Zusammenhang zwischen hydraulischer und mechanischer Bodenstabilität in Abhängigkeit von der Belastungsdauer

Interaction between mechanically and hydraulically affected soil strength depending on time of loading Soil‐deformation analysis often only considers the direct effects of mechanical stress on changes in void ratio or pore functions while the interaction between hydraulic and mechanical processes is seldomly mentioned. Thus, we analyzed the effect of mechanical stress and time of soil settlement on changes in soil strength and the corresponding interactions between stress‐dependent changes in pore water pressure on precompression stress for a clayey silt. Disturbed samples with a bulk density of 1.4 g cm^–3 and a water content of 25 g (100 g)^–1 were compressed for four time steps (10–240 min) at eight stresses (20–400 kPa) with four replications. During the experiments, the changes of pore water pressure and void ratio were registered. With increasing time of stress application, we determined an increased soil strain. The higher the stress‐application time, the smaller gets the void ratio and the precompression stress value. Parallel to these variations in settlement, we also found changes in the pore‐water‐pressure values. This is a consequence of decreasing pore diameter while the water saturation increases. Thus, the proportion of neutral stresses on total stress increases which coincides with a change of water suction (= unsaturated) conditions up to even positive pore‐water‐pressure values (from less negative to positive pore water pressure values). From our experiments, we can conclude that the changes in pore‐water‐pressure values already occur at normal stress values smaller than the precompression stress. This underlines the increasing sensitivity of soil deformation processes close to the internal soil strength. The results support the idea, that in order to quantify the mechanical strength of structured unsaturated soils, we always have to determine the changes in pore‐water‐pressure values, too.     

Soil physical properties related to soil structure

The aim of this paper is to clarify the effect of soil aggregation on soil physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Such aggregates are, most of all, more dense than the aggregated bulk soil. The intra-aggregate pore distribution consists not only of finer pores but these are also more tortuous. Thus, water fluxes in aggregated soils are mostly multidimensional and the corresponding water fluxes in the intra-aggregate pore system are much smaller. Furthermore, ion transport by mass flow as well as by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. The chemical composition of the percolating soil solution differs even more from that of the corresponding homogenized material the stronger and denser the aggregates are.

The rearrangement of particles by aggregate formation also induces an increased apparent thermal diffusivity as compared with the homogenized material. The aggregate formation also affects the aeration and the gaseous composition of the intra-aggregate pore space. Depending on the kind and intensity of aggregation, the intra-aggregate pores can be completely anoxic, while the inter-aggregate pores are already completely aerated. The higher the amount of dissolved organic carbon in the percolating soil solution, the more pronounced is the difference between the gaseous composition in the inter- and in the intra-aggregate pore system.

From the mechanical point of view, the strength single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. The more structured soils are, the higher the proportion of the effective stress on the total stress is, but even in single aggregates positive pore water pressure values can be revealed. Dynamic forces e.g. due to wheeling and/or slip processes can affect the pore system as well as the composition of the soil by: (1) a rearrangement of single aggregates in the existing inter-aggregate pore system resulting in an increased bulk density and a less aerated and less rootable soil volume, (2) a complete homogenization, i.e. aggregate deterioration due to shearing. Thus, the smaller texture dependent soil strength coincides with a more intensive soil compaction due to loading. (3) Aggregate deterioration due to shearing results in a complete homogenization, if excess soil water is available owing to kneading as soon as the octahedral shear stresses and the mean normal stresses exceed the stress state defined by the Mohr-Coulomb failure line. Consequently, normal shrinkage processes start again.

Thus, the rearrangement of particles and the formation of well defined single aggregates even at the same bulk density of the bulk soil both affect, to a great extent, various ecological parameters. Environmental aspects can also be correlated, or at least explained with the processes in soils, as a major compartment of terrestial ecosystems, if the physical and chemical properties of the structure elements and their composition in the bulk soil are understood.     

