Soil Hydrophobicity Research Articles

A machine learning framework to measure Water Drop Penetration Time (WDPT) for soil water repellency analysis

The heat from wildfires volatilizes soil’s organic compounds which form a waxy layer when condensed on cooler soil particles causing soil to repel water. Timely assessment of soil water repellency (SWR) is critical for prediction and prevention of detrimental impacts of hydrophobic soils such as soil erosion, reduced availability of water to plants, and water runoff after rainfalls leading to floods. The Water Drop Penetration Time (WDPT), i.e., the time elapsed from a drop landing on the soil surface to its complete absorption is commonly used to assess the SWR level. Its manual measurements have variability based on the used instruments and subjective observations. The goal of this work is to design an automated system to perform standardized WDPT tests and assess the SWR levels. It consists of an electronically controlled mechanism to release a water drop, and a video camera to record the water penetration process. The latter is modeled as an “action” in video and Temporal Action Localization (TAL) analytics is used for predicting the WDPT and assessing the SWR level.

The relations between soil hydrophobicity and vegetation in abandoned arable fields on sandy soil

The relations between soil hydrophobicity and vegetation in abandoned arable fields on sandy soil

Research on the treatment and utilization of hydrophobic soil in sponge city construction

This paper provides a comprehensive analysis of the characteristics of hydrophobic soil, including its strong hydrophobicity, poor water retention capacity, and unstable mechanical properties. It explores treatment technologies for hydrophobic soil such as soil amendments, bioremediation techniques, and physical treatment methods. Based on these findings, the paper proposes the utilization of hydrophobic soil as permeable paving material, filling material for rain gardens and ecological retention ponds, covering layer for green roofs, and for ecological slopes and dikes. These suggestions provide theoretical support and practical guidance for the effective use of hydrophobic soil in sponge city construction.

Atmospheric Solid Fallouts as a Source of Hydrophobicity of Urban Soils and Material for Their Formation

Atmospheric Solid Fallouts as a Source of Hydrophobicity of Urban Soils and Material for Their Formation

Effects of different tillage and cropping systems on water repellency and hydraulic properties in a tropical Alfisol of southwestern Nigeria

This study examined the effects of growing intercrops (Sorghum, SOR (Sorghum bicolor L. Moench) in between rows of cowpea, COW (Vigna unguiculata) and soybean, SOY (Glyxine max L.Merr)) under different tillage systems on soil physico-chemical properties, soil water sorptivity (Sw), hydraulic conductivity (K), and soil hydrophobicity (R) in tropical Alfisol of Southwestern Nigeria. Results demonstrated cropping systems significantly impacted soil pH under each conventional tillage (CT), no tillage (NT), and compacted no till systems (NTC) tillage system in 2019, but not in the 2020 cropping season. Sorghum-cowpea SC) and sorghum-soybean (SS) intercropping systems under the three tillage systems had higher soil organic matter (SOM) contents than the corresponding sole sorghum (SOR) at the end of the study. Soil water sorptivity differed significantly (p < 0.001) among the cropping systems in 2019 and follows the order: SC > SS > SOY > SOR > COW while in 2020, averaged over three tillage systems, mean Sw values showed an increasing trend in the order: SOR (77.67 cm h-1/2) < COW (82.42 cm h-1/2) < SOY (88.00 cm h-1/2) < SS (88.73 cm h-1/2) < SC (98.29 cm h-1/2). In general, intercropping had a lower R than sole cropping. NTC soil was 46.48% and 56.39%; 32.47% and 41.67% more hydrophobic than soils under NT and CT in 2019 and 2020. Intercropping with legumes might have contributed to the higher K levels. Organic matter in soil improves conductivity by improving soil structure. The intercrop may have improved soil water conservation because of their shading and better soil protection. The study demonstrated that conservation tillage with intercropping could effectively enhance soil hydraulic properties.

