Aggregate Breakdown Mechanisms Research Articles

A new method for disentangling the coupling effect of slaking and mechanical breakdown on aggregate stability: Validation on splash erosion

Aggregate breakdown strongly influences soil erosion process, during which slaking and mechanical breakdown are two dominant breakdown mechanisms. Despite of many methods on aggregate stability, there is a paucity of satisfactory one to disentangle how these two mechanisms interact in aggregate breakdown. Herein, on the basis of Le Bissonnais method (1996) including three treatments (i.e., fast wetting (FW), stirring after pre-wetting (WS), and slow wetting (SW)), a new treatment of water-stirring after fast wetting (SF) was proposed in order to evaluate the coupling effect of slaking and mechanical breakdown on aggregate stability. The coupling effect of slaking and mechanical breakdown was larger to a varying extent than their individual effects, and its difference with slaking showed a unimodal variation with increased aggregate stability. This new method can partition the unique and shared effects of these two breakdown mechanisms; the shared effect (0–77%) decreased logarithmically with increased aggregate stability; the unique effect of slaking (16–52%) showed a unimodal variation and much larger than that of mechanical breakdown (0–17%). As validated by previously published splash erosion data over a wide range of soils (including various soil types, land uses, textures and parent materials), the proposed aggregate stability indices in this new method showed better performance in predicting splash erosion compared to existing ones (e.g., RSI×RMI and RSI/RMI). Collectively, this new method facilitates a systematic understanding on aggregate breakdown mechanisms and exhibits great superiority in aggregate stability measurement and erosion prediction.

Mechanisms of aggregate breakdown in (sub) tropical soils: Effects of the hierarchical resistance

The soil system is a dynamic component of the earth. Aggregate stability is critical for soil quality and ecosystem services. In tropical and subtropical soils, the role of clay, soil organic carbon, aluminum, and iron oxides on aggregate stability is somewhat controversial. In addition, aggregates develop a hierarchical organization; however, this process is poorly evaluated in the context of aggregate stability on different breakdown mechanisms such as raindrop impact, slaking (entrapped air), breakdown by differential swelling, and abrasion. Therefore, the objective of this study was: 1) to evaluate the aggregate stability of (sub) tropical soils in terms of four different breakdown mechanisms; 2) to ascertain the existence of hierarchical resistance to break down mechanism of pre-selected aggregates classes; 3) to determine whether soil chemical–mineralogical attributes influence aggregate stability. Five Oxisols originated from igneous rocks along a transect were evaluated. The soil aggregate stability responded according to hierarchical resistance, but differently to each breakdown mechanism: abrasion, slaking, differential swelling, and raindrop impact. Overall, the larger aggregates (4–8 mm) corresponded to a lower resistance to abrasion and raindrop impact. In contrast, the smaller aggregates (1–2 mm) were more affected by slaking than raindrop impact. Soil organic carbon, except for large aggregates, and soil texture did not show a significant role on aggregate stability. Silicon oxides increased the resistance to abrasion, while iron oxides increased the resistance to slaking. In contrast, aluminum oxides decreased the aggregate stability (slaking). The aggregate stability was influenced by method deployed and pre-selected classes of aggregate size. In this study, rainfall simulation (raindrop impact) and wet sieving (fast wetting) were the recommended methods to evaluate (sub) tropical soil aggregate stability. An aggregate size of 2–4 mm is preferable if a single class is selected.

The relative contributions of soil hydrophilicity and raindrop impact to soil aggregate breakdown for a series of textured soils

