Seal Formation Research Articles

Interrill erosion on cultivated Greek soils: modelling sediment delivery

AbstractFor interrill erosion, raindrop‐induced detachment and transport of sediment by rainfall‐disturbed sheet flow are the predominant processes, while detachment by sheet flow and transport by raindrop impact are negligible. In general, interrill subprocesses are inter‐actively affected by rainfall, soil and surface properties. The objective of this work was to study the relationships among interrill runoff and sediment loss and some selected para‐meters, for cultivated soils in central Greece, and also the development of a formula for predicting single storm sediment delivery. Runoff and soil loss measurement field experiments have been conducted for a 3·5‐year period, under natural storms. The soils studied were developed on Tertiary calcareous materials and Quaternary alluvial deposits and were textured from sandy loam to clay. The second group of soils showed greater susceptibility to sealing and erosion than the first group. Single storm sediment loss was mainly affected by rain and runoff erosivity, being significantly correlated with rain kinetic energy (r = 0·64***), its maximum 30‐minute intensity (r = 0·64***) and runoff amount (r = 0·56***). Runoff had the greatest correlation with rain kinetic energy (r = 0·64***). A complementary effect on soil loss was detected between rain kinetic energy and its maximum 30‐minute intensity. The same was true for rain kinetic energy and topsoil aggregate instability, on surface seal formation and thus on infiltration characteristics and overland flow rate. Empirical analysis showed that the following formula can be used for the successful prediction of sediment delivery (Di): Di = 0·638βEI30tan(θ) (R2 = 0·893***), where β is a topsoil aggregate instability index, E the rain kinetic energy, I30 the maximum 30‐minute rain intensity and θ the slope angle. It describes soil erodibility using a topsoil aggregate instability index, which can be determined easily by a simple laboratory technique, and runoff through the product of this index and rain kinetic energy. Copyright © 2006 John Wiley & Sons, Ltd.

Using synthetic polymers as soil conditioners to control runoff and soil loss in arid and semi-arid regions—a review

Runoff and soil erosion are serious and widespread land degradation problems in arid and semi-arid regions. Soils in these regions are characterised by relatively high levels of salinity and sodicity, and low structural stability. One means to increase the stability of soil structure is the use of synthetic polymers as soil conditioners. Polymers consist of repeated small identical units (monomers) coupled together to form extended chains. The present review paper addresses the properties of synthetic polymers, their interactions with clays and soil particles, and their effects on infiltration rates, runoff, and soil erosion in arid and semi-arid soils exposed to rainfall and sprinkler irrigation. Because polymers increase the stability of the soil structure, they reduce the tendency of soils to form seals, thereby preventing decline in infiltration rates, reducing runoff and soil losses. The beneficial effects of various polymers with different molecular weights, charge types, and densities applied over a wide range of conditions, i.e. dryland and irrigated soils, soils of varying physical and chemical properties, and rainfall with different kinetic energies, are reviewed in the present manuscript. The addition of small amounts of the polymers, with their low cost and their high efficiency in preventing seal formation, infiltration rate reduction, and runoff and soil loss increases, suggest that the polymers can be considered as successful stabilising agents for soils that are sensitive to seal formation. However, further study on handling and field application of the synthetic polymers is needed.

Effects of Effluent Irrigation on Seal Formation, Infiltration, and Soil Loss during Rainfall