Aggregate characterization as compared to soil bulk properties

The aim of this paper is to clarify the effect of soil aggregation on the physical and chemical properties of structured soils and as compared with the homogenized material. Aggregation and aggregate strength do not only depend on biological activity and organic exudates, but also on the intensity, number and time of swelling, and drying events. Such aggregates are not only more dense than the structured bulk soil, the intra-aggregate pore distribution consists not only of finer pores, but they are also more tortuous. Thus, water and ion fluxes by mass flow as well as ion transportation by diffusion are delayed, whereby the length of the flow path in such tortuous finer pores further retards chemical exchange processes. Futhermore, the chemical composition of the percolating soil solution differs more from that of the corresponding homogenized material the stronger and denser the aggregates are. From the mechanical point of view, the strength of single aggregates, determined as the angle of internal friction and cohesion, depends on the number of contact points or the forces, which can be transmitted at each single contact point. However, internal soil parameters, like grain size distribution or chemical composition, further affect the strength. The more structured the soils are, the higher is the proportion of the effective stress on total stress, but even in single aggregates neutral stresses can be revealed. This is true because of the relationship to the smaller value of the hydraulic conductivity and higher tortuosity. Finally, some dynamic effects on aggregation and aggregate deterioration are discussed.     

Determination of pre-compression stress of a variously grazed steppe soil under static and cyclic loading

In many land use systems all over the world soil deformation is a major problem due to increasing land use intensity. On arable soils machine traffic is continuously intensified with respect to load and wheeling frequency leading to (sub-)soil compaction and deeper soil degradation concerning hydraulic or pneumatic functions. Altered soil functions, in particular reduced hydraulic conductivities and impeded aeration, may decrease crop growth and productivity as well as the filtering and buffering capacity of soils. Prevented gas exchange and longer lasting anoxia in soils due to the reduced pore continuity and pore functioning also affects global change processes. In order to evaluate potential risks for irreversible soil deformation, it is necessary to quantify their mechanical stability. A commonly applied method is the determination of the pre-compression stress, commonly under static loading conditions in oedometer tests. The determination of pre-compression stresses under static loading may not quite resemble the conditions encountered in the field where soils are loaded repeatedly with a sequence of short intermittent loading–unloading–reloading events. Such dynamic loading conditions are encountered, e.g. at multiple wheel passes or in grassland soils due to animal trampling. In this study we present a comparison of a standard (static loading) and a modified (cyclic/dynamic loading) oedometer test using data of a Calcic Chernozem from the Inner Mongolian steppe under various grazing intensities. Static loading lasted for 10 min per loading step, while the dynamic/cyclic loading was carried out by 30 s loading and following 30 s unloading (=1 cycle) for in total 20 cycles. Differences between statically and cyclically determined pre-compression stresses at an identical time of loading show lower values for the statically determined pre-compression stress values compared to those determined cyclically. Among the dynamically determined pre-compression stresses, the values decrease with increasing number of loading steps and loading time, respectively. This is particularly true for the ungrazed sites.Thus, it could also be proofed that increased grazing intensities lead to structure deformation and increased sensitivity to wind- and water erosion followed by severe land degradation of grassland soils, particularly in semi-arid areas. Furthermore, hydraulic effects, e.g. positive pore water pressure due to intense shearing and kneading processes induced by grazing animals can enhance this structural deterioration.Thus, dynamic or cyclic loading results in an intense soil deformation which also causes serious changes in ecological and soil physical properties like hydraulic conductivity or gas flux.     

Die Bestimmung der Aggregatstabilität - ein Methodenvergleich

The determination of aggregate stability - a comparison of methods The mechanical load bearing capacity and strength of aggregates against external forces can be described and quantified by indirect test (wet sieving method) and direct tests (e.g. crushing test, direct shear test). With the mean of the crushing test and the direct shear test in the shear box it is possible to achieve definite stresses, with which the influence of pore water pressure on soil stability can be explained by the occurence of water menisci forces. It is also possible to proof the pore water pressure dependent increase of strain by the particle reorientation during the process of aggregation. By the method of wet sieving, however, the summarized effect of hydraulic and mechanical processes can be described qualitatively only. A definite relation to single processes is therefore not possible.     