Soil aggregate breakdown is the first key factor that causes soil erosion. At present, research on the mechanisms of soil aggregate breakdown during rainfall is common. However, the research to on quantifying the relative contributions of internal and external forces to aggregate breakdown remains limited. This paper was conducted to analyse the relative contribution of internal and external forces to aggregate disintegration and the factors affecting aggregate stability during rainfall. Soil aggregates with a series of textures were selected as test soil samples; deionized water was employed as the soaking solution and rainfall material in static disintegration experiments and rainfall simulation tests. The effect of internal force (soil hydrophilicity) on aggregate disintegration was analysed by the static disintegration method, and the combined effects of internal force (soil hydrophilicity) and external force (raindrop impact) on soil aggregate breakdown were analysed by rainfall simulation experiments. The results indicated that external force caused more severe soil aggregate breakdown than internal force, and the crushed aggregate was mainly distributed in the range of 2–0.25 mm. With increasing rainfall kinetic energy, the degree of aggregate breakdown increased gradually, and the degree of aggregation of the soil particles decreased gradually. Furthermore, soil aggregates with a high clay content (> 30%) were more stable than medium-clay (20–30%) and low-clay (< 20%) soil aggregates, and the correlation coefficient provided a good representation of the relationship between the clay content and soil aggregate stability index (ASI). Therefore, external force contributed more to soil aggregate breakdown than internal force during rainfall, and clay plays an important role in aggregate stability. The results of this study are of great significance for elaborating the mechanism and factors affecting aggregate breakdown.

Quantifying the contribution of phyllosilicate mineralogy to aggregate stability in the East Asian monsoon region

Aggregate stability strongly influences soil function and has been a key indicator of soil quality and susceptibility to erosion. Although clay mineralogy has been recognized to have substantial effects on aggregate stability, little quantitative information is available on the specific contribution of various phyllosilicates in clay fraction. This study aimed to quantify the individual and combined effects of phyllosilicate minerals and their interaction with other soil properties on aggregate stability and breakdown mechanisms. A wide range of soil types in heavy textures from quaternary sediments were sampled under three land uses (forest, grass and arable) and at different genetic horizons along the mid-temperate to tropical climate gradient in the East Asian monsoon region. Soil aggregate stability against slaking (MWDFW), mechanical breakdown (MWDWS) and differential swelling (MWDSW) was determined, as well as phyllosilicate mineralogy by X-ray diffraction and a series of soil properties related to porosity, texture, organic matter, sesquioxides, and exchangeable cations. Aggregate stability generally exhibited a unimodal geographic trend (MWDSW in 1.85 ~ 3.49 mm, MWDWS in 0.76 ~ 3.22 mm and MWDFW in 0.14 ~ 2.74 mm), which became more remarkable with increased disruptive forces (coefficient of variation in 9 ~ 59%). Correspondingly, the dominant mechanisms of aggregate breakdown shifted from both slaking and mechanical breakdown to only slaking with increased soil weathering. The profile variation of aggregate stability and the land use effect differed with soil type and disruptive force. For phyllosilicate mineralogy, vermiculite had the largest explanatory power in aggregate stability (R2 = 54%, p < 0.001), followed by kaolilite and 1.4 nm intergrade mineral (R2 = 28 ~ 32%, p < 0.01), while illite explained little variance (R2 = 6%, p > 0.05), demonstrating the more prominent role of phyllosilicate mineralogy than other soil properties (R2 < 43%) in aggregate stabilization. Besides the dominant effect of vermiculite, soil organic carbon, exchangeable sodium and calcium exerted a complimentary effect (R2 = 12%). Collectively, aggregate stability is predominantly influenced by phyllosilicate mineralogy (especially swelling minerals) at the regional scale and by soil organic matter and exchangeable cations at the soil pedon scale.

Effects of erosion-induced land degradation on effective sediment size characteristics in sheet erosion