The use of effluent for irrigation could affect the chemical and hydraulic properties of soils due to its high salt and organic matter (OM) content, and, consequently, the rainfall–infiltration–runoff–erosion relationships during the subsequent rainy season. This study investigates the effects of long‐term effluent irrigation on soil chemical properties, seal formation, infiltration, and soil loss under rainfall. Simulated rainfall (85 mm) was applied to (i) air‐dried or (ii) prewetted clay and sandy soils from plots that had been irrigated with fresh water (FW) or effluent for >10 yr. Effluent irrigation increased the total OM content and the exchangeable sodium percentage (ESP) of the soils. The cumulative infiltration in FW‐ and effluent‐irrigated clay soils was 6.5 and 5.6 mm, respectively, in the initially dry soils, and 52.3 and 51.5 mm, respectively, in the prewetted soils. In the FW‐ and effluent‐irrigated sandy soils, the corresponding values of cumulative infiltration were 79.5 and 44.7 mm, and 85.0 and 56.3 mm, respectively. In the sandy soil, the higher sodium adsorption ratio (SAR) values in the leachate of effluent‐irrigated soil led to greater clay dispersion, which enhanced seal formation, reduced infiltration, and increased soil loss. In the clay soil, slaking was the main process involved in seal formation, neglecting the possible deleterious effect of effluent irrigation. When slaking was prevented, the SAR values in the leachate of the effluent‐irrigated soil decreased during rainfall and were similar to those of the FW‐irrigated soil at the end of the applied rainfall amount. This was probably due to the exchange of adsorbed Na with soluble Ca, which minimized the differences in clay dispersion, infiltration, and soil loss. Therefore, in the clay soil, aggregate slaking might be the main process involved in seal formation and affecting infiltration and erosion. These results show that the effect of effluent irrigation on infiltration, runoff, and soil loss depends on the soil type (amount of clay and CaCO3) and the dominant mechanisms of seal formation. Therefore, to prevent a possible deleterious effect on soil structure, it is necessary to identify sensitive areas and soils before the application of effluents for irrigation.

Formation of Low-Temperature Seals and Coatings Based on Low-Melting Lead-Tellurite Glasses

The process of formation of seals and coatings based on low-melting lead-tellurite glasses is studied. The results of a microspectral analysis of the element distribution in glass - aluminum and glass - quartz seals are specified.

Sand Seals in Coho Salmon Redds: Do They Improve Egg Survival?

AbstractWe investigated whether sand seals form in the upper egg pocket of salmonid redds and improve egg survival in a sediment‐impacted coastal stream in northern California. Sand seals can potentially reduce infiltration of detrimental finer sand and silt into the lower egg pocket. We predicted sand seals would form when the redds were exposed to streamflows high enough to entrain coarse sand and form seals. Using artificial redds of coho salmon Oncorhynchus kisutch, we conducted sediment analyses of the upper and lower egg pockets and found that protective sand seals formed in redds when discharge was two or more times the flow that entrains the median particle size of the streambed. When the coarse sand in the upper egg pocket was incorporated into a two‐stage model that predicted survival to hatching and emergence, it greatly improved predictions in years with higher flows in both natural and artificial redds. However, the sand seals provided little protection when suspended sediment flux was high from logging or road construction. We built the model used for these predictions with data from artificial redds and applied it to natural redds during 6 years of different flow regimes. Predictor variables included cumulative flow above the entrainment flow, peak discharge, coarse sand in the upper egg pocket, fine sediment in riffle gravel, coarse sand in the upper and entire egg pocket, suspended sediment flux, and presence of predaceous worms. The model explained 67% of the variance in egg survival to emergence in coho salmon natural redds. We suggest that managers consider the complex interactions of streamflow, sediment transport, formation of sand seals, and fine‐sediment infiltration when estimating salmonid reproductive success in sediment‐impacted streams.

Insignia of the Lithuania's rulers depicted on majesty seals during the end of the 13th and the middle of the 15th centuries: iconographical aspect of political programme

Development of the Lithuanian ruler's status and political context has a direct influence on the formation of seals' portrayal as well as on the seal, as insignia itself. Royal insignia expressed the ideology and symbolism of the ruler's power. They played a significant symbolic and sacral role during the coronation ceremony. The oldest royal insignia are the crown, sword, and lance, while the latter lost its significance. Because of the lack of written sources on the coronation and enthronization of Lithuanian kings and grand dukes, seals have enormous advantages over other historical witnesses. The portrayal of figures enriched by additional symbolism gives us a clear impression of what insignia were used. The majesty seals of Mindaugas, Gediminas, Vytautas, Švitrigaila, and Žygimantas Kęstutaitis not only depicted the rulers' insignia but also help us to reconstruct the political conception of these rulers. Different insignia were attributed to different ranks of rulers. According to the sphragistics materials, Mindaugas, being a king, used insignia of royal rank (an orb surmounted by a cross and a flowered sceptre). Gediminas, being a grand duke, also used insignia of royal rank (an orb and a crown) but with a slight difference: the crown is in his hand but not on his head. The sword, together with the mitra (dukes' cap), is a very important insignia of grand dukes. The portrayal figures of Vytautas, Švitrigaila, and Žygimantas Kęstutaitis seals have these insignia. The sword, being one of the oldest insignia, was already depicted on the Lithuanian rulers' seals with mounted and unmounted warriors. The sphragistic data also shows that the sword was a prerogative of rulers and was not used by those who only belonged to the ruling dynasty. Examining the Lithuanian seals' data, one could prove that the iconographical programme was influenced not only by the medieval sphragistic tradition but also by political aspirations. The seals were not only a sign of authority but also expressed the status of the Lithuanian ruler and political realities during the 13th and 15th centuries. The portrayal figure of the seal conveys, even if sometimes only symbolically, the ideal of the sovereignty of the person using it.

