Aggregate Slaking Research Articles

Microplastics alter soil structural stability as quantified by high-energy moisture characteristics

Microplastics (MiPs) can potentially influence soil structural stability, with impacts likely dependent on their chemistry, concentration, size, and degradation in soil. This study used high-energy moisture characteristics (HEMC; water retention at matric suctions from 0 to 50 hPa) to quantify the effects of these MiP properties on soil structure stabiltiy. The HEMCs of soil samples contaminated with polypropylene (PP) or polyethylene (PE) were measured and modelled. Greater MiP concentrations (2 % and 7 % w w−1) increased the volume of drainable pores (VDP). At smaller MiP concentrations (0.5 % and 1 % w w−1), larger MiP fibres (3 and 5 mm) exhibited higher VDP values compared to a smaller size (1.6 mm) across a range of concentrations. Both PE and PP MiPs increased the modal matric suction (hmodal). The impacts on VDP and hmodal were more pronounced for fast than slow wetting, likely due to MiPs fibres entangling around soil aggregates, and MiPs pores filling after aggregate slaking, respectively. Soil structural index (SI) and stability ratio (SR) values increased following MiP incorporation. Our findings revealed the detrimental impacts of MiPs on soil aggregates and pores, demonstrating that MiPs significantly influence HEMC parameters due to combined impacts on structure stability and pore distribution. Environmental implicationMicroplastics have emerged as a major anthropogenic hazardous material in the soil environment, with secondary impacts on soil structure and aggregate stability. Our study indicates that MiPs alter water retention, pore distribution, and soil hydraulic properties, affecting soil’s ability to retain and supply water. The introduction of MiPs leads to the destruction of soil aggregates and pores, compromising soil health and productivity. By characterising structural stability and pore structure dynamics using HEMC, this study highlights the sensitivity of MiP impacts, emphasizing the need for comprehensive assessment and strategies to preserve soil ecosystem functioning in the face of increasing MiP pollution.

Influence of Planting Techniques and Integrated Nutrient Management on Soil Health in Rice (Oryza sativa L.)

The study investigated the impact of different planting techniques and nitrogen management at Sardar Vallabhbhai Patel University of Agriculture & Technology, Meerut (UP) during the rainy (kharif) season of 2019-20 on soil health in rice. Soil samples were collected at two depths, air-dried, and analyzed for aggregate size distribution using wet sieving. Particulate organic carbon (POC) was determined through aggregate slaking, while total organic carbon (TOC) content was assessed using rapid titration. Microbial biomass carbon (MBC) was calculated by incubating soil samples, followed by fumigation-extraction methods. Results revealed that soil pH and electrical conductivity remained unaffected by planting techniques and fertility levels. Aggregate size distribution analysis revealed that the Furrow Irrigated Raised Bed (FIRB) treatment promoted larger macro-aggregates (>2mm) at varying soil depths, while Conventional Tillage (CT) encouraged smaller micro-aggregates. Reduced Tillage (RT) combined with Farmyard Manure (FYM) and chemical fertilizer increased POC and TOC content, emphasizing the importance of reduced soil disturbance and organic matter addition. Conversely, CT exhibited lower POC and TOC levels due to intensive soil disturbance. RT with FYM and chemical fertilizer significantly enhanced soil MBC, highlighting their role in improving soil health and fertility.

Effects of antecedent soil moisture on rill erodibility and critical shear stress

Antecedent soil moisture is known to exert a complex, perhaps controversial, effect on rill erodibility and critical shear stress. To understand their dynamic nature as a function of antecedent soil moisture, the rill erodibility and critical shear stress for sandy loam and silty loam soil, representing coarse-grained and fine-grained soil, respectively, were measured using a hydraulic flume under six antecedent soil moisture contents, i.e., 3, 6, 9, 12, 15, and 18%. The results show that antecedent soil moisture had a different effect on critical shear stress and rill erodibility for the two different soil textures. As antecedent soil moisture increased, rill erodibility for the fine-grained soil first increased and then decreased, whereas for coarse-grained soil, rill erodibility exhibited a decreasing pattern. Conversely, as the antecedent soil moisture increased, the critical shear stress decreased first and then increased for the fine-grained soil, but for coarse-grained soil, the critical shear stress decreased after a slight increase. These different patterns can be interpreted by aggregate slaking, capillary force, soil cementation, and water separation. However, when the patterns of critical shear stress and rill erodibility were neglected, the differences in critical shear stress and rill erodibility between the coarse- and fine-grained soils were not statistically significant (P < 0.05). Therefore, indiscriminate treatments of critical shear stress and rill erodibility for different textural soils under changing antecedent soil moisture regimes may cause errors in soil erosion modeling. The relationship between rill erodibility and critical shear stress for coarse- and fine-grained soils could be fitted by a power and a polynomial function, with R2 value of 0.94 and 0.78, respectively. Future work should include additional soil textures to study the influence of soil physical characteristics in combination with antecedent soil moisture on soil erosion.

