Aggregate Breakdown Research Articles

Role of free iron oxides in the physicochemical and mechanical properties of natural clay

While it is widely acknowledged that interparticle bonding due to free iron oxide (FIO) dramatically affects the behavior of natural clay, the exact role of FIO in determining soil physicochemical and mechanical properties and the underlying mechanism remain unknown. In response, this study removes FIO from natural Zhanjiang clay, a structured marine clay with cementation, to investigate how FIO affects the physicochemical and mechanical behaviors of soil. The immersion method and leaching method (with dithionite-citrate-bicarbonate solution) are used for FIO removal. The test results indicate that the removal of FIO significantly changes the soil physical indices and degrades the mechanical properties. The effects of FIO removal are explained from the perspective of microstructural alterations as revealed by scanning electron microscopy (SEM) observations, mercury intrusion porosimetry (MIP) tests, energy-dispersive X-ray spectroscopy (EDS), and zeta potential measurements. FIO is distributed at the contacts between soil particles and exists in the bridging form. Due to the attraction between the positively-charged FIO and the negatively-charged clay mineral surfaces, soil particles are bounden to form larger soil aggregates with enhanced stability. The removal of FIO leads to the breakdown of soil aggregates and the generation of large pores, thereby increasing soil compressibility and decreasing the shear strength. This study provides some new micro-level understandings of how FIO affects the behavior of natural clay.

Tracking in-situ soil aggregate turnover under raindrop impact and wetting-drying cycles using rare earth elements

Raindrop impact and wetting-drying cycles of soil strongly influence aggregate breakdown and reformation, but these mechanisms have not been examined simultaneously under field conditions. This study examined soil breakdown and aggregation in situ following rain events and inter-event soil moisture variations (referred herein as wetting-drying cycles) using rare earth elements (REEs) as tracers. Four natural aggregate size ranges (8–19 mm, 2–8 mm, 0.075–2 mm, and <0.075 mm) of soil from an agricultural field were tagged with different REEs and returned to the designated plot in the field. Soil samples were collected from the plot following rain events three days and twenty-six days after aggregate application and separated into the original four size ranges to analyze their REE concentrations. Using the change in REE concentrations from the original tagging, it was determined that there were greater proportions of aggregate breakdown (33% ± 11%) than formation (9% ± 4%) during the first three days resulting from relatively high rain power (0.051 ± 0.004 W m−2). In the next twenty-three days, more aggregates were found to reform (16% ± 22%) than break (10 ± 12%) as soil water content decreased from 33% to 23% then increased to 38% following relatively small rain events (rain power was 0.002 ± 0.005 W m−2). The turnover rate was faster for the 2–8 mm (0.014 day−1) and 0.075–2 mm (0.019 day−1) aggregates than for the 8–19 mm and <0.075 mm fractions (0.009 day−1). For a given aggregate size fraction, the turnover rate slowed down over the experiment time. This study demonstrates how REEs can track the aggregate life cycle in situ and advances the fundamental understanding of aggregate dynamics in agroecosystems in the context of raindrop impact and wetting-drying cycles.

Enhanced remediation of a real HCH-polluted soil by the synergetic alkaline and ultrasonic activation of persulfate

The desorption of hydrophobic organic compounds (HOCs) and limited mass transfer in soil systems is a significant challenge for efficient soil remediation by oxidation treatments. The utilization of sonochemistry is a promising technology to enhance the decontamination of HOCs-polluted soils. In this work, ultrasound (US) was coupled to NaOH for activating persulfate (PS) to enhance the remediation of a real soil polluted with hexachlorocyclohexanes (HCHs) (ΣHCHs = 404 mg kg−1).Batch experiments (mass aqueous/soil ratio, VL/WS = 2) were performed to evaluate the effect of US on HOCs desorption and oxidation. Moreover, the influence of US power (0–245 W, corresponding to 0–91 W L-1 of US power density) and the initial oxidant concentration (CPS = 10–60 g L-1) on pollutants abatement, dechlorination degree, and oxidant consumption have been studied. Scanning electron microscopy (SEM) images verified that the US facilitates the breakdown of soil aggregates, enhancing the desorption of trichlorobenzenes (TCBs) (generated from HCHs alkaline hydrolysis) from the soil. Moreover, their subsequent oxidation is favouring because of higher radical species concentrations and the temperature rise. An increase in the US power up to 165 W accelerates the production rate of radicals, improving the pollutants’ degradation. The difference between pollutant oxidation and dechlorination decreases with increasing US power, associated with a lower concentration of intermediate chlorinated compounds. In the same way, the initial oxidant concentration plays a fundamental role in the remediation treatment. At the selected operating conditions (CPS = 60 g L-1, NaOH/PS = 2, 165 W), a pollutants degradation and dechlorination of 0.94 and 0.74, respectively, were achieved in just 3 h of reaction time.