Geomorphic features and soil formation of arid lands in Northeastern Jordan

Arid and semiarid lands occupy about one-third of the Earth's land surface. Interpretation of soil formation and geomorphic features of arid lands is needed to assess their soil ecological potentials, limitations, problems and management needs. The objective of this paper was to study the geomorphic features and soil formation of the arid lands in northeastern Jordan, to provide information that could be used by land managers in the study area and other arid land areas. Five representative soil pedons were excavated and described in the field. Soil samples from each horizon per pedon were taken to the laboratory for chemical and physical analyses. Geomorphic features of the area were also studied. Most of studied land surfaces are plains where eolian deflation has exposed loose gravels consisting predominantly of pebbles forming desert pavements. Desert pavements cover most of the land surface, excluding the mud playas, and are composed of basalt clasts. The accumulation of calcium carbonate and gypsum within these soils create problems for their agricultural development. Accumulation of eolian fine-grained silt has resulted in the formation of a vesicular horizon. The climatic variations during the late quaternary and the late Holocene periods contributed to the development of the desert pavement and the vesicular horizons. Clay illuviation and argillic horizon development within these soils is assumed to be a relict feature from more humid climates during the Quaternary. Sustainable agricultural development of such arid lands may not be easy. In general, these soils have high erodibility, high runoff generation potential, high susceptibility to seal and crust formation, poor water-holding capacity, pedon hardening and structural instability.

Soil Wetting and Texture Effects on Aggregate Stability, Seal Formation, and Erosion

Previous studies about the effect of antecedent moisture content (AMC) on seal formation have shown contradictory results. We hypothesize that this controversy is related to differences in slaking during wetting. The objectives were to analyze the effects of: (i) clay content on aggregate stability and slaking; (ii) clay content and slaking on seal formation and interrill erosion under various wetting rates (WR) and AMC under simulated rain. Aggregate stability was determined on six smectitic soils from Israel with clay content from 80 to 630 g kg−1 In the rain simulator, soils with 230, 410 and 620 g kg−1 clay were prewetted with WR = 1 and 5 mm h−1 to AMC = 0.25 and 0.5 of field capacity (FC), prior to the application of 80 mm of rain. Aggregate stability and slaking by fast WR increased with increase in clay content. In soils with 230 and 410 g kg−1 clay, raindrop impact was enough to disintegrate the aggregates and sealing was not affected by WR and AMC. Conversely, in the soil with 620 g kg−1 clay, seal formation increased with slaking caused by fast wetting. Thus, final infiltration rate of the clay soil with AMC = 0.5 FC and WR = 1 mm h−1 was 11.1 mm h−1 compared with 6.0 mm h−1 in the air‐dry soil (fast wetting by rain). The effects of WR and AMC on soil loss were similar to their effect on runoff but more pronounced. The relation between wetting process and clay content should be considered when predicting soil erosion in smectitic soils.

Lipid- and mechanosensitivities of sodium/hydrogen exchangers analyzed by electrical methods.