No-tillage and soil physical environment

Implications of no-till (NT) management on soil C dynamics, soil fertility, and crop yields have been discussed, but an up-to-date synthesis of NT impact on soil physical properties based on a comprehensive compilation of global published studies is not available. Yet, an understanding of changes in soil physical properties after NT adoption is important to manage soils, agricultural production, and environmental quality. We compared data on soil physical properties among NT, reduced till (RT), and conventional till (CT) systems, discussed factors influencing tillage system effects, and underscored research needs. No-till had mixed effects on soil bulk density and penetration resistance but reduced Proctor bulk density (compactibility) by 4 to 13% in the 0 to 15 cm depth, suggesting that NT can reduce the susceptibility of the soil to compaction. No-till increased wet aggregate stability by 1 to 97%, water infiltration by 17 to 86%, and available water by 44%. It reduced or had no effect on soil temperature during the growing season but increased soil thermal conductivity. The latter indicates that NT can increase the soil's ability to conduct heat. No-till induced some slight water repellency, which can reduce soil aggregate slaking and enhance C storage. However, NT had no consistent effects on saturated hydraulic conductivity, soil consistency, and shear strength. Reduced till effects on soil properties were intermediate in values between NT and CT. No-till benefits for reducing compaction risks and improving structural quality increased in the long term. Changes in soil physical properties appear to be mainly confined to the upper 10 cm depth. Reviews on NT and soil C have also concluded that NT can cause stratification of soil organic C in the upper 5 or 10 cm depth. Thus, NT-induced increases in near-surface (<10 cm depth) soil organic C concentration most likely improved wet aggregate stability and available water capacity, and reduced compactibility. One-time tillage of NT soils does not seem to negatively affect soil physical properties. Addition of companion practices (i.e., cover crops, C amendments) can enhance NT performance. Overall, NT management can have positive effects on soil physical properties with the extent depending on soil textural class and management duration.

Effect of environmental pollution on susceptibility of sesquioxide-rich soils to water erosion

This work assessed the impact of environmental pollution on soils susceptibility to water erosion within South-eastern Nigeria. Sources of pollutants which could possibly affect the chemical composition of runoff; hence its pH, were first determined by remote sensing and field observations. Rain and runoff water samples collected within the study area were analysed for its physicochemical compositions. Soil samples were also collected and analysed for their geotechnical properties, and chemical composition of their fine fractions. An empirical method was then employed to determine the effect of change in chemical composition of runoff on the susceptibility of the studied soil to water erosion. This was achieved by conducting soil aggregate slaking, dispersion and dissolution tests on aqueous solutions of varying pH. Results from the experiment shows that the fine particle fractions of the soils are chiefly composed of sesquioxides. The slaking of these sesquioxide-cemented soils is not affected by the variations in pH of the solutions, but rather by the plasticity index of the soils. However, dispersion and dissolution of the soil samples where dependent on variations in the pH of the solutions. It was therefore concluded that environmental pollution has the potential of increasing runoff erosivity.

Quantifying contributions of slaking and mechanical breakdown of soil aggregates to splash erosion for different soils from the Loess plateau of China

The information of aggregate disintegration mechanisms during splash erosion is scant. This study was conducted to quantify contributions of the mechanisms of aggregate disintegration to splash erosion. Six soils with five soil textures were used. Soil aggregate stability was determined by the Le Bissonnais (LB) method. Deionized water was used to simulate the combined effect of slaking and mechanical disaggregation, while ethanol was used to estimate the sole contribution of the mechanical breakdown. Simulated rainfall with intensity of 60 mm h−1 was applied at five fall heights (0.5 m, 1 m, 1.5 m, 2 m and 2.5 m) to achieve different levels of rainfall kinetic energy. The results indicated that slaking caused the most severe aggregate breakdown, and followed by mechanical breakdown, while chemical dispersion in slow wetting with deionized water was the weakest breakdown mechanism. The splash erosion rates due to the effects of slaking and mechanical breakdown increased with an increase in rainfall kinetic energy. The contributions of the slaking (mechanical breakdown) to splash erosion decreased (increased) as rainfall kinetic energy increased. The contribution of mechanical breakdown had a power function relation with rainfall kinetic energy, and had the most significant correlation with RSI (relative slaking index)/RMI (relative mechanical breakdown index). A power and a linear function could be used to describe the relationships between the contributions of mechanical breakdown with rainfall kinetic energy and RSI/RMI, respectively, which could be used to estimate the contribution of mechanical breakdown. The results of this research would be helpful to improving the soil erosion prediction models.