Land degradation by erosion alters soil properties to a varying extent and definitely influences the subsequent soil erosion. However, little is known about the impact of erosion-induced land degradation on the erosion process from the aspect of sediment size characteristics, which may not only indicate aggregate breakdown and sediment transport mechanism, but also influence sediment deposition. Here, the effective sediment size distribution (undispersed) and its dynamics during sheet erosion on different eroded lands (non, moderately and very-severely eroded) were studied under field rainfall simulation on five types of soils (Calcic Luvisols, Ferric Luvisols, Plinthic Alisols, Plinthic Acrisols and Acric Ferralsols) formed separately on loess deposits, quaternary red clays and basalt from temperate to tropical climate. The rainfalls at the intensities of 45 and 90 mm h−1 were simulated on pre-wetted bare fallow plots (3 × 0.8 m2, 10° slope). The effect of erosion degree on the effective sediment size showed a unimodal trend from temperate Luvisols to tropic Ferralsols, coupled with the influence of rainfall intensity to a varying extent. Under the low-intensity rainfall, except for Ferralsol, the moderate erosion degree increased the sediment size by −49% to 72% relative to non-eroded lands, in contrast to a decrease of 6–64% for the very-severe erosion degree. The high-intensity rainfall was shown to lessen the erosion degree effect for Luvisols, magnify it for Alisol, and change it to positive for Acrisol. Sediment transport was dominated by suspension-saltation of <0.10 mm size fraction for Luvisols regardless of erosion degree due to the weak soil structure, and shifted from both rolling (0.5–2 mm) and suspension-saltation on the non/moderately eroded lands to only suspension-saltation on the very-severely eroded lands for the other soil types. Rainfall intensity, pH and bulk density contributed jointly to sediment size (Adj-R2 = 0.76, p < 0.001), and exchangeable sodium showed a positive effect on its temporal variation (Adj-R2 = 0.36, p < 0.001). The varying effects of land degradation by erosion on sediment size and transport mechanism at the soil pedon and regional scales merit consideration in land management and rehabilitation as well as erosion modeling.

Estimation of aggregate stability of some soils in the loam belt of Belgium using mid-infrared spectroscopy

Rapid assessment of soil aggregate stability (AS) is vital for a better understanding of the processes of soil aggregate breakdown, which is necessary for effective soil erosion control planning. This study explored the possibility of quantifying the mean weight diameter (MWD) of soil aggregates after applying the three disruptive treatments of fast wetting (FW), slow wetting (SW) and mechanical breakdown (MB), representing the dominant aggregate breakdown mechanisms, using a handheld mid-infrared (MIR) spectrophotometer on soils of the Belgian loam belt. Partial least squares regression (PLSR) was used to build calibration models for the three treatment methods, using a calibration set accounting for 70% (75% in the case of MB) of the samples, which were validated by using a validation set of 30% (25% in the case of MB) of the total samples. Results showed very good prediction accuracies with coefficient of determination (R2) values of 0.73, 0.85 and 0.79, root mean square error of prediction (RMSEP) values of 0.29, 0.46 and 0.26 mm, ratio of performance to interquartile range (RPIQ) values of 2.16, 4.98 and 3.64 and residual prediction deviation (RPD) values of 1.99, 2.60, and 2.22, for FW-MWD, SW-MWD, and MB-MWD, respectively. The best prediction was obtained for SW-MWD. The results of this study suggest that, for soils where soil organic carbon is a relevant indicator of aggregate strength, the three AS indices can be estimated to a high accuracy with spectra information in the MIR region of the electromagnetic spectrum.

Modeling aggregate size distribution of eroded sediment resulting from rain-splash and raindrop impacted flow processes

Soil susceptibility to detachment and transport sub-processes of erosion is generally controled by the aggregate breakdown mechanism. Measuring particle size and aggregation to the estimate erodibility potential of soils is important under erosive rainfall conditions. The Aggregate Size Distribution (ASD) is one of the most important determinants of soil structure along with soil organic matter content for describing the efficiency of applied, sustainable management strategies. This study aimed to compare the performances of three different aggregate size distribution models to predict the characteristic aggregate size parameter (median diameter, D50) for eroded sediment from interrill erosion processes of Rain-Splash Transport (RST) and Raindrop Impacted Flow Transport (RIFT). The ASDs of 1143 collected sediment samples from the RST and RIFT processes were measured and modeled by the Log-normal, Fractal, and Weibull approaches. The D50 value, as a characteristic parameter for aggregate size distributions, derived from the cumulative ASD curve was compared for soils from different land use types and different slope and rainfall intensity conditions. The performance of each model was evaluated using the Mean Square Error (MSE) and Coefficient of Determination (R2). The Weibull approach was the most accurate model showing the best fit with the lowest MSE values (0.0002 ≤ MSE ≤ 0.0048) and having the greatest R2 values (0.936 ≤ R2 ≤ 0.998) when compared with the Log-normal and Fractal models. Herewith, for semi-arid land use and soil, specific shape and scale parameters for the Weibull distribution, the respective ASDs were successfully re-generated for modeling the eroded sediment of the simulated RST and RIFT interill processes.