Sodium/hydrogen exchangers (NHEs) are ubiquitous ion transporters that serve multiple cell functions. We have studied two mammalian isoforms, NHE1 (ubiquitous) and NHE3 (epithelial-specific), by measuring extracellular proton (H+) gradients during whole-cell patch clamp with perfusion of the cell interior. Maximal Na(+)-dependent H+ fluxes (JH+) are equivalent to currents >20 pA for NHE1 in Chinese hamster ovary fibroblasts, >200 pA for NHE1 in guinea pig ventricular myocytes, and 5-10 pA for NHE3 in opossum kidney cells. The fluxes are blocked by an NHE inhibitor, ethylisopropylamiloride, and are absent in NHE-deficient AP-1 cells. NHE1 activity is stable with perfusion of nonhydrolyzable ATP [adenosine 5'-(beta,gamma-imido)triphosphate], is abolished by ATP depletion (2 deoxy-D-glucose with oligomycin or perfusion of apyrase), can be restored with phosphatidylinositol 4,5-bisphosphate, and is unaffected by actin cytoskeleton disruption (latrunculin or pipette perfusion of gelsolin). NHE3 (but not NHE1) is reversibly activated by phosphatidylinositol 3,4,5-trisphosphate. Both NHE1 and NHE3 activities are disrupted in giant patches during gigaohm seal formation. NHE1 (but not NHE3) is reversibly activated by cell shrinkage, even at neutral cytoplasmic pH without ATP, and inhibited by cell swelling. NHE1 in Chinese hamster ovary fibroblasts (but not NHE3 in opossum kidney cells) is inhibited by agents that thin the membrane (L-alpha-lysophosphatidylcholine and octyl-beta-D-glucopyranoside) and activated by cholesterol enrichment, which thickens membranes. Expressed in AP-1 cells, however, NHE1 is insensitive to these agents but remains sensitive to volume changes. Thus, changes of hydrophobic mismatch can modulate NHE1 but do not underlie its volume sensitivity.

Functional Mapping of Ca2+ Signaling Complexes in Plasma Membrane Microdomains of Polarized Cells

Many cells cluster signaling complexes in plasma membrane microdomains. Polarized secretory cells cluster all Ca2+ signaling proteins, including GPCRs, at the apical pole. The functional significance of such an arrangement is not known because of a lack of techniques for functional mapping of signaling complexes at plasma membrane patches. In the present work, we developed such a technique based on the use of two patch pipettes, a recording and a stimulating pipette (SP). Including 20% glycerol in the SP solution increased the viscosity and the hydrophobicity to prevent leakage and formation of tight seals on the plasma membrane. This allowed moving the SP between sites to stimulate multiple patches of the same cell and with the same agonist concentrations. Functional mapping of Ca2+ signaling in pancreatic acinar cells revealed that the M3, cholecystokinin, and bombesin signaling complexes at the apical pole are much more sensitive to stimulation than those at the basal pole. Furthermore, at physiological agonist concentrations, Ca2+ signals could be evoked only by stimulation of membrane patches at the apical pole. [Ca2+](i) imaging revealed that Ca2+ waves were invariably initiated at the site of apical membrane patch stimulation, suggesting that long range diffusion of second messengers is not obligatory to initiate and propagate apical-to-basal Ca2+ waves. The present studies reveal a remarkable heterogeneity in responsiveness of Ca2+ signaling complexes at membrane microdomains, with the most responsive complexes confined to the apical pole, probably to restrict the Ca2+ signals to the site of exocytosis and allow the polarized functions of secretory cells.

Assembly of a Ca2+-dependent BK channel signaling complex by binding to beta2 adrenergic receptor.

Large-conductance voltage and Ca2+-activated potassium channels (BKCa) play a critical role in modulating contractile tone of smooth muscle, and neuronal processes. In most mammalian tissues, activation of beta-adrenergic receptors and protein kinase A (PKAc) increases BKCa channel activity, contributing to sympathetic nervous system/hormonal regulation of membrane excitability. Here we report the requirement of an association of the beta2-adrenergic receptor (beta2AR) with the pore forming alpha subunit of BKCa and an A-kinase-anchoring protein (AKAP79/150) for beta2 agonist regulation. beta2AR can simultaneously interact with both BKCa and L-type Ca2+ channels (Cav1.2) in vivo, which enables the assembly of a unique, highly localized signal transduction complex to mediate Ca2+- and phosphorylation-dependent modulation of BKCa current. Our findings reveal a novel function for G protein-coupled receptors as a scaffold to couple two families of ion channels into a physical and functional signaling complex to modulate beta-adrenergic regulation of membrane excitability.