Effect of high-intensity ultrasound on the structural, rheological, emulsifying and gelling properties of insoluble potato protein isolates

The denaturation and lower solubility of commercial potato proteins generally limited their industrial application. Effects of high-intensity ultrasound (HIU) (200, 400, and 600 W) and treatment time (10, 20, and 30 min) on the physicochemical and functional properties of insoluble potato protein isolates (ISPP) were investigated. The results revealed that HIU treatment induced the unfolding and breakdown of macromolecular aggregates of ISPP, resulting in the exposure of hydrophobic and R–SH groups, and reduction of the particle size. These active groups contributed to the formation of a dense and uniform gel network of ISPP gel and insoluble potato proteins/egg white protein (ISPP/EWP) hybrid gel. Furthermore, the increase of solubility and surface hydrophobicity and the decrease of particle size improved the emulsifying property of ISPP. However, excessive HIU treatment reduced the emulsification and gelling properties of the ISPP. Meanwhile, HIU treatment changes the secondary structure of ISPP. It could be speculated that the formation of a stable secondary structure of ISPP initiated by cavitation and shearing effect might play a dominant role on gel strengthens and firmness. Meanwhile, the decrease in relative content of β-turn had a positive effect on the formation of small particle to improve emulsifying property of ISPP.

Fate of Soil Carbon Transported by Erosional Processes

The accelerated process of soil erosion by water and wind, responsible for transport and redistribution of a large amount of carbon-enriched sediments, has a strong impact on the global carbon budget. The breakdown of aggregates by erosivity of water (raindrop, runoff) and wind weakens the stability of soil C (organic and inorganic) and aggravates its vulnerability to degradation processes, which lead to the emission of greenhouse gases (GHGs) including CO2, CH4, and N2O, depending on the hydrothermal regimes. Nonetheless, a part of the eroded soil C may be buried, reaggregated and protected against decomposition. In coastal steep lands, (e.g., Taiwan, New Zealand) with a short distance to burial of sediments in the ocean, erosion may be a sink of C. In large watersheds (i.e., Amazon, Mississippi, Nile, Ganges, Indus, etc.) with a long distance to the ocean, however, most of the C being transported is prone to mineralization/decomposition during the transit period and is a source of GHGs (CO2, CH4, N2O). Land use, soil management and cropping systems must be prudently chosen to prevent erosion by both hydric and aeolian processes. The so-called plague of the soil, accelerated erosion by water and wind, must be effectively curtailed.

Breakdown of dry aggregates by water drops after applications of poultry manure and spent mushroom wastes