Effect of drying\u2013wetting cycles on aggregate breakdown for yellow\u2013brown earths in karst areas

BackgroundDrying and rewetting process, frequently occurred during climatic changes, is an important process in soil aggregate slacking and dissolution. The severer interference of human activities on global climate makes the extreme climate scenarios like drought and rainstorm occur frequently. Therefore, there is necessity to further our understanding on the impact of the drying–wetting cycles and initial water content on the breakdown of soil aggregates. The typical yellow–brown earth composed of water–stable and water–unstable aggregates is selected. Variations of water-stable aggregate size distributions after drying-wetting cycles are measured by wet sieving, under variable initial water content and cycles respectively.ResultsDrying-wetting cycles cause a significant aggregate slaking, especially within the first two cycles. After that, most aggregates show more slacking resistant. The variation curves of the proportion of water-stable aggregates with the size 1–5 mm shows a coexistence of slaking process and supplement. The critical initial water content (about 24%) and turning point (with the aggregate size of 0.3 mm) are proposed to describe the effects of initial water content on size distribution of water-stable aggregates. Overall, the increase of initial water content strengths the water stability. In addition, the mathematical model for the relative leakage ratio based on the drying–wetting cycle, initial water content and size distribution are established.ConclusionsThe findings reported in this paper may be capable of supporting the intensive study for the breakdown mechanism and assessing the leakage potential under the influence of climate change. However, there exists a certain mismatch between the drying-wetting cycles in the tests and in practice, mainly in the frequency and intensity, which should be paid more attention.

Soil quality and aggregation in runoff water harvesting forestry systems in the semi-arid Israeli Negev

Water availability is a major limiting factor for dryland afforestation. Earthworks that modify natural landforms for the formation of runoff harvesting systems are prevalent in the Israeli drylands, with the aim of establishing afforestation projects. However, serious concerns alarm that such earthworks have detrimental effects on the geo-ecosystem functioning. Therefore, the objective of this study was to examine the impact of the establishment of contour bench terrace (also called shichs/shychs/shikim) runoff harvesting systems on selected soil properties, with particular focus on soil aggregation. Thus, we assessed the effect of the establishment of contour bench terraces in a multi-aged forestry land, comprised of 2-year-old and 9-year-old afforestation areas, and compared them to ‘natural’ hillslopes as a reference treatment. The study was implemented in the Ambassadors' Forest, located in the semi-arid northern Negev of Israel, where we sampled the surface soil (at a depth of 0–5cm) in north- and south-facing hillslopes. Considerable differences were recorded for the afforestation systems between the source (inter-terrace area) and sink (terrace-bottoms) areas. Data normalizing according to the relative cover percentage of the terraces and inter-terrace areas showed that the mean values of mean weight diameter (MWD), stable aggregate content, and particulate organic carbon in the natural hillslopes were 1.4%, 32.4%, and 20%, respectively, greater than in the 9-year-old afforestation systems, and 12.1%, 28.9%, and 31%, respectively, greater than in the 2-year-old systems. Means of clod content, aggregate slaking index, and clay dispersion index in the natural hillslopes were 62.3%, ~twofold, and 35.0%, respectively, smaller than in the 9-year-old systems, and almost threefold, nearly twofold, and 46.2%, respectively, smaller than in the 2-year-old areas. The soil calcium carbonate content was similar in soils of the natural hillslopes and 9-year-old afforestation lands, which was ~17% smaller than in the 2-year-old afforestation systems. Considerable differences among the land-uses were also recorded for the soil texture. Mean coarse root biomass, despite revealing only a marginally-significant effect (p-value=0.0765), was 40.0% greater in the natural hillslopes than in the 9-year-old systems, and more than twofold greater than in the 2-year-old systems. Hillslope aspect affected some of the measured properties, revealing only slightly better soil conditions in the (mesic) north- than in the (xeric) south-aspects. The data revealed that forestry-related earthworks degrade the soil quality and geo-ecosystem functioning in the short term. Yet, despite some discrepancies, the data also showed the occurrence of self-restoration processes of the geo-ecosystem over the long term.