Rainfall‐Induced Soil Surface Sealing: A Critical Review of Observations, Conceptual Models, and Solutions

Rainfall‐induced soil surface sealing can have severe agricultural, hydrological, and environmental effects. Seal formation is a complex phenomenon dominated by a wide variety of factors involving soil properties, rainfall characteristics, and flow conditions. It has been studied through extensive experimental investigations as well as simulation models. This study reviews some of the main issues, in terms of morphology, phenomenology, and both conceptual and empirical modeling approaches to improve our perception of the phenomenon and our ability to simulate its effects on flow processes. The effects of different factors on infiltration during seal formation and in sealed soil profiles are highlighted, including seal representation, soil and rainfall properties, and field heterogeneity. New research opportunities toward a generalized formulation of the processes involved in soil sealing and a reliable quantitative prediction of its effects on flow processes are identified. These are related mainly to the ability to quantify and predict the relationships between soil hydraulic properties and physical, chemical, and biological factors that affect the soil resistance to destruction.

Organic Matter and Aggregate Size Interactions in Infiltration, Seal Formation, and Soil Loss

The objectives of this study were to investigate (i) the effect of soil organic matter (OM) content on the mechanisms that form a seal, and (ii) the OM content and aggregate size interactions in seal properties, infiltration rate (IR), and soil loss. Two samples of sandy loam (Humic Dystrudept) designated low‐OM soil (2.3% by weight OM) and high‐OM soil (3.5% OM) were studied. Aggregate sizes <2, 2 to 4, and 4 to 6 mm of each soil were exposed to 80 mm of simulated rainfall with an intensity of 42 mm h−1 The final IR increased with increasing aggregate size from 4.2 to 5.2 mm h−1 in the low‐OM soil, and from 5.8 to 10.8 mm h−1 in the high‐OM soil. There was a significant interaction between OM content and aggregate size in seal formation and final IR. The low aggregate stability and the high dispersivity of the low‐OM soil allowed the development of a dense and thick crust for all the aggregate sizes. Conversely, the high aggregate stability and the low dispersivity of the high‐OM soil limited the seal formation. Consequently, the IR values were high and differences in IR among the three aggregate sizes in this soil were relatively high. The soil losses increased with increasing aggregate size, ranging from 4.5 × 10−3 to 6.6 × 10−3 kg m−2 mm−1 in the low‐OM soil, and from 1.2 to 3.8 × 10−3 kg m−2 mm−1 in the high‐OM soil. In this case, no significant interaction was found between soil OM content and aggregate size.

Mulching with composted municipal solid wastes in Central Negev, Israel: I. effects on minimizing rainwater losses and on hazards to the environment

Dry-land farming in arid and semiarid regions requires minimization of rainwater losses. Major causes for the loss of rainwater are: (i) runoff due to seal formation by raindrop impact, and (ii) evaporation from the wet soil surface. Mulching the soil surface is an effective way to prevent seal formation and water losses. We hypothesized that composted municipal solid waste (CMSW) could be used for mulching arable lands and minimizing rainwater losses without posing a hazard to the environment. Our objective was to study the effects of annual application of CMSW at the soil surface on rainwater retention in the soil, crop production and some hazards to the environment. The experiments were conducted during 1996–2000 in commercial rain fed wheat ( Triticum aestivum L.) field of a xerosol (Calcic Haploxeralf) in the Central Negev, Israel. Amounts of 0, 100 and 300 m 3 ha −1 CMSW were added annually to the soil surface prior to the rainy season. Water content in the soil was determined four times, soil salinity, and sodicity were determined twice and heavy metals in the soil and the yield of wheat were determined once every year. Yield was determined at the end of each growing season. Application of CMSW increased available water in the root zone mainly due to reduction in evaporation, and almost doubled the yields. No considerable increase in salinity, sodicity and heavy metals was noted in the root zone following CMSW application. Our observations suggested that annual application of CMSW at the rate of 100 m 3 ha −1 was enough to significantly minimize rainwater losses and increase yield under dry-land conditions, without posing specific hazards to the environment.

Soil mineralogy and slope effects on infiltration, interrill erosion, and slope factor