Evaluation of soil properties that enable aggregates to resist breakdown from impacting water drops is useful in modeling soil erosion process and tillage of some tropical soils. The effect of applications of poultry manure and spent mushroom wastes on soil properties related to resistance of aggregates of a degraded ultisol to water drops detachment energy (D) in southern Nigeria was studied. The area was under continuous field trials for maize crop. The treatments were: control (PM0SM0), 5 and 10 t ha−1 poultry manure (PM5 and PM10, respectively), 5 and 10 t ha−1 spent mushroom wastes (SM5 and SM10, respectively), and 5 t ha−1 poultry manure plus 5 t ha−1 spent mushroom wastes (PM5 + SM5) applied in seven consecutive years. Experimental design was a randomized complete block design (RCBD) in five replications. Soil samples were analyzed for texture, organic matter, aggregate stability, water retention, bulk density, clay mineralogy and water drop test. Results revealed that seven-year applications of PM5, SM5, SM10, and PM5 + SM5 treatments had significant positive effects on soil structural and hydraulic properties than PM10. However, in comparison to PM0SM0, applications of 10 t ha−1 PM had significant positive effects on these properties. Saturated hydraulic conductivity was 46.6, 35.4, 32.6 and 25.5 cm h−1 in PM5 + SM5, SM10, SM5 and PM10 soils, respectively; compared to 9.6 cm h−1 in PM0SM0. A low aggregate ratio (AR) obtained for PM10 compared to higher values for SM10 and PM5 + SM5 suggest that PM10 in this case had a disaggregating effect on the soil aggregates. Sand content accounted for 81.2% of negative effect on water drop detachment energy (D), while clay content accounted for 86.8% of the positive correlation with D. Only water-stable aggregates (WSA) 0.5–0.25 mm had a significant positive correlation with D (r = 0.661, p < 0.05). Clay dispersion index (CDI) had a highly negative correlation with D (r = − 0.896, p < 0.01). Volumetric water content at FC (− 10 kPa) and PWP (− 1500 kPa) correlated positively with D (r = 0.618, P < 0.05 and r = 0.614, p < 0.05). There was a non-significant positive correlation of Muscovite and Berlinite with D. Significant positive correlations of calcite with D (r = 0.694, p < 0.05), and between mica and D (r = 0.731, p < 0.05), revealed their strong linkage with macro-aggregate stability. It was found that high rates of poultry manure alone could increased erodibility and decreased water retention at FC and PWP, while a combination of poultry manure and spent mushroom wastes could reduce erodibility. These properties could be used to make a quick inference on the resistance of soil aggregates to water drop impact.

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.

Consolidation properties and structural alteration of Old Alluvium

We present experimental observations and a conceptual model for understanding the compression and swelling characteristics of Old Alluvium (OA) from San Juan, Puerto Rico. Prior studies have classified OA as a transported, in situ weathered tropical soil whose intact macrostructure comprises a cemented, pseudo-silt with a mixture of quartz grains and aggregated clay particles. The aggregates include mixtures of kaolinite and smectite coated by Fe-oxides. The Fe-oxides also act as cementing agents between the particles. This study describes results from a series of high-pressure (up to 63 MPa) incremental consolidation tests. Breakdown of the clay aggregates and cemented structure is observed through changes in the compression properties, significant swelling during unloading, and an extraordinary reduction (i.e., by three orders of magnitude) in the coefficient of consolidation. These experimental observations are explained by a combination of mechanical processes, comminution and breakdown of cementing bonds, and physicochemical changes linking pore fluid in the intra- and inter-aggregate pore space. These processes alter the fundamental particle size distribution and macro-porosity of the soil and activate the swelling potential of the smectites concealed by the Fe-oxides coating in the intact material. The experimental observations provide the basis for the formulation of a constitutive model to describe macroscopic compression and swelling behavior of Old Alluvium and offer a framework to understand the response of piedmont transported residual soils found elsewhere.

Soil physical degradation and rill detachment by raindrop impact in semi-arid region

Tillage along slopes creates furrows similar to rills which exposes the soil surface directly to raindrop impact (RI) especially in semi-arid regions. In order to quantify the effect of RI on soil physical degradation and rill detachment we determined the change of soil aggregate stability, bulk density, surface resistance and rill detachment in semi-arid soils. Toward these some dominant soils including silt, silty clay, clay, sandy clay and loamy sand were exposed under rainfall impact at four slope gradients (5, 10, 15 and 20%). The laboratory experiments were set up at two soil surface conditions: with and without RI in a flume under simulated rainfalls with an intensity of 90 mm h−1. Four rills with parabolic cross sections, 10 cm width and 4 m length were installed along the flume. Results indicated that the soils are physically degraded (aggregate breakdown, soil compaction and surface resistance) by RI. The rate of physical degradation processes was significantly influenced by the soil type and slope gradient. The soils with higher water-stable aggregates such as sandy clay showed lower physical degradation symptoms. The susceptibility of these soils to physical degradation processes increased with increasing slope gradient. Soil detachment through concentrated flow under RI was affected by the soil physical degradation rate, which in turn was controlled by the two variables i.e. soil type and slope gradient. However the direct effect of RI act in rill detachment (rainfall-driven force) increased rill detachment. As a conclusion, the rill detachment showed higher dependency to RI than soil physical degradation processes. This study revealed that the importance of maintaining the soil surface cover for controlling the negative effects of RI in the soil physical degradation as well as rill detachment especially in semi-arid soils with lower aggregate stability and under steep slopes.