Die Bestimmung der Wasserspannungs- /Wasserleitfähigkeits-Beziehung von Bodenaggregaten

Determination of the unsaturated hydraulic conductivity of soil aggregates by use of microtensiometers The hydraulic properties of single aggregates were measured with the use of microtensiometers. They are small enough (tip diameter 1 mm, length 1–2 mm) that two of them can be installed inside an aggregate within a distance of 1–3 mm. The changes of water suction are measured with pressure transducers and recorded by a micro-computer. Results obtained for different aggregates show, that at the same water suction, the hydraulic conductivity of single aggregates is up to 2 orders of magnitude smaller than that of the bulk soil. The cross-over-suction value for aggregates can also be derived.     

Effect of compaction on pore functions of soils in a Saalean moraine landscape in North Germany

Soil compaction and related changes of soil physical parameters are of growing importance in agricultural production. Different stresses (70, 230, 500, and 1000 kPa) were applied to undisturbed soil core samples of eight typical soils of a Saalean moraine landscape in N Germany by means of a confined compression device to determine the effect on (1) total porosity/pore‐size distribution, (2) saturated hydraulic conductivity, and (3) air conductivity to assess the susceptibility towards compaction. Different deformation behaviors after exceeding the mechanical strength particularly resulted from a combination of soil characteristics like texture and initial bulk density. The saturated hydraulic conductivity, as an indicator for pore continuity, was largely affected by the volume of coarse pores (r² = 0.82), whereas there was no relationship between bulk density and saturated hydraulic conductivity. Since coarsely textured soils primarily possess a higher coarse‐pore fraction compared to more finely textured soils, which remains at a high level even after compaction, only minor decreases of saturated hydraulic conductivity were evident. The declines in air conductivity exceeded those in hydraulic conductivity, as gas exchange in soils is, besides the connectivity of coarse pores, a function of water content, which increases after loading in dependence of susceptibility to compaction. A soil‐protection strategy should be focused on more finely textured soils, as stresses of 70 kPa may already lead to a harmful compaction regarding critical values of pore functions such as saturated hydraulic conductivity or air capacity.     

Auswirkung unterschiedlicher Bodenbearbeitung auf die mechanische Belastbarkeit von Ackerböden

Effect of different tillage systems on the mechanical compressibility of arable soils The influence of different tillage systems (Zero-tillage, minimum tillage with rotary tiller and conventional tillage with moldboard plow) on the mechanical compressibility of two soils, a Regosol from loess-colluvium, and a Vertisol from mesocoic clay has been investigated as well as the causes and consequences for penetration resistance and air permeability. In addition the bulk density and the pore size distribution of aggregates have been investigated with regard to a possible explanation for the often described difference in stability in untilled or minimum tilled soils when compared to tilled ones. The results clearly demonstrate the higher mechanical stability of the untilled or minimum tilled soils as compared to tilled soils, which may result in better growing condition for plants. The higher stability is the result of a more vertical oriented pore geometry and stress distribution and can be explained by measured differences in the bulk density and pore size distribution within the aggregates too.     

Mechanisms of soil vulnerability to compaction of homogenized and recompacted Ultisols

Soil compaction is a main cause of soil degradation in the world and the information of soil compaction in subtropical China is limited. Three main Ultisols (quaternary red clay, sandstone and granite) in subtropical China were homogenized to pass through 2 mm sieve and recompacted into soil cores at two bulk densities (1.25 and 1.45 g cm⁻³). The soil cores were equilibrated at different matric potential values (−3, −6 and −30 kPa) before subjected to multi-step compaction tests. Objectives of this study were to determine how different initial soil conditions and loading time intervals influence pre-compression stress and to evaluate an easy measure to determine soil vulnerability to compaction. It became evident that the soil strength indicator, pre-compression stress, was affected by soil texture, initial soil bulk density and matric potential. The coarser the soil texture, the lower the bulk density and the higher the matric potential, the lower was the pre-compression stress. The pre-compression stress decreased exponentially with increasing initial soil water content. Soil water content and air permeability decreased after compaction. The amount of water loss was affected not only by soil texture, bulk density and initial water content but also by loading time interval. These results indicate soil pore structure and hydraulic conductivity changed during compactions. The applied stress corresponding to the highest changes of pore water pressure during compaction had a significant linear relationship with the pre-compression stress (R=0.88, P<0.001). The correlation was ascribed to that the changes in pore water pressure describe the dynamics of the interactive effects of soil pore characters and soil water movement during compaction. The results suggested the evaluation of soil vulnerability to compaction have to consider the initial soil condition and an easy method to measure the changes in pore water pressure can be applied to compare soil strength and soil vulnerability to compaction.     