Stabilization of micaceous residual soil with industrial and agricultural byproducts: Perspectives from hydrophobicity, water stability, and durability enhancement

Micaceous residual soil is prevalent in the tropical regions of southern China. However, it is generally deemed unsuitable as a construction material in earthen structures due to its inferior engineering properties. Despite previous research into stabilizing micaceous soils, there remains an urgent demand for sustainable alternatives to traditional stabilizers to achieve circularity. Furthermore, uncertainties regarding the long-term performance of stabilized soils under hydraulic conditions complicated their use in flood-susceptible areas. To provide a sustainable and low-cost solution for creating stabilized soil for earthen structures in waterlogged regions, this study explores the impact of coir fiber and fly ash, agricultural and industrial byproducts, on the hydrophobicity, durability, and water stability of micaceous residual soil from Hainan, China. The study commenced with the preliminary mechanical tests to ascertain the optimal dosage of coir fiber and fly ash, which were determined to be 1% and 15%, respectively. Then, hydrophobicity, durability, and disintegration tests are conducted on fiber-reinforced fly-ash-treated soil under various stabilization conditions. The microstructural evolution of the soil fabric in response to the stabilization process was meticulously traced using scanning electron microscopy and energy-dispersive spectroscopy. Results indicated that the addition of fly ash could impart severe and persistent water repellency to the studied soil, with the contact angle improving from 16.7° to 102.9°, and the water drop penetration time significantly extending from 3.6 s to approximately 600 s. While incorporating coir fibers into the cemented soil presented negligible improvement on soil hydrophobicity, it markedly enhanced the soil’s durability, mitigated moisture-related deterioration and decelerated disintegration. Notably, the strength residual ratio of fiber-reinforced cemented sample maintained higher than 90% even after 20-day water immersion, and the disintegration rate decreased by 57% compared to unreinforced sample. Microstructural analysis revealed that bonding, friction, and interlocking among fibers, hydration products, and soil particles are the main contributors to the stable and robust matrix, ensuring improved mechanical behavior and longevity of micaceous residual soil. This study provides an innovative method for utilizing waste byproducts in stabilizing micaceous residual soil, as a viable option for earthworks in rain-soaked or flood-prone areas.

Evaluating the occurrence and spatial patterns of soil water repellency in the Deschutes National Forest, Oregon

AbstractHigh levels of soil water repellency (SWR) can hinder water infiltration and increase surface soil erosion risk and runoff. Although SWR occurs naturally in many areas, it is often patchy and does not impede water movement. However, fire can increase the connectedness and extent of SWR leading to topsoil loss, nutrient limitations, increased root water stress, and ultimately slower ecosystem recovery. This study examines the naturally hydrophobic soils of Oregon's Deschutes National Forest following the 2020 Green Ridge Fire to (1) examine the relationship between SWR and management‐relevant burn severity classes, (2) quantify an appropriate spatial scale over which to evaluate SWR properties, and (3) determine which environmental factors drive patterns in SWR. We found that the top 1–3 cm of soil became less hydrophobic after fire, while the profile to about 10 cm became more uniformly hydrophobic. This could indicate that surface soil is more prone to post‐fire erosion in burned areas. However, predicting SWR is still challenging. While burn severity and environmental metrics did statistically account for some variation in SWR following fire, the range of SWR spatial autocorrelation was at best a few meters. Due to this fine‐scale variation, future work should focus on determining an efficient post‐fire soil evaluation protocol with adequate density and scale of sampling while also incorporating the influence of environmental factors to inform management decisions.

Differential effects on soil water repellency of Eucalyptus and Pinus plantations replacing natural pastures

