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.

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.

Boreal forest regions are a focal point for investigations of coupled water and biogeochemical fluxes in response to wildfire disturbances, climate warming, and permafrost thaw. Soil hydraulic, physical, and thermal property measurements for mineral soils in permafrost regions are limited, despite substantial influences on cryohydrogeologic model results. This work expands mineral soil property quantification in cold regions through soil characterization from the discontinuous permafrost zone of interior Alaska, USA. Values extend beyond the range of prior measurement magnitudes in analogous regions, highlighting the importance of this data set. Rocky and silty upland soil landscape classifications and wildfire disturbance provided guiding frameworks for the sampling and analysis for potential implications for the hydrologic response to thawing permafrost. Bulk density (rho(b)), soil organic matter, soil-particle size distributions (sand, silt, and gravel fractions), and soil hydraulic properties of van Genuchten parameters alpha and N had moderate evidence of differences between silty and rocky classifications. Burned and unburned sites had only moderate evidence of differences for silt fraction. Field-saturated hydraulic conductivity (K-fs) was more variable at burned sites compared to unburned sites, which corresponded to observations of greater rooting depths at burned sites and observations of root paths in soil cores for K-fs measurement. Soil thermal properties suggested that gravel content may reduce the accuracy of commonly used estimation methods for thermal conductivity. This work provides soil parameter constraints necessary for hypothesis testing and site-specific prediction with cryohydrogeologic models to examine controls on active layer and permafrost dynamics in upland boreal forests.