Effect of repeated tractor wheeling on stress/strain properties and consequences on physical properties in structured arable soils

The discussion about the effect of repeated short time wheeling on long-term changes in soil structure and pore functioning reveals a great uncertainty. On the one hand it is told that soil structure elements are rigid and do not undergo intense changes in pore functions as a consequence of the short loading interval during each single wheeling. On the other hand, the complete deterioration of the structure elements and pore functions is assumed to occur, which also results in changes of the shrinkage pattern, soil strength including even strength regain. Consequently, the effect of wheeling on soil deformation and stress/strain distribution was investigated in a soil bin which contained Hiwassee clay at the NSDL, Auburn. If the soil is very strong due to aggregation, plow pan formation or dryness, soil stress applied by repeated wheeling results in an increased primarily vertical soil particle displacement in the Hiwassee clay soil while during repeated wheeling (up to 10×) a more pronounced displacement linked with a more intense movement of particles can be proofed. With increasing number of wheeling events, new platy or again coherent structure elements are formed, which create a very different pore system. The more intense is soil wheeling, the smaller is the saturated hydraulic conductivity and the higher is the unsaturated one at a given pore water pressure value. Such changes are the more pronounced the more completed is the rearrangement of the still existing aggregates into new units like plates. Due to shear because of the three-dimensional soil displacement even under dry conditions such aggregates can be redisturbed and a coherent but very compacted soil horizon can be formed. Under those conditions the values of bulk density are even higher than the Proctor density.     

Soil Water Relations and Aeration Status of Single Soil Aggregates,Taken from a Gleyic Vertisol

Clayey soils have the potential to swell and to shrink depending on their hydraulic and hydrological status. Thus bulk density values vary in a range of 1.0 to 2.0 g cm^?3 in the case of a gleyic Vertisol, by which also other soil physical properties e.g. the pore size distribution of the bulk soil as well as of the soil aggregates are affected. Intraaggregate airfilled porosities are reduced by shrinkage and are relatively low. Thus it appeared to be difficult to determine the airfilled porosity of the aggregates below pF 1.5. For that reason and because of the influence of pore forms we were not able to get a clear relation of diffusion constant K with airfilled porosity. Regarding soil aeration status, the existence of anoxic microsites in the interior of unsaturated soil aggregates has been proved by microelectrode measurements of oxygen partial pressure and redox potential distribution in single soil aggregates. We verified restrained oxygen supply to the aggregate center as well as reduced redox potentials only for aggregates of the A horizon. There the microbial activity, measured as soil respiration as well as the source for C and N was by a factor 2 to 4 higher than in the subsurface horizons.     

Soil Structural Stability and Water Retention Characteristics Under Different Land uses of Degraded Lower Himalayas of North‐West India

The lower Himalayan regions of north‐west India experienced a severe land‐use change in the recent past. A study was thus conducted to assess the effect of grassland, forest, agricultural and eroded land uses on soil aggregation, bulk density, pore size distribution and water retention and transmission characteristics. The soil samples were analysed for aggregate stability by shaking under water and water drop stability by using single simulated raindrop technique. The water‐stable aggregates (WSA) >2 mm were highest (17·3 per cent) in the surface layers of grassland, whereas the micro‐aggregates (WSA < 0·25 mm) were highest in eroded soils. The water drop stability followed the similar trend. It decreased with the increase in aggregate size. Being lowest in eroded soils, the soil organic carbon also showed an adverse effect of past land‐use change. The bulk density was highest in eroded lands, being significantly higher for the individual aggregates than that of the bulk soils. The macroporosity (>150 µm) of eroded soils was significantly (p < 0·05) lower than that of grassland and forest soils. The grassland soils retained the highest amount of water. Significant (p < 0·05) effects of land use, soil depth and their interaction were observed in water retention at different soil water suctions. Eroded soils had significantly (p < 0·05) lower water retention than grassland and forest soils. The saturated hydraulic conductivity and maximum water‐holding capacity of eroded soils were sufficiently lower than those of forest and grassland soils. These indicated a degradation of soil physical attributes due to the conversion of natural ecosystems to farming system and increased erosion hazards in the lower Himalayan region of north‐west India. Copyright © 2013 John Wiley & Sons, Ltd.     