ABSTRACT Land-use changes from native pastures to forest plantations in humid temperate areas have raised concerns about their potential impact on the environment. This study aimed to assess the effects of such changes on soil water properties, focusing on the impact of the forest species planted and their relationship with changes in soil C content. Specifically, we aimed to identify the development of surficial soil hydrophobicity and changes in soil water holding capacity. A long-term forest experiment with variable planting densities (816, 1111, and 2066 trees ha -1 ) of Eucalyptus grandis Hill ex Maiden and Pinus taeda L. was established in 2004 on native pasture vegetation. Undisturbed soil samples (0.00-0.03 m soil layer) were extracted from the experiment and surrounding pastures and soil water repellency was determined by the water drop penetration time (WDPT) method at three soil matric potential levels (SMP). Bootstrapping was used to test if the sample size was sufficient to obtain robust results. Replacing native pastures with forest plantations significantly increased surficial soil hydrophobicity, which was more pronounced under Eucalyptus grandis than under Pinus taeda. Soil water repellency increased with decreasing SMP, particularly in land-uses that generated higher initial hydrophobicity. Additionally, the soils under forest cover had less water retention capacity than those under pastures at each SMP, with larger differences when the soil was dried to more negative SMP. More research is necessary to determine if soil alterations from converting native pastures to forest plantations in temperate climates will lead to a significant decrease in soil water holding capacity and an increase in hydrophobicity at deeper depths.

Exploring soil-root interactions: A comparative study of wheat species and soil types

Soil-plant interactions of different wheat species (e.g., common, synthetic, durum and wild emmer ancestor) have not been studied yet. Therefore, our study was conducted to examine the interactive effect of 39 treatments including four wheat species (twelve varieties) and three soil types (loamy sand, sandy loam and sandy clay loam) on soil physical and biophysical quality indicators in a greenhouse pot experiment. The rhizosphere data were compared with the no plant (control) soil as well. The results revealed that the rhizosphere soils had a greater soil organic carbon (SOC) storage compared to the no plant soil, with the synthetic wheat species possessing the highest SOC content (order of SOC: synthetic > durum > emmer > common > control). The highest and the lowest SOC values (0.79 and 0.69 kg 100 kg−1) were observed in the rhizosphere of synthetic wheat S88 and common wheat CK varieties, respectively. The SOC showed a positive correlation with root morphological traits such as root volume and root dry weight. Basal soil respiration (BSR) showed significant variation among the four wheat species, and common and emmer ones had the highest BSR values. The maximum and minimum values of BSR (21.9 and 10.9 mg CO2 kg−1 soil day−1) were observed in the rhizosphere of the common species planted in sandy loam soil and in the loamy sand bulk soil (control), respectively. The presence of plant roots significantly increased BSR compared to the control (bulk soil); on average, the BSR in the rhizosphere was 21% greater than the control. Both the wheat species and soil type significantly affected the soil water repellency index (RI) and wettability parameters, but the interactive effects of treatments (i.e., wheat species and soil type) were not significant. The soils in all of the studied treatments were sub-critically or slightly water repellent (i.e., contact angle lower than 90°). The RI values were measured in the order emmer > durum > common > control ≥ synthetic; however, the difference between the synthetic species and bulk soil was not significant. Plant roots increased the soil hydrophobicity by increasing the SOC storage and significant variations in RI among the wheat species indicated that the emmer, durum and common species with greater RI might have better soil physical quality in the rhizosphere. The wheat plants substantially increased water-stable aggregates and decreased water-dispersible clay in the rhizosphere compared to the bulk soil. These findings showed that wheat roots were able to alter the structural stability of the rhizosphere soil. Correlation analysis indicated that soil structural stability was closely related to the SOC, microbial activity, water repellency and root morphological traits. Principal component analysis was used to enhance visualization, identify interrelationships between soil quality indicators and categorize wheat varieties.

Effect of surfactant surface and interfacial tension reduction on infiltration into hydrophobic porous media

Surfactant molecules increase the infiltration rate into hydrophobic porous media by lowering the infiltrating water's surface tension and the interfacial tension between hydrophobic surfaces and water molecules. We investigated the relative effect of these rate-limiting processes on the infiltration rate of aqueous surfactant solution into hydrophobic porous media. Two surfactants at various concentrations were applied at a constant pressure head to 1D columns filled with hydrophobic soil, and water was applied at the same pressure head to columns filled with surfactant-pretreated hydrophobic soil. Based on the measured contact angle, surfactant pretreatment significantly reduced the hydrophobic soil's interfacial surface tension, which increased the infiltration rate compared to the direct aqueous surfactant application to the hydrophobic soil. The latter's slower infiltration rate was attributed to the depletion of surfactant molecules due to its adsorption to the hydrophobic molecules near the advancing wetting front, yielding an increase in the surface tension of the infiltrating solution. Surfactant pretreatment increased the opportunity time for surfactant adsorption to the hydrophobic molecules, resulting in interfacial tension reduction and infiltration rate increase. Diffusion-limited surfactant adsorption on the hydrophobic surfaces, leading to reduced interfacial tension between the surface and infiltrating liquid, had a greater impact on limiting infiltration into hydrophobic porous media compared to the reduction in surface tension of the infiltrating liquid due to surfactant presence.