Under-Vine Vegetation Mitigates the Impacts of Excessive Precipitation in Vineyards.

Excessive precipitation events have greatly increased in several grape growing regions due to human-caused climate change. These heavy downpours result in a myriad of problems in the vineyard including soil aggregate breakdown, soil runoff, nutrient leaching, excessive vine vegetative growth, and diseased fruit. The negative impacts of excessive precipitation events on vineyards are exacerbated by the maintenance of bare soil under the vines. Exposure of bare soil results in soil erosion and runoff which pollutes nearby watersheds; raindrops weaken and break apart soil aggregates, leading to increased soil erosivity and contributing to the formation of surface crusts. In addition to excessive precipitation events, some grape growing regions can be characterized by fertile soils. The availability of ample water and nutrients can lead to highly vigorous vines with shoot growth continuing through harvest. Long shoots and large leaves result in shaded fruit, a humid vine microclimate, and excessive cluster rot. In this review, we examined how either natural (i.e., resident) or seeded under-vine vegetation (UVV) can help mitigate many of the problems associated with excessive precipitation. Through providing vegetative coverage to reduce the force of raindrops, increasing soil organic matter and enhancing soil microbial diversity, UVV can reduce the soil degradation and off-site impacts caused by excessive precipitation events. Through competition for soil resources, UVV can reduce excessive vegetative growth of vines and decrease cluster rot incidence and severity, although grapevine response to UVV can be highly variable. We discussed recent advances in understanding below and aboveground vine response and acclimation to UVV and presented current evidence of factors influencing the impact of UVV on vine growth and productivity to assist practitioners in making informed decisions and maximize the ecosystem services provided by UVV.

Exposure of humic acid-coated goethite colloids to groundwater does not affect their adsorption of metal(loid)s and their impact on Daphnid mobility

Engineered humic acid-coated goethite (HA-Goe) colloids find increasing application in in situ remediation of metal(loid)-polluted groundwater. Once introduced into the subsurface, the colloids interact with groundwater altering their physicochemical properties. In comparison to freshly synthesized, unreacted HA-Goe colloids, such alterations could reduce the adsorption affinity towards metal(loid)s and also result in altered ecotoxicological effects. In our study, HA-Goe colloids were exposed to two groundwaters (low vs. high concentrations of metal(loid)s) from two metal(loid)-contaminated sites for 87 days. We investigated (i) the course of HA-Goe ecotoxicity (Daphnia magna immobilization tests), (ii) HA-Goe adsorption properties (multi-element solutions containing As, Cu, Zn, Ni and Co), and (iii) changes in the chemical composition as well as in the mineral and aggregate properties of HA-Goe. The adsorption affinity of HA-Goe decreased in the order As ≈ Cu ≫ Zn > Ni ≈ Co. The metal(loid) adsorption occurred rapidly after mixing prior to the first sampling, while the duration of ongoing exposition to groundwater had no effect on the adsorption of these metal(loid)s. We neither observed a desorption of humic acids from the goethite surface nor alterations in the mineralogy, crystallinity, and surface properties of HA-Goe. Standardized Daphnia magna immobilization tests showed an increased number of mobile organisms with increasing exposure time of HA-Goe to both groundwaters. The decrease in HA-Goe-mediated immobilization of D. magna was strongest within the first 30 d. We attribute this to a shift to smaller sizes due to the breakdown of large HA-Goe aggregates, particularly within the first 30 d. The breakdown of these μm-sized aggregates may result mainly from the repeated shaking of the HA-Goe suspensions. Our study confirms within this particular setting that the tested HA-Goe colloids are suitable for the long-term immobilization of metal(loid)s, while lethal effects on D. magna were negligible.

