive strength of improved silty soil continued to improve and eventually tended to stabilize. The optimal dosage and curing period were 2% and 7 days, respectively. In addition, Xanthan gum could greatly improve the permeability and disintegration of low liquid limit silty soil. The permeability coefficient of improved silty soil with a content of 0.75% Xanthan gum and a 7-day curing period was 4.73 x 10-4 ms-1, which was only 1.10% of that of plain silty soil at the same curing period. After immersion in water for 12 h, the soil only experienced slight disintegration. The scanning electron microscope image showed that the gel generated by the hydration reaction of Xanthan gum could improve the compactness and integrity of the soil by filling the voids, thus significantly improving the mechanical and hydraulic properties of the low liquid limit silty soil.

Investigating the impact of train-induced vibration loads on soil hydraulic properties, this study conducted experiments using a self-designed indoor soil seepage platform that incorporates vibration loads. The experiments were complemented with scanning electron microscopy to analyze the influence of train-induced vibration loads on soil hydraulic conductivity and its evolutionary characteristics under different vibration frequencies. The experimental results indicated that as the vibration frequency increases from no vibration (0 Hz) to 20 Hz, the time required for the soil volumetric moisture content to reach its peak and stabilize decreases rapidly. However, after the vibration frequency exceeds 20 Hz, the rate at which the time required for the volumetric moisture content to reach its peak and stabilize decreases slows down. Furthermore, the soil pore water pressure increases with the increase in vibration frequency. At a vibration frequency of 80 Hz, the peak value of pore water pressure increases by 105% compared to the non-vibration state, suggesting that higher vibration frequencies promote the development and acceleration of soil pore moisture migration. Additionally, as the vibration frequency increases, the soil hydraulic conductivity initially experiences a rapid increase, with a growth rate ranging from 40.1 to 47.4%. However, after the frequency exceeds 20 Hz, this growth rate significantly decreases, settling to only 18.6% to 7.8%. When the soil was subjected to a vibration load, the scanning electron microscopy test revealed alterations in its pore structure. Micropores and small pores transformed into macropores and mesopores. Additionally, the microstructural parameters indicated that vibration load decreased the complexity of soil pores, thereby speeding up the hydraulic conduction process. This, in turn, affected the hydraulic properties of the soil and established a relationship between pore structure complexity and soil hydraulic properties.

The presence of mica is considered to be why the physical and mechanical properties of micaceous soil differ significantly from those of common clay and sand, such as the difficulty in compacting micaceous soil and its high compressibility and resilience. However, despite micaceous residual soil being widespread in transportation engineering, the specific role played by mica in determining its geotechnical properties remains ambiguous, as does the underlying mechanism for doing so. Presented here is a systematic investigation of the compaction, compression, and hydraulic characteristics of residual soil with varying mica content, with scanning electron microscopy used to scrutinize the structural features of mica and the resulting microstructural changes within the soil under different compression stress levels. The findings show that adding mica to residual soil results in coarser gradation, and the distinctive flaky layered structure and interlayer pores of mica particles characterize the residual soil with high compressibility, water retention, swelling potential, and inadequate compaction. The specific effects of particle arrangement between mica and soil particles-such as bridging and ordering-contribute to the high porosity and permeability of soil. Also, the breakdown of the mica stacking structure and the degradation of bridging and ordering effects under high consolidation stresses decrease the resilience and permeability of soil, leading to further compression deformation. This study elucidates the microscopic mechanism whereby mica influences the physical and mechanical properties of micaceous residual soil, thereby contributing to enhanced insights into the compaction, compression, and hydraulic properties of such soils.

This paper aims to provide a state-of-the-art review on the effects of vegetation roots on the soil structure and soil hydraulic properties. After a thorough review of current studies, the effects of vegetation roots are summarized into four: root exudation, root penetration, root water uptake and root decay. Root exudates alter the size and stability of aggregates, the contact angle of soil, and the viscosity and surface tension of pore fluid; root exudates of crops always increase the soil water retention capacity and decrease the soil saturated hydraulic conductivity. Root penetration creates new pores or clogs existing pores during root growth, and root parameters (e.g., root biomass density, root diameter and root length density) are well correlated to soil hydraulic properties. Root water uptake can apparently increase the soil water retention capacity by providing an additional negative pressure and induce micro-fissures and macropores in the rhizosphere soil. Root decay modifies the pore structure and water repellency of soil, resulting in the increase of soil macro-porosity, soil water retention, and the saturated hydraulic conductivity or steady infiltration rate. Some of the above four effects may be difficult to be distinguished, and most importantly each is highly time-dependent and influenced by a multitude of plant-related and soil-related factors. Therefore, it remains a significant challenge to comprehend and quantify the effects of vegetation roots on the soil structure and soil hydraulic properties. Unsolved questions and disputes that require further investigations in the future are summarized in this review.