Process based modelling of plants–fungus interactions explains fairy ring types and dynamics

Many mushroom-forming fungi can develop circular colonies affecting the vegetation in a phenomenon named fairy rings. Since the nineteenth century, several hypotheses have been proposed to explain how fairy ring fungi form ring-like shapes instead of disks and why they produce negative or positive effects on the surrounding vegetation. In this context, we present a novel process-based mathematical model aimed at reproducing the mycelial spatial configuration of fairy rings and test different literature-supported hypotheses explaining the suppressive and stimulating effects of fungi on plants. Simulations successfully reproduced the shape of fairy rings through the accumulation of fungal self-inhibitory compounds. Moreover, regarding the negative effects of fungi on vegetation, results suggest that fungal-induced soil hydrophobicity is sufficient to reproduce all observed types of fairy rings, while the potential production of phytotoxins is not. In relation to the positive effects of fungi on plants, results show that the release of phytostimulants is needed to reproduce the vegetation patterns associated to some fairy ring types. Model outputs can guide future experiments and field work to corroborate the considered hypotheses and provide more information for further model improvements.

Straw Inputs Improve Soil Hydrophobicity and Enhance Organic Carbon Mineralization

The mechanism of the influence of soil water repellency (SWR) and agglomeration stability on soil organic carbon (SOC) mineralization has not been thoroughly studied following different methods of returning straw to the field. The research background in this study was ordinary black soil, and the addition of straw was accomplished via straw mixing (CT), straw mulching (CM), straw deep burying (CD), and straw tripling deep burial (CE). A 120-day long-term incubation test was used to measure the contact angle between water droplets and soil, the particle size distribution of aggregates and their organic carbon (OC) content, organic carbon pool (OCP) content, OC contribution, and soil CO2-C release, the extent of SWR and the direct effect of agglomerates on SOC mineralization were assessed under different straw return methods. The results revealed that the water-droplet–soil contact angle (CA) was much greater and the rate of CA decline was significantly lower in the CD treatment compared to the CT, CM, and CE treatments, the rate of water droplet penetration on the soil surface was slower, and the SWR was improved. The CD treatment significantly increased the content of macroaggregates and their OCP content, and also significantly increased the content of microaggregates’ OC. The CO2-C emission rate and cumulative emissions were enhanced by adding the same amount of straw, with the most significant enhancement in the deep straw treatment. The cumulative CO2-C emission rate and SOC mineralization significantly increased with increases in SWR, macroaggregates content, and microaggregates OC content, but significantly decreased with increases in macroaggregates’ OC content, according to principal component analysis and Pearson’s correlation analysis. These results highlight the extent of SWR and the direct effect of agglomerate particle size distribution and OC content on SOC mineralization under different straw return methods. This will help to consolidate soil structural stability and nutrient management to support productivity and SOC sequestration in different agricultural systems.

Representation of three-dimensional unsaturated flow in heterogeneous soil through tractable Gaussian random fields

The representation of the spatial variability of soil properties is required to model non-uniform hydrological flow processes such as fingering and preferential moisture migration, which are prominent in both unsaturated and hydrophobic soils. This paper presents a new method for simulating three-dimensional hydrological flow processes in soil masses in which random fields of soil properties are conveniently generated through the numerical solution of a set of partial differential equations that have the same structure as those used for computing moisture transport. The method uses the Whittle–Matérn autocorrelation function to generate the required correlated Gaussian random field. A new method for representing statistical boundary effects on the degree of moisture infiltration is also presented. The statistical model component is integrated into a finite-element code for moisture transport and applied to the simulation of moisture infiltration and flow within an in situ layer of sandy loam. The transport properties and associated statistical measures employed to generate the random field are those reported in the experimental study. The results from the numerical simulations show that the model represents fingered flow behaviour with good accuracy, with the numerical and experimental infiltration fronts having similar characteristics and infiltration rates.