Positive impact of moderate stubble grazing on soil quality and organic carbon pool in dryland wheat agro-pastoral systems

Stubble grazing by livestock in post-harvest wheat fields is common in drylands. Previous studies have shown that this practice causes land degradation. Therefore, the objective of this study was to examine the effect of long-term stubble grazing, by comparing soil quality indicators in continuous wheat croplands of two rain-fed farming systems: with moderate stubble grazing during the summer (GR) vs. entire stubble retention (NO). Multi-annual averaged dry organic matter residue retained on the ground surface was ~0.8Mgha−1 in NO, as opposed to ~0.3Mgha−1 in GR. the same soil characteristics were also studied in ‘natural’ lands (NAT), to assess land-use change impact. The study was implemented in the semi-arid, northern Negev of Israel. Sampling of soil at depths of 0–5 and 5–10cm was conducted in summer 2013. Some of the results suggest the degradation of soil quality following land-use change from NAT to croplands, as well as in GR, compared to NO. This included the coarse root biomass, which was 67% to ~two times greater under NAT than that under NO and GR. This impact was also revealed by the aggregate slaking index which was 18% to two times greater under the two cropland treatments than that under NO, as well as for the clay dispersion index which was ~two to three times greater under the two cropland treatments than that under NO. At the same time, unexpectedly, the majority of soil characteristics showed better soil quality under GR than that under NO. For example, hygroscopic moisture content under NAT was only 10% greater than that under GR, but 22% greater than that under NO. Also, the soil organic carbon pool was similar between NAT and GR, which had 16–22% greater value than that under NO. Overall, soil aggregation properties also suggested negative impact of land-use change, but, at the same time, showed a positive impact on soil quality by GR compared to that under NO. These aggregation properties included the micro-aggregate content, stable aggregate content, contents of the aggregates >2000μm, the aggregate size fraction 1000–5000μm, mean weight diameter of aggregates, and mean weight diameter of the 1000–5000μm aggregate size fraction. An unexpected effect was recorded for the content of clods >8000μm, which was ~two times greater under NAT than that under the two cropland treatments. The soil organic carbon's stratification ratio was marginally affected by treatment (P=0.1015), and was 4% and 17% greater under NAT than that under NO and GR, respectively, revealing the clearest layering of soil organic carbon under NAT and the least clear layering under the GR. This suggests that mixing of organic residues in soil is smallest under NAT and greatest under GR. It is proposed that, in the long term, together with the input of animal excretion, the mixing of stubble residue in soil imposed by the livestock trampling compensates for the quantitative loss of stubble (through its consumption by the grazing animals), increasing soil organic carbon pool, and improving macro-aggregation processes and overall soil quality. A conceptual model is proposed to summarize these effects and relate them to soil conservation issues.

Soil aggregate stability within morphologically diverse areas

Knowledge of spatial distribution of soil aggregate stability as an indicator of soil degradation vulnerability and its possible prediction are required for many scientific and practical environmental studies. The goal of our study was to provide a model for predicting soil aggregate stability within morphologically diverse areas, where soil properties have been affected by soil material redistribution due to erosion. The study was performed on a study site (6ha area) in the loess region of Southern Moravia, Czech Republic. Haplic Chernozem, which is an original dominant soil unit, has been transformed into different soil units (eroded phases of Chernozem, Regosol, colluvial Chernozem and Colluvial soil). 36 sampling spots were selected in order to represent diverse soils. The following soil properties were measured: oxidable organic carbon content (Cox), CaCO3 content, pHH2O, pHKCl, soil particle density (ρs), bulk density (ρd), porosity (P), actual field soil-water content (θfield), content of iron and manganese (in ammonium oxalate extract, Feo and Mno, and dithionite–citrate extract, Fed and Mnd) and mass specific magnetic susceptibility (χlf and χhf). The aggregate stability was assessed using various tests to study different disruption mechanisms. Terrain attributes were derived from a digital elevation model.In general, the lowest soil aggregate stability was observed on steep slopes, which were highly impacted by soil erosion. The highest aggregate stability was measured on soils sampled at relatively flat upper parts, which were less influenced by erosion processes. Higher stability was also obtained on toe slopes, where the sedimentation of previously eroded soil material occurred. The simple correlations revealed that characteristics resulting from the tests studying aggregate slaking due to the compression of the entrapped air (Water Stable Aggregate index and coefficient of vulnerability from fast wetting test) were positively impacted by the Cox, P, Feo, Mno, Fed, Mnd, χlf and χhf values, and negatively by the ρd value. The soil aggregate stability was also negatively influenced by the plan and total terrain curvatures, i.e. larger aggregate stability was measured at concave parts in comparison with that at convex parts. Almost no statistically significant relationships were found in the case of the tests evaluating either aggregate disintegration caused by the micro-cracking due to the different swelling, or by the physico-chemical dispersion due to the osmotic stress or the mechanical aggregate breakdown. The multiple linear regressions resulted in the model for estimating the WSA index using the Cox content, total terrain curvature and actual field soil-water content (θfield). In this model the Cox content positively and the total terrain curvature and θfield value negatively influenced the value of the WSA index. Since Cox was positively related with iron content and thus also with the magnetic susceptibility, the alternative model was proposed for less costly and time consuming WSA estimation. The WSA index may be predicted by combining the mass specific magnetic susceptibility (χlf and χhf), total terrain curvature and actual field soil-water content (θfield).