Aspects of transport processes in aggregated soils

The way in which water and solutes move in aggregated soils depends on the mode of saturation of the pore space that is made up of the micropore region within the aggregates and the macropores surrounding them. When both regions are saturated, a hydraulic-head gradient causes water to flow preferentially in the macropores with little flow within the aggregates, so that movement of solutes into or out of the aggregates is mainly by diffusion caused by the difference between the solute concentrations of the water in the two regions. When macropores full of water surround unsaturated aggregates, water is imbibed by the aggregates giving rise to convective movement of solutes with the moving water. When the macropores are empty, the aggregates become almost isolated so that redistribution of water and solutes occurs only within the aggregates with very little transport of water and solutes between them. The movement of water and solutes in the micropore region within the aggregates can be considered to behave as if in a continuum, and can be described by Darcy's law and the dispersion equation, with boundary conditions imposed by conditions in the macropores. These physical considerations of transport behaviour in aggregated soils can be used to give guidance on soil management practices concerning drainage and leaching.     

Wirkung mechanischer Belastung auf Gefüge und Ertragsleistung einer Löss‐Parabraunerde mit zwei Bearbeitungssystemen

Effect of mechanical stress on structure and productivity of a loess‐derived Luvisol with conventional and conservation tillage In Germany farmers are committed to caring for the land by a soil protection law. Yet vehicles with ever increasing axle load endanger productivity and environmental quality of arable soils. In spring of 1995 a field experiment was startet on a wet silty Luvisol to test the effect of single mechanical loading on soil and crop characteristics, when managed by mouldboard ploughing (PL) or conservation tillage (CT). CT soils are considered to be more resistant against compactive stresses and to recover from degeneration more rapidly than PL soils. Beside an unwheeled control the loading treatments were light (2 × 2.5 t; number of wheel passes times wheel load); medium (2 × 5 t) and high (6 × 5 t). In 1995 even light loading of the PL soil caused a significant yield decline by 50% in spring barley, but this happened on CT soil only with high loading. In subsequent years with winter wheat and winter barley yield decline was less distinct. Loading of PL soil reduced total root length (from 4 to 1 km m⁻²) and rooting depth (from 70—90 to 40—70 cm), but on CT soil only root length was diminished by high loading. A tillage‐traffic pan (30—35 cm) hindered subsoil rooting in PL, which was favored in CT by earthworm channels. High loading caused compaction to at least 50 cm depth. Within the pan of the PL soil, penetration resistance attained 5 MPa and bulk density 1.65 g cm⁻³. In the CT soil the zone of maximum compaction was closer to the surface (15—25 cm). In PL soil the saturated hydraulic conductivity and the O₂‐diffusion coefficient gradually decreased with loading, but in CT soil only with heavy loading. The compacted top soil was broken in subsequent years by ploughing (PL: 25 cm) or rotary implements (CT: 5—8 cm). With PL, structure in the pan layer and subsoil did not recover, and rooting depth was still limited. Some restoration, however, was indicated with CT. Here transmitting properties increased in time, which was attributed to the reconstruction of root and earthworm channels, as demonstrated by computer tomography. We conclude that in silty soils compacted layers below ploughing depth will hardly be regenerated by internal processes. CT soils are less susceptible to loading, but high stresses are harmful too. Therefore recommending CT as a measure for protecting soil from compaction would not be enough, considering the present development towards heavy field machinery.     

根土界面的行为及小麦品种对其的影响

Water movement into and out of roots depends on the water potential difference between the bulk soil and the root xylem and the total hydraulic conductance of the pathway,which can be divided into three parts, i.e.soil conductance,soil-root conductance and root conductance .The values and relative importance vary with soil water content .The general rule is that water uptake by roots is mainly limited by radial hydraulic conductance of root in wet soils, the soil-root interface becomes a major limiting factor water uptake in moderately dry soils,and the water uptake is limited by the rapidly decreasing soil hydraulic conductance in seriously dry soils .Meanwhile these limiting factors vary with crop variety,and these variations can be used to evaluate the drought-resistance and water use efficiency of crops.     