Use of dynamic ballast testing equipment for analysis of rail aggregate breakdown

The granulometric range used has a direct influence on the life of the ballast layer when subjected to the load generated by rail traffic. However, in Brazil, we have only two size ranges to be used. The publications related to the subject matter under study express the need for a continuous grain size strip, different from the ones usually used in our railways. This more continuous band would provide less deformation and a reduction of degradation, granting a longer ballast life. To verify the influence of granulometry, a Dynamic Ballast Test Equipment was used to simulate railway traffic. The results show that if we use a more uniform granulometry, a direct influence on the deformation and the breakage index is evidenced, and consequently, a shorter useful life. The study highlights the need to study continuous granulometric bands for the rail ballast.

Measurement of particle agglomeration and aggregate breakdown of reclaimed asphalt pavement

Cold milling of old asphalt pavement is widespread in pavement resurfacing. It produces reclaimed asphalt pavement (RAP) and changes the state of the old asphalt pavement; the pavement is cut into particles, known as RAP particles. RAP particles are composed of many smaller aggregates bonded by the aged binder (particle agglomeration). The aggregates contained in the old pavement can be crushed into smaller pieces in the milling operation (aggregate breakdown). Particle agglomeration increases the risk of insufficient moisture stability and fatigue resistance of the recycled asphalt mixture; aggregate breakdown often prevents the use of a higher RAP content in the new mixture. In this study, the effect of these two problems is studied through a pavement maintenance project. The effect of milling speed on the RAP particle agglomeration rate is evaluated. The size-susceptibility of the agglomerate d particles, gradation deviation of the recycled mixture, and morphology classification of RAP particles are discussed. The effect of milling speed on aggregate breakdown is also investigated. The results show that approximately 30%–50% of RAP particles are made of aggregates smaller than the corresponding particle size. The agglomeration rate increases with an increase in milling speed and RAP particle size. High milling speed produces a lower rate of aggregate breakdown, resulting in a higher coarse aggregate content and a lower filler and fine aggregate content than a low milling speed.

A new method for weakening slaking of collecting eroded aggregates by water erosion

In most of previous studies about aggregate breakdown caused by water erosion, a critical process that water inevitably fragments the eroded aggregates by water erosion again is generally ignored. This will reduce the accuracy of research results. Hence, alcohol has been added to the collecting device of eroded aggregates to weaken the slaking of eroded aggregates by the researchers, but the optimal alcohol-to-water ratio for protecting the size fraction distribution of eroded aggregates remains unclear. Aggregate with a size fraction of 2–5 mm obtained by dry sieving of black soil in Northeast China was selected as the research object to analyse the effects of five alcohol-to-water ratios on aggregate characteristics and their variation with soaking time. The results showed that the size distribution in terms of mass percentage under different soaking times was similar. For the same soaking time, the mass percentage of 2–5-mm aggregates gradually decreased with decreasing alcohol ratio, but the aggregates with the second-highest mass percentage were gradually distributed among smaller size fractions. Compared with the alcohol soaking aggregates, the mean weight diameter (MWD) of the aggregates soaked in the other ratio of alcohol-to-water decreased gradually with the increase of soaking time, and the disintegration of the aggregates was the most evident at the initial stage of soaking. A percentage of aggregate destruction (PAD) of 20% served as the dividing line to evaluate the stability of soil aggregates. In this case, eroded aggregate collection should be completed within 12 min at an alcohol-to-water ratio of 1:1. Alternatively, eroded aggregate collection should be completed within 6 min at an alcohol-to-water ratio of 2:3. The above results can provide a reference for improving the accuracy of data collection regarding eroded aggregates. Especially in the study of water erosion which needs to collect eroded aggregates, alcohol can be added to the collecting device before the experiment. The amount of alcohol should be added according to the amount of water that may be generated, so as to protect eroded aggregates and weaken the slaking of eroded aggregates.