Impact of drying-wetting cycles on the soil aggregate stability of Alfisols in southwestern China

Drying-wetting cycles are important environmental factors controlling potential changes in aggregate stability. However, the relationships between soil aggregate stability and drying-wetting cycles under different breakdown mechanisms are not clear. We conducted a simulation study to investigate the effects of drying-wetting cycles on aggregate stability under the three main breakdown mechanisms (slaking, microcracking, and mechanical breakdown) without soil organic matter inputs. Four initial aggregate size classes (i.e., 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm) within a clay-loam soil, which undergoes intense drying-wetting cycles, were selected and subjected to eight levels of drying-wetting cycles (i.e., 0, 1, 2, 3, 5, 7, 10, and 15 cycles). Aggregate stability was measured with three treatment methods involving fast wetting (FW), slow wetting (SW), and shaking after prewetting treatments (ST). Our study showed higher resistance to mechanical breakdown and lower resistance to slaking. Compared with the mean weight diameter (MWD), the normalized mean weight diameter (NMWD) was a better indicator of comparing the susceptibility of different initial size aggregates to breaking up into smaller aggregates. The values of NMWD for 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm aggregates were 0.1, 0.1, 0.09, and 0.09, respectively, for FW; 0.35, 0.34, 0.33, and 0.32, respectively, for SW; and 0.46, 0.45, 0.44, and 0.42, respectively, for ST. This indicated that as initial aggregate size increased, aggregates were more prone to breaking down. The effects of drying-wetting cycles on aggregate stability varied greatly among different aggregate breakdown mechanisms. For 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm size aggregates, drying-wetting cycles decreased MWD by 61%, 67%, 70%, and 53%, respectively, for SW, and 69%, 72%, 80%, and 78%, respectively, for ST. For FW, MWD increased by 5%, 7%, 8%, and 36% after the first two cycles, respectively, while it decreased by 36%, 36%, 44%, and 15% after 15 cycles. The results showed that different aggregate breakdown mechanisms influence the effects of drying-wetting cycles on aggregate stability. This study provides an important basis for better soil management practices and yields insights for an improved understanding of the role of drying-wetting cycles on aggregate stability.

Effects of slaking and mechanical breakdown on disaggregation and splash erosion

SummaryThe contributions of different mechanisms of aggregate breakdown to splash erosion are still obscure. This study was designed to investigate the effects of various mechanisms of soil disaggregation on splash erosion. Loamy clay, clay loam and sandy loam soil types were used in this research. Soil aggregate stability was determined by the Le Bissonnais method. Deionized water was used to simulate the combined effect of slaking and mechanical disaggregation, whereas alcohol was used to estimate the contribution of mechanical breakdown only. Simulated rain with an intensity of 60 mm hour−1 was applied at five heights (0.5, 1, 1.5, 2 and 2.5 m) to achieve different amounts of rainfall kinetic energy. The results indicated that the rate of splash erosion increased with the increase in rainfall kinetic energy in tests with both deionized water and alcohol. The rates of splash erosion for three types of soil followed the order of loamy clay soil &lt; clay loam soil &lt; sandy loam soil, but the mean weight diameter (MWD) of disintegrated aggregates followed the reverse order. The rates of splash erosion from the effects of slaking and mechanical breakdown increased with an increase in rainfall kinetic energy. The contributions of slaking and mechanical breakdown to splash erosion decreased for the former, whereas it increased for the latter as rainfall kinetic energy increased. The slaking effect contributed more than 50% of splash erosion. The rates of contribution of slaking and mechanical breakdown to splash erosion depended on rainfall kinetic energy and soil type.Highlights Contributions of different mechanisms of aggregate breakdown to splash erosion remain obscure. Alcohol was used to simulate the effect of mechanical breakdown only. Slaking contributed more than 50% of splash erosion. Contributions by mechanisms of aggregate breakdown depend on rainfall kinetic energy and soil type.