Hydraulic characterization and modeling of hydrophobic substrates

The hydraulic characterization of hydrophobic substrates, which strongly depends on the water content, is an understudied topic in the field of soil physics. Based on laboratory experiments, this work presents a physically-based model, denoted as Soil Water Repellency Infiltration Model (SWRIM), to describe the double-slope cumulative infiltration curves commonly observed in hydrophobic substrates, whose behavior is conditioned by the dual hydraulic behavior (under dry and saturated conditions) of the water repellency materials. The experimental setup consists of 1D infiltration tests on hydrophobic substrates and tension of 0 cm, following by a free drainage. The SWRIM model assumes that water flow in the hydrophobic substrate is gravity driven, representing the pore system as a set of disconnected pathways or capillaries with dual behavior. Each capillary is governed by two hydraulic conductivity values corresponding to dry or partially saturated and saturated conditions, Kd and Ks respectively. During infiltration, water flow within each pathway is regulated by Kd, but once the pathways are filled and the substrate is fully saturated, the water flow is governed by Ks, whose value is directly measured from the slope of the drainage curve. In order to incorporate the variability of the pore system, unimodal and bimodal probability distributions of Kd values were considered. The application of the SWRIM model allowed determining the distribution of Kd and the percentage of water-conducting pores, δ, of the hydrophobic layer. Additional determinations of the soil hydraulic conductivity, K, and sorptivity under water repellent conditions were also obtained from the inverse analysis of the initial times of the infiltration test. The proposed model was validated on experimental soil columns of 5 cm internal diameter and 1.0 and 2.5 cm height filled with pine forest organic material, Pine, rosemary leaf litter, RS, and, blond peat, Peat. Overall, convex-to-linear curves with a monotonously increasing infiltration, typical of hydrophobic substrates, were obtained. SWRIM model allowed robust fits of the infiltration curves, showing in all cases a distribution of Kd with a bimodal shape. Pine and Peat presented the largest values of δ. The estimated average Kd was within the same order of magnitude as K values. Overall, the results showed that the proposed physically-based model allows to satisfactory describe and characterize the dynamic hydraulic behavior of hydrophobic soil substrates.

Effect of soil hydrophobicity on soil-water retention curve of a silt loam soil

Soil wettability is an important property that affects the behaviour of fine-grained soils. Previous studies have shown that hydrophobicity induced by organic or silane additives may affect the soil-water retention curve (SWRC). However, current findings regarding the effect of hydrophobicity on the SWRC are controversial. Previously, organic or silane additives were assumed to change only the soil wettability. Nevertheless, this study shows that polyvinyl alcohol (PVA) can change the hydrophobicity, pore size distribution (PSD), and specific surface area (SSA) of soil, thus affecting the SWRC. PVA-treated soil is hydrophobic when cured at 20 ℃, but hydrophilic when cured at 100 ℃. No thermal degradation is indicated in the PVA at 100 ℃ based on thermogravimetric analysis and a comparison of mass loss, and the sole difference between the PVA-treated soils cured at 20 and 100 ℃ is their wettability. This study managed to make the hydrophobicity caused by PVA to become an independent variable separated from the PSD and SSA by controlling the ambient temperature. The wetting and drying curves are measured under isopiestic humidity control in the high-suction domain (2.7–298.7 MPa), whereas pressure plates are used for the drying curves in the low-suction domain (0–800 kPa). The results obtained indicate that soil hydrophobicity significantly accelerates drainage when the matric suction is between 100 and 400 kPa, and an increase in PVA content increases the water retention capacity of soil. When the matric suction exceeds 2.7 MPa, the effect of soil hydrophobicity on the SWRC of the soil weakens. At this time, the SWRC is dominated by the van der Waals force, and the SSA contributes significantly to the process. Additionally, this study enables some controversial findings from previous studies to be explained, which is of great significance in soil and agriculture sciences.