Interactions between the effects of soil mineralogy and of slope gradient on seal formation, infiltration rate (IR), runoff, and soil loss were evaluated, and the slope factor (Sf) functions for various soils with differing mineralogy were examined. Three different soils, on slopes of 9, 15, 20, and 25%, were subjected to 80 mm of simulated rainfall. The final IR was ≥20.5 mm h−1 in a clayey kaolinitic soil and ≤15 mm h−1 in smectitic soils. The total runoff, as a percentage of rainfall, ranged from 24 to 18%, 65 to 43%, and 50 to 35% from the kaolinitic, clayey smectitic, and sandy loam smectitic soils, respectively. The total soil loss ranged from 0.32 to 0.45 and from 1.14 to 3.93 kg m−2 for the kaolinitic and smectitic soils, respectively. In all three soils, increasing the slope gradient increased the detachment of coarse particles (>0.1 mm) more sharply than that of small particles (<0.1 mm), especially in the smectitic soils. Combining these results with previous findings for six soils indicated that soils could be divided into two groups according to their Sf values: (1) soils that contained smectite and were therefore dispersive and (2) soils that did not contain smectite and were therefore stable. For the former soils the regression Sf = 0.47exp(7.7 sin ) defined the Sf values significantly, whereas for the latter group a linear regression Sf = 0.81 + 1.77 (sin ) was required. It was suggested that the difference in the Sf functions was mainly due to seal formation enhancement by the smectite.

Organic Matter and Aggregate Size Interactions in Infiltration, Seal Formation, and Soil Loss

The objectives of this study were to investigate (i) the effect of soil organic matter (OM) content on the mechanisms that form a seal, and (ii) the OM content and aggregate size interactions in seal properties, infiltration rate (IR), and soil loss. Two samples of sandy loam (Humic Dystrudept) designated low-OM soil (2.3% by weight OM) and high-OM soil (3.5% OM) were studied. Aggregate sizes <2, 2 to 4, and 4 to 6 mm of each soil were exposed to 80 mm of simulated rainfall with an intensity of 42 mm h−1 The final IR increased with increasing aggregate size from 4.2 to 5.2 mm h−1 in the low-OM soil, and from 5.8 to 10.8 mm h−1 in the high-OM soil. There was a significant interaction between OM content and aggregate size in seal formation and final IR. The low aggregate stability and the high dispersivity of the low-OM soil allowed the development of a dense and thick crust for all the aggregate sizes. Conversely, the high aggregate stability and the low dispersivity of the high-OM soil limited the seal formation. Consequently, the IR values were high and differences in IR among the three aggregate sizes in this soil were relatively high. The soil losses increased with increasing aggregate size, ranging from 4.5 × 10−3 to 6.6 × 10−3 kg m−2 mm−1 in the low-OM soil, and from 1.2 to 3.8 × 10−3 kg m−2 mm−1 in the high-OM soil. In this case, no significant interaction was found between soil OM content and aggregate size.

Fabrication of Patch Pipets

Patch clamping refers to a wide range of electrophysiological measurements, all of which have in common the use of patch pipets and the formation of gigaohm seals. The purpose of this unit is to describe the fabrication of patch pipets. The aspects of the pipet geometry that are important to different applications and the different procedures that have been found to most reliably and simply achieve these results are described. Parameters for glass selection are detailed in the beginning of the unit. Pulling patch and whole-cell pipets, elastomer coating, fire polishing, pipet filling, and pipet testing in an experimental setup are highlighted. Additional support protocols describe alternative ways to optimize pipet geometry and cleaning the glass before pulling. Considerations for noise and dynamic performance are emphasized as these two requirements for single-channel and whole-cell current measurements dictate how the pipets must be fabricated.

Soil mineralogy effects on seal formation, runoff and soil loss

Interaction between clay particles is one of the main factors responsible for soil aggregation. Therefore, soil mineralogy, which has substantial effects on clay dispersion, may also influence aggregate stability, seal formation, runoff and soil loss. In spite of this, the effects of soil mineralogy on these phenomena have received much less attention in the literature than those of other soil properties. This paper reviews the effects of soil mineralogy on aggregate stability, seal formation and micromorphology, and on the associated infiltration rate (IR), runoff and soil loss. The paper is focused mainly on the authors' previous studies. In order to determine these effects, various soils with different mineralogy were collected from Israel, South Africa and Kenya. In these soils, the aggregate stability was determined under fast wetting conditions, and the IR and the interrill soil loss were determined using rainfall simulator of a rotating disk type. The micromorphology of the crust, which was developed at the soil surface by the rainstorm, was determined by scanning electron microscope. Clay mineralogy was found to be a dominant factor in controlling aggregate stability, seal formation, soil IR and interrill soil loss. The phyllosilicate soils, which were review in this paper, were divided into two main groups: (i) stable soils with final IR>8.0 mm h −1 and (ii) unstable soils with final IR<4.5 mm h −1. These two soil groups differ in their mineralogy. Kaolinitic and illitic soils that do not contain smectite were stable soils and less susceptible to seal formation. In contrast, kaolinitic and illitic soils that contain some smectite and smectitic soils were unstable. Examination of the susceptibility of 21 phyllosilicate soils to interrill erosion indicated that these soils could be divided into three groups. Soil loss was higher for unstable soils than for stable soils, but the soil loss of the smectitic soils was significantly higher than that of the unstable soils, which contained kaolinite or illite as the dominant clay.