Einfluss des Porenwasserdruckes auf die Zugfestigkeit von Bodenproben

Effect of pore water pressure on tensile strength Direct tensile testing with measurements of the pore water suction was used to investigate the relationship between tensile strength and suction. The tests were conducted on a till and a clayey soil, both homogenized. A closer view is focused on the relationship between material strain and the development of suction. Beyond, the factor χ of the effective stress equation for unsaturated soils by Bishop (1959), which was calculated by the data of tensile strength and corresponding matric suction is compared to the volumetric χ of the tested soil specimens. It could be shown, that the pore water pressure changes with strain. Therefore, not the initial suction of a soil is relevant for its failure but the actual one that can be measured in the failure zone at the moment of fracture. In addition the application of the volumetric χ in the effective stress equation compared to the χ derived from tensile testing leads to an 1.6 to 2.8 fold overestimation of the contribution of matric suction to soil tensile strength.     

A method for predicting soil susceptibility to the compaction of surface layers as a function of water content and bulk density

Identifying the vulnerability of soils to compaction damage is becoming an increasingly important issue when planning and performing farming operations. Soil compaction models are efficient tools for predicting soil compaction due to agricultural field traffic. Most of these models require knowledge of the stress/strain relationship and of mechanical parameters and their variations as a function of different physical properties. Since soil compaction depends on the soil's water content, bulk density and texture, good understanding of the relations between them is essential to define suitable farming strategies according to climatic changes. In this work we propose a new pedotransfer function for 10 representative French soils collected from cultivated fields, a vineyard and forests. We investigate the relationship between soil mechanical properties, easily measurable soil properties, water content and bulk density. Confined compression tests were performed on remoulded soils of a large range of textures at different initial bulk densities and water contents. The use of remolded samples allowed us to examine a wide range of initial conditions with low measurement variability. Good linear regression was obtained between soil precompression stress, the compression index, initial water content, initial bulk density and soil texture. The higher the clay content, the higher the soil's capacity to bear greater stresses at higher initial water contents without severe compaction. Initial water content plays an important role in clayey and loamy soils. In contrast, for sandy soils, mechanical parameters were less dependent on initial water content but more related to initial bulk density. These pedotransfer functions are expected to hold for the soils of tilled surface layers, but further measurements on intact samples are needed to test their validity.     

The effect of aggregate size on water retention and pore structure of two silt loam soils of different genesis

Aggregate size distribution and pore structure affect many soil functions and root growth. The aggregate structure is associated with soil genesis and management practices applied. In this study the effects of various size ranges of aggregates (<0.25, 0.25–0.5, 0.5–1, 1–3, 3–5, and 5–10 mm) and undisturbed soil from the plough layer (0–15 cm) of two types of soils (Haplic Phaeozem and Eutric Fluvisol) of the same silty loam textural group on water retention curves (WRC) and pore size distribution (PSD) were assessed. A greater bulk density and lower humus content characterized the Eutric Fluvisol as compared to the Haplic Phaeozem. The WRC was determined using standard Richards chambers in drying process and expressed as the degree of saturation. Equivalent PSD was derived from the WRC. Resin impregnated sections from the layer of 0–8 cm showed that the Eutric Fluvisol, compared with the Haplic Phaeozem, had coarser pores and aggregates. The degree of saturation in beds of aggregates <0.25, 0.25–0.5 and 0.5–1 mm compared to beds of aggregates 1–3, 3–5 and 5–10 mm was greater at higher values of pressure head for both soils, and for undisturbed soil it was greater for the Haplic Phaeozem than for the Eutric Fluvisol at lower values of pressure head. The inverse relationship was true at higher values of pressure head. The derivative curves of PSD showed that the beds of aggregates and undisturbed soils exhibited multi-peak PSD. The pore radius peaks within the textural (primary) pore system were more defined in beds of aggregates <0.25 mm than in beds of coarser aggregates, whereas in the case of the structural and macropore peaks it was often the reverse. Greater magnitude and narrower shape of the peaks in the undisturbed Haplic Phaeozem compared to the Eutric Fluvisol indicated a more heterogeneous nature of the pore system in the former. The PSD data are discussed in relation to aggregate size distribution and stability of the soil aggregates. The results of this study can be helpful in predicting storage and transmission functions of surface aggregated soils.     