Aggregate strength in calcareous soil fertilized with pig slurries

Effects of applications of pig slurry on soil aggregate stability are not well understood in dryland agriculture. This research aims to (i) identify aggregate stability tests that give a reliable description of the soil's behaviour when pig slurry (PS) is applied to calcareous soil and (ii) interpret them in terms of chemical, biological, morphological and physical soil properties for soil quality assessments. Soil samples from eight fertilizer treatments (mineral fertilizers and PS), applied over seven growing seasons were analysed. We applied five methodologies to examine different mechanisms of aggregate breakdown. Porosity was characterized by image analyses. There was minimum resistance to the mechanical breakdown of aggregates when slurries had been applied 12 months before analysis. Recent applications of slurry (3 months before the analysis) improved resistance to implosion caused by the penetration of water into dry aggregates (slaking), although the opposite result can occur if the method of evaluation is not chosen properly. Recent applications of PS also enhanced soil respiration and increased soil porosity in the 25–100‐µm size range (packing pores between aggregates) and in the 100–400‐µm size range (interaggregate or faunal pores). In dryland systems and in the winter cereal cropping season, the resistance of dry aggregates to slaking is improved temporarily if PS is applied at N rates equivalent to around 1.7 Mg OM ha−1 year−1.HighlightsHow does soil aggregate strength change in response to pig slurry (PS) applications? Improve understanding of PS as a fertilizer and its effect on soil quality assessments Changes in aggregate stability are controlled by time since pig slurry application The effects of PS on the increase in packing pores between aggregates are transient

Evaluation of methods for determining soil aggregate stability

Aggregate stability is widely used as an indicator of soil health and erosion risk, however its use appears to be greatly hindered by a lack of standardised procedures and instrumentation. Aggregate stability can be measured using a number of different approaches each applying different types and levels of disruptive energy, which may or may not adequately match the type and level of disruptive energy experienced by soil aggregates in the field. This paper explores the effects of various approaches for the measurement of aggregate stability, and reports on the mechanisms responsible for the breakdown of intensively cultivated sandy clay loam and sandy loam in Tasmania, Australia. Stability was measured by exposing soil aggregates to rainfall simulation (RS), wet sieving (WS), ultrasonic vibration (UV) and clay dispersion (CD). Analysis was conducted on 2.00–4.75mm aggregates that were either (i) air-dry (DRY) or (ii) re-moistened to field capacity at −10kPa (MOIST). The mechanisms responsible for aggregate breakdown were explored by comparing values of aggregate stability determined using different fluids. Correlations between the various methods for determining aggregate stability were lower than expected. The highest correlation existed between the DRY—RS and DRY—WS (R2=0.749, P<0.001) procedures, whilst some procedures were not significantly correlated at all, especially for aggregates pre-wetted to field capacity. Low correlation between procedures suggests that the mechanisms of aggregate breakdown and thus the type of disruptive energy applied to the aggregates significantly differed between procedures. Efforts to standardise measurement of aggregate stability are not supported as use of a single approach may not adequately predict the behaviour of aggregates in the field. Choice of procedure should aim to mimic the types and levels of disruptive energy experienced under field conditions.

Aggregation agents and structural stability in soils with different texture and organic carbon contents

The flat pampas in the state of Santa Fe in Argentina have soils with high silt content, variable carbon content, and diverse degrees of structural degradation. Aggregate stability has been used as an indicator of the structural condition of the soil. This study aimed to quantify the effect of the addition of crop residues and root activity on the agents of aggregation and mechanisms of aggregate breakdown in soils with different carbon contents and textures cultivated under no-till. An experimental trial was conducted on a loamy soil (Typic Hapludoll, Santa Isabel series) and a silty soil (Typic Argiudoll, Esperanza series) under controlled conditions for 112 days with the following treatments: (i) with and without wheat plant growth and (ii) with and without addition of wheat residues. Soil structural stability by a method allowing for differentiation of aggregate breakdown by slaking, mechanical effect and microcracking, total organic carbon content, particulate organic carbon, glomalin and carbohydrate fractions was assessed. In general, the addition of residues and the presence of plant with active roots increased the presence of all aggregation agents and decreased aggregate breakdown processes in both soils. Soluble carbohydrates and proteins related to glomalin were the most important aggregating agents and their function was to reduce the magnitude of breakdown mechanisms, slaking and microcracking, evidencing a greater impact on the silty soil.