Stability analysis of aggregates in relation to the hydrophobicity of organic manure for Sri Lankan Red Yellow Podzolic soils

Aggregate slaking is linked with rapid pressure buildup within aggregates. Soil water repellency may help hamper the pressure buildup within aggregates by reducing their wetting rates. We examined the effects of animal manure in improving aggregate stability, the hydrophobic effects of green manure, and the possibility of using organic manure mixtures to increase the aggregate stability for Sri Lankan red yellow podzolic soils using model aggregates. Almost all the cow dung (CD) added samples showed extremely low percentages of water stable aggregates (%WSA) demonstrating rapid destruction of aggregates. Although the addition of ≥ 10% goat dung (GD) improved the %WSA, aggregate floating occurred, showing the risk of aggregate floating with runoff water. Addition of 5% GD would be an acceptable solution if the %WSA can be improved. Casuarina equisetifolia L. leaves (CE) was found to be a hydrophobic green manure. Although addition of ≥ 5% CE increased the %WSA up to about 90%, aggregate floating occurred. The possibility of improving %WSA using 1–2% hydrophobic green manure in organic manures mixtures was tested. Samples with 5% GD + 2% CE manure mixture showed the highest and the most stable %WSA without showing aggregate floating. Additions of compost and poultry litter were found not to be effective in improving aggregate stability with or without CE. Strong or higher water repellency was not observed in any of the samples with manure mixtures, showing that the addition of 1–2% hydrophobic CE would not induce detrimental effects of water repellency. There was no clear correlation between %WSA and the hydrophobicity of soils. However, the %WSA can be considered to show a tendency to increase with increasing hydrophobicity, because the %WSA was very high in samples with hydrophobic CE, the %WSA increased when mixed with 1–2% CE, and samples with highest water drop penetration time (WDPT) among all the manure mixtures showed the highest %WSA.

Quantitative analysis of liquid penetration kinetics and slaking of aggregates as related to solid–liquid interfacial properties

Summary Aggregate stability is frequently shown to be enhanced by strong soil water repellency, however, there is limited systematic evidence on this effect for moderately (subcritically) water repellent soils. This study aimed to investigate the specific effects of interfacial properties on the liquid penetration kinetics in relation to the stability of subcritically water repellent aggregates (4–6.3 mm) from various arable and forest soils against breakdown by slaking. In contrast to many other studies, where aggregate stability was determined by wet sieving, we here assessed the stability by immersion of air-dry aggregates in water–ethanol solutions with surface tensions ranging from 30 to 70 mN m −1 . This approach allowed a highly sensitive discrimination of different stability levels and the determination of breakdown kinetics also for less stable aggregates. Interfacial properties were characterized in terms of contact angle measured on crushed aggregates, θ c , and calculated for intact aggregates, θ i , based on infiltration measurements with water and ethanol. Aggregate stability turned out to be higher in forest soils compared to arable soils with topsoil aggregates generally found to be more stable than subsoil aggregates. For water repellent aggregates, characterized by contact angles >40° and low water infiltration rates ( 3 s −0.5 ), the fraction of disrupted aggregates after 30 s of immersion was generally below 10%, whereas in case of the more wettable aggregates, characterized by contact angles 0.25 mm 3 s −0.5 ) more than 80% of the aggregates were disrupted. In accordance, we found a close relationship between aggregate stability and wettability with differences between θ c and θ i being generally small. In addition, aggregate stability turned out to be related to organic carbon content. However, correlation analysis revealed that both persistence of aggregate stability and kinetics of aggregate breakdown were more strongly affected by the contact angle, θ c ( r = 0.90 and r = −0.83, respectively) and θ i ( r = 0.89 and r = −0.76, respectively) than the organic carbon content ( r = 0.62 and −0.52, respectively), suggesting that stability was primarily controlled by aggregate interfacial properties. Calculation of liquid penetrativity as a function of surface tension and contact angle clearly demonstrated the importance of both solid and liquid interfacial properties in determining the stability of subcritically water repellent aggregates against slaking.