Soil types differentiated their responses of aggregate stability to hydrological stresses at the riparian zones of the Three Gorges Reservoir

Purpose

The aim of this study was to investigate the resistance of aggregates to flooding stresses for different soil types and present implications for the restoration of eroded soils.

Materials and methods

Twelve field sites for three soil types were selected and separated into four hydrological stress levels at the riparian zones of the Three Gorges Reservoir. Soil samples were collected randomly, followed by lab analysis of soil mechanical composition, soil aggregate and stability, and soil carbon and nitrogen contents in the bulk soil and different sizes of aggregates.

Results and discussion

Clay and silt migrated from the upper water level sites to lower water level sites for Regosols under hydrological stresses; however, the mechanical compositions were not changed for Anthrosols and Luvisols. Total carbon content (TC), total nitrogen content (TN), and carbon and nitrogen ratio (C/N) were highest under strong hydrological stress for all-sized aggregates and bulk soils. Aggregate disintegration under hydrological stresses made organic matter exposed, but the anaerobic environment created by flood avoided organic matter from being decomposed. Most TC and TN in aggregates and bulk soils were negatively correlated with stability. Compared with Anthrosols and Luvisols, Regosols had lower aggregate stability due to its low large macro-aggregate proportions for each stress level. Therefore, much attention should be given to Regosols which has a high potential for erosion. Resistances of aggregates to strong and intermediate hydrological stress were higher for Anthrosols than other tested soils. However, Luvisols had the highest resistance to hydrological stresses because of its higher stability above the elevation of 165 m, due to its highest small macro-aggregate proportion. Therefore, anthropogenic restorations are recommended to stabilize the structure of Anthrosols and Luvisols under weak and strong hydrological stress, respectively.

Conclusions

The operation of the Three Gorges Reservoir forced the riparian ecosystem to undergo periodical flooding stresses. The resistance of soil aggregates to hydrological stresses was lowest for Regosols, which should be concerned urgently to reduce soil losses. Under strong and intermediate hydrological stresses, Anthrosols had greater stability to maintain its original structure. However, the aggregate stability of Luvisols was higher for weak and none hydrological stress levels. Hence, anthropogenic restorations are recommended to take priorities for Anthrosols and Luvisols to reduce soil erosion under weak and strong hydrological stress, respectively.

     

Eine Methode zur Ermittelung der wasserspannungsabhängigen Änderung des Sauerstoffpartialdruckes und der Sauerstoffdiffusion in einzelnen Bodenaggregaten

A method to determine oxygen partial pressure and oxygen diffusion in single soil aggregates as a function of soil moisture tension Anaerobic zones occur even in unsaturated soils of silty or clayey texture, that are aerated sufficiently in their macropore system. These zones can be related to the inner parts of soil aggregates. To describe the oxygen balances in soils it is necessary to measure not only in soil profiles but as well in single soil aggregates within a range of soil matrix potentials. Therefore oxygen partial pressure in single soil aggregates of different texture was measured continuously as a function of soil matrix potential. For that purpose we developed an oxygen sensitive microelectrode with a tip diameter of 0.5 mm, that is sturdy enough to measure even in sandy soils. One microtensiometer (diameter of the tip < 0.5 mm) and one oxygen microelectrode were placed in water saturated soil aggregates. Soil water potential and oxygen partial pressure were measured continuously during soil drying. The results show an aeration of primarily anoxic soil aggregates at different soil matrix potentials due to different texture and structure. The clayey polyhedral aggregates of the Vertisol were aerated at significantly lower soil matrix potentials than the loamy prisms of the Fluvisol. These show higher values of oxygen partial pressure even at soil water potentials less than 150 hPa. In the aggregates of the Vertisol, that have a fine texture, values of rel. aparent diffusion D_s/D_o were in the range of 1 · 10^?3 at soil water potentials < ?