Evaluation of soil aggregate microstructure and stability under wetting and drying cycles in two Ultisols using synchrotron-based X-ray micro-computed tomography

Soil structure and the soil pore system are important for many soil environmental processes. However, little is known about the influence of intra-aggregate microstructure on aggregate stability. The objective of this study was to evaluate the effect of soil pore characteristics on wet aggregate stability and aggregate tensile strength (TS) using synchrotron-based X-ray micro-computed tomography (SR-μCT) under wetting and drying cycles. Undisturbed topsoils (200cm3) derived from shale (S) and Quaternary red clay (Q) were submitted to three wetting and drying treatments under controlled laboratory conditions. 3–5mm aggregates were collected from different treatments and scanned at 3.7μm voxel-resolution for the reconstruction of 3D micro tomography images. The wet aggregate stability and TS were measured after each treatment. The relationships among pore characteristics, wet aggregate stability, and TS were analyzed using partial least squares regression (PLSR). The results indicated that porosity (P), percentage of pores >100μm (Pd>100), and fraction of elongated pores (FEP) all significantly increased with an increasing number of wetting and drying cycles, while the opposite trends were observed for the total number of pores (TNP), the percentage of pores with a diameter of 75–100, 30–75, and <30μm. Decrease in wet aggregate stability and TS of both soils was mainly due to higher P, a more extensive and complex pore network, which caused increased air pressure, increased rate of water entry, and high probability of crack propagation and interaction. The TNP, Pd75–100, P, and Pd>100, were identified as the primary factors controlling the wet aggregate stability and TS according to PLSR. The pore characteristics and soil clay content accounted for as much as 99% of the variation in wet aggregate stability and TS. This study provides insights for improved understanding of the change in topsoil microstructure; however, the effect of pore characteristics on the aggregate breakdown mechanism requires further investigation.

Aggregate breakdown mechanisms as affected by soil texture and organic matter in soils dominated by primary minerals

Soil aggregates breakdown mechanisms depend on soil properties such as texture, clay mineralogy and organic matter content. Little is known about the effect of soil properties on aggregate breakdown mechanisms in South African soils. The objective of this study was to establish the relationship between aggregate breakdown mechanisms and selected soil properties in some South African soils dominated by primary minerals. Soil samples with varying properties were collected from the surface 0–0.2 m from 14 ecotopes in Eastern Cape province. Aggregate stability was determined following the fast wetting (FW), slow wetting (SW) and wet stirring (WSt) methods. Soils with high quartz were the least stable due to its inability to bond with other clay minerals or soil organic matter (SOM). In contrast, soils with high kaolinite were the most stable. Slaking was the dominant aggregate breakdown mechanism in most of the studied soils. Aggregate stability was significantly correlated with particulate organic matter (POM) for FW (r2 = 0.74, P < 0.05) and SW (r2 = 0.64, P < 0.05). It was concluded that increasing POM content in soils through SOM inputs and farming methods that reduce POM mineralisation can reduce slaking during rapid wetting by raindrops.