Land use effect on soil and particulate organic carbon, and aggregate stability in some soils in Tunisia

Conversion of native forests to cultivation is usually accompanied by a decline in soil organic carbon and nutrients, and deterioration of soil structure. Our knowledge of this process must increase if we are to optimise the physical regeneration of degraded soils in semiarid areas. In this work we aim to (i) determine the relationship between aggregate stability and soil and particulate organic carbon (ii) to study the evolution of this relationship with land use. Samples were collected from a cropland cultivated and an adjacent natural forest and pasture from tree toposequence in North Tunisia. Change in land use from forest soil to pasture and cultivation induced significant losses of soil and particulate organic carbon. The soil organic carbon loss from forest soils to agriculture soils reached 83 and 74% from forest soils to pasture lands. However, the average of particulate organic carbon in forest ranged from 15 to 27 g kg-1 and 3 to 14 g kg-1 in agriculture soils. Across 13 sites, the average of mean weight diameter significantly decreased from (2.79 mm) natural forest soil to (2.10 mm) to pasture soil, being more significant in the cultivation soil (1.70 mm). Fast wetting treatment, causing aggregate slaking, was the process that decreased aggregate stability the most. Significant correlation was found between aggregate stability and soil organic carbon fraction. However, the greater association was observed between aggregate stability and particulate organic carbon. Soil where the organic matter was the principal aggregation agent (soil with sandy texture and zero carbonate calcium content) showed greater degradation sensitivity to soil organic carbon loss. Soil aggregate stability and soil organic carbon fraction could be used as indicators to apply the most appropriate management practices to increase soil sustainability or productivity. Key words: Land use, soil organic carbon, particulate organic carbon, aggregate stability, North Tunisia.

Salinity and Sodicity of Weathered Minesoils in Northwestern New Mexico and Northeastern Arizona

Evolving relationships between electrical conductivity (EC) and sodium adsorption ratio (SAR) in reconstructed soils at surface mines have been insufficiently documented in the literature. Some minesoils (i.e., rootzone material) are classified as saline, sodic, or saline-sodic and are considered unsuitable for revegetation. Weatherable minerals such as calcite and gypsum are common in alkaline minesoils and on dissolution tend to mitigate elevated SAR levels by maintaining or increasing electrolytes in the soil and providing sources of exchangeable calcium and magnesium. Topsoils (i.e., coversoils) contribute to mitigation of sodic conditions when soluble cations are translocated from coversoils into the underlying minesoils. This study evaluated the weathering characteristics of minesoils sites from three surface coal mines in northwestern New Mexico and northeastern Arizona. Minesoils were grouped into 11 classes based on EC and SAR. After 6 to 14 yr, differences between upper and lower halves of the coversoils suggest general increases occurred with EC, SAR, chloride (Cl(-)), and sulfate (SO(4)(2-)) with depth. Within the reclaimed minesoils, there were several significant (P < 0.05 or < 0.10) relationships among EC and SAR that related to Minesoil Class. Lower SAR levels with corresponding increases in EC compared to baseline minesoils were more apparent in upper minesoil depths (0-5 and 5-15 cm). Minesoil anion concentrations suggested coversoil leachates and gypsum dissolution influenced EC and SAR chemistry. Over time, chemical changes have increased the apparent stability of the saline and sodic reclaimed minesoils studied thereby reducing risks associated with potential aggregate slaking and clay particle dispersion.

Primary particle size distribution of eroded material affected by degree of aggregate slaking and seal development

SummaryPrimary particle size distribution (PSD) of eroded sediment can be used to estimate potential nutrient losses from soil and pollution hazards to the environment. We studied eroded sediment PSDs from three saturated soils, packed in trays (20 × 40 × 4 cm), that had undergone either minimal aggregate slaking (MAS) or severe aggregate slaking (SAS) prior to a 60 mm simulated rainstorm (kinetic energy, 15.9 kJ m−3; droplet diameter, 2.97 mm) and collected runoff at regular intervals. The degree of aggregate slaking was controlled by the rate at which soils were wetted to saturation. The PSDs of eroded materials and of parent soils were determined using a laser particle size analyser. For each soil, PSD frequency curves of eroded sediments and parent soils were generally of a similar shape but most eroded sediments had larger clay contents than their parent soils. In the SAS treatment, cumulative clay enrichment in the eroded materials was inversely related to the parent soil clay content, these being 28.5, 26.6 and 22.8% richer in clay than their parent soils for the loam, sandy clay and clay, respectively. Generally, total clay loss was greater from soils with SAS than from those with MAS because of erosion rates; however, clay enrichment of sediments, compared with parent soil clay contents, was mostly greater in samples with MAS. Greater clay enrichment took place during the early seal development stage in the loam, but could not readily be associated with specific stages of seal development for the clay. In the sandy clay, the relation between seal development and clay enrichment in the eroded material depended on the initial degree of aggregate slaking. The observed large preferential loss of clay by erosion in cultivated soils re‐emphasizes the need to employ erosion control measures.