Effect of raindrop impact and its relationship with aggregate stability to different disaggregation forces

A soil surface exposed to rainfall is subjected to processes of wetting and drop impact which can lead to the formation of a seal during the rainfall, reducing infiltration and increasing erosion by increasing runoff. The objective of this research was to evaluate the relationship between the effect caused by the drop impact and the aggregate stability of the soils when they are subjected to different disaggregation forces. The aggregates were subjected to cracking (by slow wetting), slaking (by fast wetting) and mechanical breakdown (by mechanical stirring after pre-wetting in ethanol). The effect of each process was evaluated by measuring the mean weight diameter (MWDsl, MWDf and MWDst, respectively) calculated as the sum of the mass fraction of soil left in the sieve after fractionation into four size classes, ranging from <0.25 to 2 mm, multiplied by the mean aperture of the sieve meshes and divided by the initial soil weight. The effect of water impact plus wetting was quantified by the saturated hydraulic conductivity of the seal (Ks) and the time necessary to reach this value. A relative sealing index (RSI) that measured the reduction of water intake caused by sealing was defined as the relationship between the minimum value of saturated hydraulic conductivity of the seal and that reached when the drop impact was avoided. The air-dry material rupture was evaluated with a penetrometer. The main soil characteristics that determine all these processes for the study soils were analysed. Most of the studied soils were very sensitive to slaking and mechanical breakdown, while they were stable when they were subjected to slow wetting. A significant relationship was found between the minimum saturated hydraulic conductivity (Ks) and the MWDst ( R 2=0.40, p<0.005), and between Ks and the MWDf ( R 2=0.69, p<0.05). In both treatments, slaking and mechanical stirring, the percentage of aggregates retained in the larger sieve mesh was also significantly correlated with Ks. This result could indicate that both processes are implicated in the disaggregation produced by drop impact, which contribute to seal formation process. The less stable soils had the lowest Ks value (<1 mm h −1), which was reached in a short period of time (<10 min). The high silt content and the low organic matter control the loss of aggregation by mechanical breakdown and the formation of the seal. The RSI values indicated a 200-fold reduction in water infiltration for some soils, caused by the formation of a seal.

Seal formation and interrill erosion on a smectite-rich Kastanozem from NE-Mexico

On-site and off-site effects of interrill erosion can be highly significant for river and reservoir sedimentation and can cause serious water contamination. Interrill erosion changes during storms, reflecting the effect of seal formation on soil resistance to interrill erosion. Swelling clay soils appear to be particularly complex in their response to rainfall. Assessment of interrill erosion hazards on such soils requires effective quantitative description of the effects of seal formation on interrill erosion. However, short-term changes of seal properties are not reflected well in current event-based erosion models, leading to poor prediction quality. The objective of this study was to identify the changes in seal condition relevant to temporal change in interrill erosion on a smectite-rich Kastanozem from NE-Mexico. Formation and removal of a non-coherent washed-out layer, consisting of stable aggregates, was the most apparent change in seal condition associated with interrill erosion dynamics. The progressive exposure of a more cohesive washed-in layer resulted in reduced interrill erosion, which is attributed to lower erodibility of the washed-in layer. The results demonstrate that seal formation and interrill erosion dynamics of a freshly tilled Kastanozem are strongly linked. The amount of rainfall required for the removal of the washed-out layer (180 mm) shows that seal formation lasts for a significant part of the rainy season. The changes of interrill erosion caused by seal formation on the Kastanozem are not represented adequately in current methods of erodibility assessment and improving the quality of event-based erosion prediction remains difficult for two reasons. First, relationships between soil properties, seal formation and erosion dynamics are not well understood. Second, the quantitative description of seal property dynamics relevant for interrill erosion is characterized by strong spatial and temporal variability. Therefore, it is likely that even with best modeling efforts, the quality of event-based erosion prediction will be limited by the high uncertainty about erodibility.