Compaction Characteristics of Organo-silane Treated Soils

Abstract The extent to which hydrophobic soils can be used in geotechnical engineering practice depends upon a commensurately increased understanding of how fundamental relationships apply in hydrophobic systems, such as moisture-density. This research is to determine the compaction characteristics of organo-silane (OS) treated soils using conventional geotechnical laboratory equipment. The intrinsic difficulty in lubricating hydrophobic soils, which would allow for rearrangement of particles and subsequent compaction, arises from their inherent low surface energy. This work describes a method for compacting OS treated soils that leverages the necessary conditions such as reaction time and drying conditions for achieving hydrophobicity. Procedural steps for compacting OS treated soils are detailed by making use of a water-soluble hydrophobizing agent added to a fine-grained soil. Based on the critical dosage ratio of 1:100 (hydrophobizing agent: soil) identified, a molding water content is defined constituting of a fraction of the hydrophobizing agent. Soil water content and dry density curves are developed using the standard Proctor and Harvard miniature to contrast the resulting effect of OS. Compared to the untreated soil, a decrease in optimum water content was observed with the OS treated soil regardless of compaction technique used. For the standard Proctor test, a decrease in optimum water content from 12.0 to 9.2 % was observed, whereas compaction with the Harvard miniature showed a marginal decrease from 9.3 to 9.0 %. With the untreated soil, a relatively larger maximum dry density (2.17 g/cm3) was obtained with the standard Proctor compared to the Harvard miniature (2.05 g/cm3). The protocol defined to compact OS treated soils has shown to induce hydrophobicity spatially within the sample depth. These results suggest that engineered water repellency can be implemented in so far as treatment and compaction are largely synchronous and prior to reaction and hydrophobization.

Photochemical mobilization of dissolved hydrocarbon oxidation products from petroleum contaminated soil into a shallow aquifer activate human nuclear receptors

Elevated non-volatile dissolved organic carbon (NVDOC) concentrations in groundwater (GW) monitoring wells under oil-contaminated hydrophobic soils originating from a pipeline rupture at the National Crude Oil Spill & Natural Attenuation Research Site near Bemidji, MN are documented. We hypothesized the elevated NVDOC is comprised of water-soluble photooxidation products transported from the surface to the aquifer. We use field and laboratory samples in combination with complementary analytical methods to test this hypothesis and determine the biological response to these products. Observations from optical spectroscopy and ultrahigh-resolution mass spectrometry reveal a significant correlation between the chemical composition of NVDOC leached from photochemically weathered soils and GW monitoring wells with high NVDOC concentrations measured in the aquifer beneath the contaminated soil. Conversely, the chemical composition from the uncontaminated soil photoleachate matches the NVDOC observed in the uncontaminated wells. Contaminated GW and photodissolution leachates from contaminated soil activated biological targets indicative of xenobiotic metabolism and exhibited potential for adverse effects. Newly formed hydrocarbon oxidation products (HOPs) from fresh oil could be distinguished from those downgradient. This study illustrates another pathway for dissolved HOPs to infiltrate GW and potentially affect human health and the environment.

Soil moisture observations and machine learning reveal preferential flow mechanisms in the Qilian Mountains