Soil structure changes: aggregate size and soil texture effects on hydraulic conductivity under different saline and sodic conditions

Hydraulic conductivity of soil is strongly dependent on soil structure, which can be degraded during wetting and leaching. It was hypothesised that this structural degradation is dependent on initial aggregate size distribution and soil texture. The general aim of this study was to investigate the effects of aggregate sizes and soil textures, and their interactions, on the structural degradation and saturated hydraulic conductivity (Ks) of smectitic soils under different saline and sodic conditions. The studied soils were clay and loamy sand soils with low (~4.5) or high (~10) exchangeable sodium percentages (ESP), and with aggregate sizes in the ranges: (i) &lt;1 mm (small aggregates); or (ii) 2–4 mm (large aggregates). The Ks values of the samples in a column after slow or fast pre-wetting were determined by means of a constant head device. Different wetting rates and leaching under various saline and sodic conditions had no effect on the Ks of the loamy sand; however, the Ks values of this soil with large aggregates were an order of magnitude greater than those of the soil with small aggregates. In contrast, in the clay soil with large aggregates, the Ks values after fast pre-wetting were significantly smaller than those after slow pre-wetting, probably because of aggregate slaking. No significant effects of the wetting rates on Ks were found in clay soil with small aggregates. An increase in the ESP in the clay soil decreased the Ks by a factor of 1.5 for the large aggregates and by an order of magnitude for the small aggregates, mainly as a result of increased clay swelling. Leaching the clay soil with deionised water significantly decreased the Ks values, partly because of clay dispersion. Although significant structural degradation of the clay soil occurred during leaching, the Ks values were smaller in the soils with small aggregates than in those with large aggregates, indicating the importance of the initial aggregate size on Ks even in soils that are prone to structural damage.

Changes in soil hydraulic conductivity, runoff, and soil loss due to irrigation with different types of saline–sodic water

Irrigation with saline–sodic water causes sodic conditions in the soil which reduces soil productivity. We evaluated the changes in a number of important indices related to soil structural stability when treated wastewater (TWW), albeit with higher loads of organic matter and suspended solids, was used instead of more saline–sodic irrigation water, under different degrees of aggregate slaking. We studied soil saturated hydraulic conductivity (HC) using disturbed samples packed in columns, and soil infiltration rate, runoff and erosion under simulated rainfall. Aggregate slaking was manipulated by wetting the samples prior to all tests at either a slow (1–2 mm h − 1 ) or a fast (50 mm h − 1 ) rate. Samples of a calcareous silty clay (Typic Calciorthids) from the Bet She'an Valley, Israel, were taken from plots irrigated for three years with either TWW, saline–sodic Jordan River water (JRW), or moderately saline–sodic spring water (SPW), and also from a non-cultivated area (control). With little or no aggregate slaking (use of slow wetting), higher HC values and lower amounts of total runoff and soil loss were measured compared to when more severe aggregate slaking was induced (use of fast wetting). The HC values for the TWW treatment were similar to, or lower than, those for the control and significantly higher than those for the JRW treatment. For the runoff and soil loss data, differences among the water quality treatments were, generally, more pronounced when aggregate slaking was substantially reduced, and were related to soil sodicity. Under the latter condition, runoff and soil loss from the TWW treatment were comparable with those from the control and significantly lower than those from the JRW treatment. Our results suggested that replacing saline–sodic irrigation water with TWW could have favorable effects on soil structural stability, especially under conditions where aggregate slaking can be reduced (e.g., in regions with low to moderate rain intensities; and/or use of low intensity irrigation systems).