Synthesis of Ni(OH)2, suitable for supercapacitor application, by the cold template homogeneous precipitation method

α-Ni(OH)2 obtained by template homogeneous precipitation exhibits high electrochemical activity in supercapacitors. The main disadvantage is the high energy consumption for maintaining a high temperature during synthesis. To reduce energy consumption, it is proposed to lower the synthesis temperature. In the study, α-Ni(OH)2 was obtained by the method of cold template homogeneous precipitation using Culminal C8465 (0.5 %) as a template for 6 months at t=20–35 °С. The electrochemical characteristics of the sample were studied by cyclic voltammetry and galvanostatic charge-discharge cycling of a pasted binder-free electrode made without introducing an external binder in the supercapacitor mode. It was determined that low-crystalline α-Ni(OH)2 was formed, consisting of agglomerates of spherical particles. Low specific characteristics of nickel hydroxide were revealed at the beginning of cycling due to blocking of the active surface. It was shown that the specific capacity of the sample increased with further cycling due to the breakdown of aggregates into smaller particles; specific capacities of 80 F/g and 38 mA⋅h/g were obtained. However, the lack of binding properties of the template residues was revealed, resulting in a decrease in specific characteristics. It was concluded that it was necessary to introduce an external binder. A previously undescribed effect of a significant increase in the specific capacity during drying of an alkali-impregnated electrode caused by the disintegration of particle agglomerates during alkali carbonization (the maximum capacity is 135 F/g and 69 mA⋅h/g) was revealed. It was concluded that using the revealed effect of any nickel hydroxide samples obtained by various methods of bulk template synthesis was promising

Erosion effects on soil carbon and nitrogen dynamics on cultivated slopes: A meta-analysis

Soil carbon (C) redistribution within cultivated landscapes is strongly controlled by soil erosion and sedimentation and it is widely appreciated that C is preferentially transported during erosion. In contrast, it remains elusive whether erosion induced transport of C and N is coupled although changes in the balance between C and N strongly affect soil C and N dynamics. We therefore reviewed the literature on carbon and nitrogen redistribution by erosion.Twenty-nine studies reported results on C and N enrichment of freshly eroded sediments after erosion events. Thirty-nine studies reported results on C and N contents and stocks along eroded slopes.Eroded sediments were enriched in C and N by 51.3% and 50.6% indicating that both elements are stored in soil fractions that are preferentially eroded. Slope gradient and soil texture strongly affected C and N enrichment. Decreasing C and N enrichment in fine-textured soils was counterbalanced by increasing erosion rates in these soils. This suggests similar SOM losses independent of textural class. The C/N ratio increased by 9.9%, pointing to preferential movement of C-rich particulate organic matter (POM) compared to N-rich mineral associated organic matter (MAOM). Breakdown of aggregates by rainfall energy possibly released POM which is then preferentially eroded. Soil C and N contents and total stocks showed similar percentage increases from upslope to depositional sites, indicating that downslope C and N stocks were largely driven by enrichment, rather by than increases in depth or bulk density.Altogether, our findings confirm that quantification of soil loss alone is not sufficient to estimate erosion-induced changes in soil fertility, because soil organic matter and plant nutrients are selectively moved during erosion. This leads to a shift in C and N dynamics in different slope positions and thus to an increase in the spatial variability of the C and N along the slope.

The high content of mineral-free organic matter in soils increases their vulnerability to wildfire in humid-temperate zones

Under a new scenario of global warming, in which wildfire is predicted to affect new forest areas in temperate and boreal climates, information about the properties of soil organic matter (SOM) may be crucial for assessing the impact of wildfire. Unlike Mediterranean soils, the soils in temperate and boreal areas are richer in SOM in the form of unprotected, mineral-free SOM, which is presumably particularly sensitive to heating.To address this topic, three recently burned, SOM-rich Atlantic forest soils, in which different levels of soil burn severity were distinguished, were selected for study. Soil organic matter density fractions (free particulate OM [fPOM], occluded particulate OM [oPOM] and mineral-associated OM [MAF]) and different particle size fractions of MAF (200–63, 63–20 and <20 µm) were obtained and analyzed by thermal analysis (DSC-TG) and solid state nuclear magnetic resonance (13C CPMAS NMR). The effect of heating on SOM was also studied in relation to the changes observed in the water repellency and aggregate size of the soil.Heating led to important losses of the three SOM density fractions, particularly the fPOM. The SOM remaining after fire was thus dominated by oPOM and MAF. Although recalcitrance increased in all physical SOM fractions, the effect was greatest in the fPOM, which can be attributed to differences in the combustion conditions and to the composition of these OM pools.Loss and changes in the composition of fPOM explained the reduction in water repellency usually observed in these soils after intense heating. The loss of MAF in severely burned soils seems to be the main driver of the aggregate breakdown. Depletion of this SOM fraction may affect the resilience of the system because of the low rate of replenishment. Although the high degree of recalcitrance of mineral-protected OM may be important for C sequestration in soil, the present findings revealed that the potential effect is relatively small.