The complexity of the spatial distribution and temporal occurrence of preferential flow (PF) makes it challenging to understand the mechanisms of PF. This study aims to identify the spatial and temporal patterns of PF occurrence using machine learning (Classification and Regression Trees and Random Forests) in the Qilian Mountains, Northwest China. Our results show that detected PF events transport much more rainfall down to the subsoil than non-PF events. Different vegetation types exhibit variations in the main soil layers where PF occurs, which is closely related to the distribution of roots. The PF proportion varies significantly both vertically and horizontally. Based on the Random Forests, we found that the spatial distribution of the PF proportion is mainly controlled by the saturated hydraulic conductivity and residual soil moisture, which cannot be identified by conventional correlation analysis methods. With these soil properties, the spatial distribution of the PF proportion can be estimated with reasonable performance. Using the Classification and Regression Trees method, we identified the temporal occurrence pattern of the PF for different vegetation types and all observation stations. Results indicate that the dominant factors controlling the temporal occurrence of the PF varied for different vegetation types. The thresholds at which these factors initiate the PF also varied. Finally, we found that the PF occurs particularly under wet conditions (except for hydrophobic soils), under denser vegetation, and under conditions of high rainfall amount and intensity, regardless of vegetation type. Our study confirms that both site factors (e.g., soil properties and vegetation) and temporal factors (e.g., initial soil moisture and rainfall characteristics) control the occurrence of the PF in mountainous regions such as the Qilian Mountains and that the Classification and Regression Trees has great potential to study the temporal occurrence of the PF.

Greywater irrigation and soil quality: An assessment framework adjustment and application

The on-site use of greywater is increasingly popular for alleviating water stress in various parts of the world, particularly as a water source for irrigation. However, greywater can contain a range of pathogenic bacteria that may compromise public health as well as substances with the potential to induce environmental consequences, such as soil hydrophobicity, accumulation of salts, and damage to plants. While the health issues are being addressed by greywater legislation, its environmental risks are largely ignored. Therefore, the main objective of the current study was to quantify the impacts of greywater irrigation on soils by developing a soil quality index (SQI) using a 14-month planter experiment. The sum of the absolute value of all indicator scores represents the final score of the integrated SQI, which ranges from 0 to 100. Three threshold values were used: <30 represents deteriorated soil quality, 50–70 indicates intermediate quality, and >70 represents optimal quality. The results based on the planter experiment revealed that, after 14 months, the SQI of all raw greywater-irrigated soils was lower than 70, indicating soil functions and plant health might be compromised. The use of scoring functions was a useful tool for quantifying and comparing the effects of greywater irrigation on different soil quality indicators. Integration of all indicator scores into a single SQI quantifies and summarizes the overall beneficial and detrimental effects of greywater irrigation. However, for better understanding and management decisions, SQI scores should be used and interpreted in conjunction with the scores of the single indicators constituting the index. In our experiment, treated ​greywater ​did ​not compromise ​soil quality even after 14 months of irrigation. As such, based on the fact that irrigation with raw greywater might compromise soil quality, treatinggreywater prior to its use is recommended.

How does calcium and sodium in saline water affect the potential of different rates and size of sugarcane biochar to soil water repellency remediation?

Soil water repellency (SWR) is a growing problem in arid and semiarid regions with non-saline water limitations. The objective of this study was to investigate the potential of different rates and sizes of sugarcane biochar on reducing soil water hydrophobicity when using saline and non-saline water. Eleven sugarcane biochar application rates from 0 to 10% with two different sizes including <0.25 and 0.25–1 mm were studied. The experiment was conducted in two strongly and extremely water-repellent soil. In addition, to investigate the effect of electrolyte concentration on biochar potential for SWR reduction, calcium chloride and sodium chloride electrolyte solutions with 5 concentrations including 0, 0.15, 0.3, 0.45 and 0.6 mol L−1 were considered. The results showed that both sizes of biochar reduced the soil water repellency. While in strongly repellent soil the 4% biochar was enough to change the soil from strongly repellent to hydrophilic, in extremely water-repellent soil, the use of 8% of fine biochar and 6% of coarse biochar changed the extremely water-repellent soil to slightly hydrophobic and strongly hydrophobic respectively. Increasing the electrolyte concentration caused the expansion of soil hydrophobicity and reduced the positive effect of biochar to water repellency management. Increasing the electrolyte concentration in sodium chloride solution has a greater effect on increasing hydrophobicity than in calcium chloride solution. In conclusion, biochar could suggest as a soil-wetting agent in these two hydrophobic soils. However, the salinity of water and the its dominant ion could increase the amount of biochar for soil repellency reduction.