Effects of stability, transport distance and two hydraulic parameters on aggregate abrasion of Ultisols in overland flow

During runoff on soil surface, aggregates travel, abrade, and produce finer and more transportable particles, resulting in a significant effect on sediment transport, water infiltration and soil erosion. This study was to assess the effects of stability, transport distance and two hydraulic parameters on aggregate abrasion using Ultisols in subtropical China. Aggregate stability was investigated for soils from two parent materials (Shale and Quaternary red clay) with different land uses using LB methods. They were studied with a series of aggregate abrasion experiments in a 3.8m long flume with a fixed bed. Results indicated that slaking and mechanical breakdown were probably the main mechanisms of aggregate breakdown in these study soils. Two stages of aggregate abrasion process could be distinguished via the coefficient α in different transport distances. Aggregates were first rapidly abraded, became round, and was predominantly broken apart into smaller fragments, followed by weakly abrasion of smaller fragments and round aggregates, making them even light and less angular. A good multiple regression equation was obtained for estimating aggregate abrasion in overland flow with consideration of mechanical breakdown index RMI and transport distance x. From two selected Ultisols (SX3 and QX1) and under the combination of five slopes and five discharges, exponential relationships were found between aggregate abrasion and flow depth or friction factor. This research makes a good attempt at analyzing the aggregate abrasion in overland flow from simple laboratory experiments settings, and the results of the analysis could be valuable for the development of adequate soil erosion models.

Interrill erosion from disturbed and undisturbed samples in relation to topsoil aggregate stability in red soils from subtropical China

Abstract The assessment of soil erodibility to water erosion in the field is often expensive and time-consuming. This study was designed to reveal the effects of aggregate breakdown mechanisms on interrill erosion dynamics and develop an improved model for assessing interrill soil loss, which incorporated the soil aggregate stability tests as a substitute for the interrill erodibility parameter, from both disturbed and undisturbed samples for red soils in subtropical China. Six cultivated areas of sloping land with red soils were selected, and topsoil aggregate stability was analyzed using the Le Bissonnais method to determine the different disaggregation forces. Laboratory rainfall simulations were designed to distinguish the effects of slaking (at different wetting rates) and mechanical breakdown (with and without screening) on soil erosion characteristics. Field rainstorm simulations with medium and high rainfall intensities were conducted on runoff plots (2 m 1 m) with slope gradients varying from 10% to 20% for each soil type. A new instability index, Ka , which considers aggregate breakdown mechanisms in interrill erosion processes, was proposed based on the disturbed sample results. Ka showed a close relationship with erosion rates in both disturbed and undisturbed samples. Following from the results of undisturbed sample experiments, Ka was used as a substitute for the erodibility factor, and introduced into the WEPP model, establishing a new erosion predication formula for red soils which had a good correlation coefficient ( R 2 = 0.89**). This research made a good attempt at estimating the interrill erosion rate on the basis of aggregate stability from simple laboratory determinations. These results extend the validity of soil aggregation characterization as an appropriate indicator of soil susceptibility to interrill erosion in red soils from subtropical China. The formula based on the instability index, Ka , has the potential to improve the methodology used for assessing interrill erosion rates.

Estimating interrill soil erosion from aggregate stability of Ultisols in subtropical China

During raindrop impact soil, aggregates breakdown and produce finer, more transportable particles and micro-aggregates. These particles and micro-aggregates appreciably affect the processes of infiltration, seal and crust development, runoff, and soil erosion. Aggregate stability is, therefore, an important property that may explain, quantify, and predict these processes. This study was designed to develop improved formulae for assessing interrill erosion rate by incorporating the aggregate stability index ( A s) in the prediction evaluations for soil erodibilites of Ultisols in subtropical China. Field experiments of simulated rainfall involving rainstorm simulations with medium and high rainfall intensity were conducted on six cultivated soils for which the soil aggregate stability was determined by the LB-method. This study yielded two prediction equations D i = 0.23 A s I 2(1.05 − 0.85 exp −4sin θ ) and D i = 0.34 A s qI(1.05 − 0.85 exp −4sin θ ) that allowed a comparison of their efficiency in assessing the interrill erosion rate. A s is an aggregate stability index, which reflected the main mechanisms of aggregate breakdown in interrill erosion process, θ is the slope angle, I is the rainfall intensity, and q is the runoff rate. Relatively good agreement was obtained between predicted and measured values of erosion rates for each of the prediction models ( R 2 = 0.86 **, and R 2 = 0.90 **). It was concluded that these formulae based on the stability index, A s, have the potential to improve methodology for assessing interrill erosion rates for the subtropical Chinese Ultisols. Considering the time-consuming and costly experimentation of runoff rate measurements, the equation without runoff rate ( q) was the more convenient and effective one to predict interrill erosion rates on Ultisols of subtropical China.