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.

Chemical weathering and progressing alteration as possible controlling factors for creeping landslides.

Landslides can behave as dynamic processes, which emerge from the complex interplay of tectonics, erosion, weathering and gravitational influences, triggered by various hydrological, mineralogical, biological and geotechnical factors. Integral studies to assess the mechanisms underlying landslide initiation and progression are mainly focussed on specific cases with high geohazard potential. The landslide near Stadtschlaining (Austria) represents a key study site to elucidate the impacts of pelitic sediment composition, weathering regime, alteration patterns and hydrochemistry on recurrent damage progression in the local infrastructure. Based on field work, soil-mechanical logging (Atterberg limits, undrained strength, friction angles), water chemistry (ICP-OES, IC, hydrochemical modeling), solid-phase characterization (XRD, XRF, SEM) and sorption experiments we establish a conceptual model for initiating and progressing of landslides: Infiltration of low mineralized meteoric water (EC: <200μS/cm) in permeable limonitic gravels triggers chemical weathering of greenschist-derived detritus and promotes its transformation into kaolinite and smectite. The clayey strata (>50wt% of clay minerals) create zones of mechanical and chemical weakness in the underground (~4-6m below ground level), which are characterized by particle disintegration/delamination, slip bedding and deformations, and development of porous layers depicting water flow paths. Subsequent Na+ exchange for bivalent ions in the smectite interlayer delivered by percolating, highly mineralized water (EC: 1600-5100μS/cm) is caused by de-icing salt and fertilizer applications during winter and late summer, and yield in i) decohesion and physical breakdown of the particle aggregates and ii) swelling of the clay matrix in early spring and autumn. These processes reduce the shear strength of the pelitic sediments, resulting in failure and initiation of landslides (deformation: ~500mm within a month) and subsequent steady creeping motion (deformation: ~100mm in 6months). Customized engineered solutions to prevent landslides in this area are presented, which can be conveyed to analogous landslide-affected areas worldwide.

Effect of soil internal forces on fragment size distributions after aggregate breakdown and their relations to splash erosion

AbstractSoil aggregate breakdown is the key step of splash erosion. Recent studies have shown that it is the soil internal forces that induce aggregate breakdown and subsequent splash erosion. However, little is known about the size characteristics of soil aggregate breakdown and corresponding effects on splash erosion. In this study, Cinnamon soil and Heilu soil were adopted to investigate the effects of soil internal forces on the characteristics of aggregate fragments and splash erosion through wet sieving and rainfall simulation. The results showed the following. (1) With a decrease in electrolyte concentration in soil solution, soil net repulsive force increased; meanwhile, aggregate stability decreased and splash erosion rate increased. Additionally, when the electrolyte concentration was less than 10−2 mol L−1, soil net repulsive force hardly changed, and aggregate stability and splash erosion rate accordingly varied very little. (2) With the increase in the soil net repulsive force, the content of fragments &lt;0.053 mm released from aggregates increased, whereas the content of fragments &gt;0.25 mm decreased, thus suggesting that the aggregates tended to break into fine fractions as soil net repulsive forces increased. (3) Regression analyses showed that soil splash erosion rate decreased linearly with increases in the aggregate stability; moreover, correlation analyses showed that splash erosion rate was significantly and positively correlated with the fragments with a diameter of &lt;0.053 mm, and negatively correlated with the fragments with a diameter of &gt;0.25 mm. This study suggests that soil internal forces induce the aggregate crush and determine the size distribution of fragments available for mobilization by the raindrop impact force. Also, the raindrop impact force is the driving mechanism leading to the movement of fragments. This study provides new insights into the mechanism responsible for splash erosion and could be valuable for controlling soil erosion.Highlights Soil aggregates breakdown dynamics and splash erosion were quantitatively investigated. Soil internal forces determined the size distribution of fragments after aggregates breakdown. The movement of soil fragments largely depended on raindrop impact force. Splash erosion was driven by the soil internal forces and raindrop impact force.