Aggregate slaking during rapid wetting: Hydrophobicity and pore occlusion

The slaking process after rapid wetting is a key factor controlling soil structural stability in dry soil, and an understanding of the relative importance of the different mechanisms involved in slaking may help in the design of management strategies aimed at maintaining a stable surface soil structure. Slaking has been linked to, among other factors, rapid pressure build-up in aggregate, and previous work has emphasized the role of organic matter to hamper that pressure build-up, possibly due to hydrophobicity, reducing rapid water entry within aggregates and hence the build-up. This study emphasizes this latter aspect linked to slaking. The evolution of the intra-aggregate pressure, the matter lost by slaking and the expelled air after rapid wetting of two soils of different textures (clay loam soil and silty-clay loam soil) amended with different types of paper sludge were studied. Hydrophobicity effects were also studied using a tensio-active solution. The results of these experiments showed that when aggregates were submitted to sudden wetting, those treated with paper sludge had an improved resistance to the destructive action of rapid wetting. The lower pressures measured in the aggregates from the amended soils and having less slaking resulted most likely from slow water entry and reduced swelling. Detailed investigation on the link between hydrophobicity and water entry revealed that the true hydrophobic effect (modification of contact angle) was non-existent for the silty-clay loam and minor for the clay loam. This study, rather, suggests that changes in the water potential at the wetting front following organic matter addition and aggregate immersion most likely depend on pore occlusion and on changes in pore surface roughness. Key words: Aggregate stability, organic matter, slaking, pressure, swelling, wettability

Effect of soil wetting conditions on seal formation, runoff, and soil loss in arid and semiarid soils—a review

Soil surface sealing is one of the main causes for low infiltration rate (IR) and high runoff and soil loss under raindrop impact conditions in arid and semiarid regions. Many studies have focused on the effects of soil properties on seal formation under fast wetting conditions. However, in the field, soils can be exposed to different wetting conditions, before an intense rainfall event, which can affect the role of the soil properties on seal formation. The present paper reviews the effects of different initial wetting conditions and their interactions with soil properties on seal formation, IR, runoff, and soil loss in smectitic soils. Fast wetting of soil causes aggregate slaking, which enhances seal formation, runoff, and soil loss under rainfall, mainly in soils with &gt; 40% clay content. An increase in clay content of the soil increases aggregate strength, but at the same time increases the slaking forces. Hence, in soils with low clay content (&lt;40%) and low aggregate stability, raindrop impact alone was sufficient to break down the aggregates and to develop a seal. In contrast, in soils with &gt; 40% clay content and high aggregate stability, slaking plays an important role in aggregate breakdown and seal formation. An increase of raindrop kinetic energy, from 8 to 15.9 kJ/m3, decreased the effect of the slaking forces on seal formation and runoff. It was suggested that the effects of raindrop kinetic energy and of the slaking forces on aggregate disintegration and seal formation are complementary. An increase in soil exchangeable sodium percentage (ESP), from 0.9 to 20.4%, decreased the effect of slaking forces on seal formation and runoff production under rainfall with 15.9 kJ/m3 kinetic energy. Probably, increasing the ESP increased the soil dispersivity, and therefore diminished the effect of the slaking forces on aggregate disintegration and seal formation. Aging (the time since wetting) of soil increased the stability of soil structure, decreased the seal formation, maintained high IR, and diminished soil loss amounts. These effects of soil aging depend on both the prewetting rate of the soil and soil texture.

Influence of Sodicity, Clay Mineralogy, Prewetting Rate, and Their Interaction on Aggregate Stability

Sodicity adversely affects soil physical conditions reflected by weak structural stability. Soil clay mineralogy influences the degree of aggregate disruption induced by sodicity. The main objective of this research was to evaluate the interactive effects of clay mineralogy, prewetting rate (PWR), and sodicity on soil aggregation. Three soils with predominantly clay minerals of smectite, vermiculite‐smectite, and kaolinite were equilibrated with NaCl–CaCl2 solutions having sodium adsorption ratio (SAR) values of 0, 20, and 50 and electrical conductivity (EC) = 3.0 dS m−1 After air‐drying, the treated samples were packed and prewetted at rates of 2 and 30 mm h−1 with the NaCl–CaCl2 solutions. Aggregate‐size distribution, macroscopic swelling, and surface soil dispersion were determined after the packed samples were equilibrated to a matric potential of −0.1 MPa. The kaolinitic soil showed the lowest inherent aggregate stability when subjected to slow PWR and the lowest SAR. Furthermore, aggregate stability of the kaolinitic loam soil was not significantly affected by increasing SAR. The Millox (smectitic) soil, on the other hand, was most susceptible to aggregate slaking, whereas the Malibu (vermiculitic) soil was most susceptible to differential swelling (at slow PWR). When SAR was low, aggregate slaking by fast PWR was the main cause of the aggregate breakdown. At SAR ≥ 20, swelling and dispersion became more important to the structural stability.