Comparison of two methods for aggregate stability measurement - a review

Soil structure is a very important soil property, which influences many processes in the soil. There are many methods for aggregate stability measurement varying in the energy applied in the treatment. The aim of this paper is to compare two aggregate stability measurement methods on a set of reclaimed dumpsite soils. Method proposed by Le Bissonnias (1996) is composed of three tests, which allow distinguishing the particular aggregate breakdown mechanisms. Results can be expressed by a coefficient of vulnerability (Kv). Results of the second method, assessment of water stable aggregates, can be expressed by WSA index. WSA indexes mainly correspond to the results of the first test, which qualify the aggregate breakdown during the fast wetting. A strong statistically significant relationship was found between WSA and Kv for each test. Correlation coefficients were &amp;ndash;0.767, &amp;ndash;0.806, and &amp;ndash;0.741 for linear models. Our conclusion is that results of both methods are comparable.

SW—Soil and Water: Linking Clay Enrichment and Sediment Delivery Processes

Recent research has directed attention to the size distribution of eroded material because of its influence on deposition mechanics and in carrying capacity of pollutant materials. At first, in this paper the relationship between aggregate breakdown mechanism and erosion processes is briefly reviewed. Then the link between the clay enrichment ratio and the sediment delivery ratio at morphological unit scale is investigated. For 129 soil samples well distributed over the Sicilian Sparacia basin, the values for the experimental clay enrichment ratio calculated by the measured ultimate grain-size distributions are compared with the theoretical clay enrichment ratio values obtained by a procedure based on effective grain-size distribution.The clay content measurements, carried out in sediment samples drawn out from the sedimentation tank located at the basin outlet during the period February 1996–February 1997, are used to calculate the coefficient appearing in the relationship applied to estimate the sediment delivery ratio of each morphological unit.A relationship for estimating this coefficient using sediment clay content, measured by a conventional ultimate particle-size distribution, is also proposed.Finally, a Monte Carlo technique and caesium-137 measurements carried out in the experimental basin are used for verifying the accuracy of the sediment delivery distributed model which is strongly dependent on the estimate of the coefficient appearing in the sediment delivery ratio relationship.

Mechanisms of aggregate stabilization in Ultisols from subtropical China

The quantification and interpretation of aggregate stability depend on internal soil properties and external factors such as measurement method and aggregate size. The objectives of this study were to: (i) determine the aggregate stability in Ultisols from subtropical China applying the Le Bisssonais Method; (ii) determine the effect of initial aggregate size on its stability, and (iii) interpret mechanisms of aggregate stabilization in the soils. Three aggregate-size ranges (5–3, 3–2 and 2–1 mm) were obtained by dry sieving. After the wetting treatments, the dominant fraction of fragments for each soil was 2–1 mm or 0.63–0.2 mm. The mechanisms of aggregate breakdown was in the order, slaking>mechanical breakdown>micro-cracking. They differed with soil type and composition. The normalized mean weight diameter (NMWD) of the aggregates after fast wetting and wet stirring were more correlated with soil properties, such as degree of micro-aggregation (DOA), cation exchange capacity (CEC), K 2O, Fe 2O 3 or Al 2O 3 rather than clay and soil organic carbon (SOC) content. The binding force by soil organic matter was smaller than the force caused by entrapped air or the force of combination of mechanical stress by stirring and differential swelling of minerals. The smaller the aggregate, the larger was the aggregate stability according to NMWD. The rankings of the soils differed with the soil aggregate sizes and the wetting treatments. Sandy loams from sandstone (Sc and Sw) were the weakest soils while the purple mudstone (Pp) was the strongest. All the cultivated soils decreased in aggregate stability compared with the comparable uncultivated soils or parent materials irrespective of the cultivation time and the changes in SOC content after cultivation.