Vertical distribution patterns and drivers of soil bacterial communities across the continuous permafrost region of northeastern China

Background Soil microorganisms in the thawing permafrost play key roles in the maintenance of ecosystem function and regulation of biogeochemical cycles. However, our knowledge of patterns and drivers of permafrost microbial communities is limited in northeastern China. Therefore, we investigated the community structure of soil bacteria in the active, transition and permafrost layers based on 90 soil samples collected from 10 sites across the continuous permafrost region using high-throughput Illumina sequencing. Results Proteobacteria (31.59%), Acidobacteria (18.63%), Bacteroidetes (9.74%), Chloroflexi (7.01%) and Actinobacteria (6.92%) were the predominant phyla of the bacterial community in all soil layers; however, the relative abundances of the dominant bacterial taxa varied with soil depth. The bacterial community alpha-diversity based on the Shannon index and the phylogenetic diversity index both decreased significantly with depth across the transition from active layer to permafrost layer. Nonmetric multidimensional scaling analysis and permutation multivariate analysis of variance revealed that microbial community structures were significantly different among layers. Redundancy analysis and Spearman’s correlation analysis showed that soil properties differed between layers such as soil nutrient content, temperature and moisture mainly drove the differentiation of bacterial communities. Conclusions Our results revealed significant differences in bacterial composition and diversity among soil layers. Our findings suggest that the heterogeneous environmental conditions between the three soil horizons had strong influences on microbial niche differentiation and further explained the variability of soil bacterial community structures. This effort to profile the vertical distribution of bacterial communities may enable better evaluations of changes in microbial dynamics in response to permafrost thaw, which would be beneficial to ecological conservation of permafrost ecosystems.

Active earth pressure against flexible retaining wall for finite soils under the drum deformation mode

A reasonable method is proposed to calculate the active earth pressure of finite soils based on the drum deformation mode of the flexible retaining wall close to the basement’s outer wall. The flexible retaining wall with cohesionless sand is studied, and the ultimate failure angle of finite soils close to the basement’s outer wall is obtained using the Coulomb theory. Soil arch theory is led to get the earth pressure coefficient in the subarea using the trace line of minor principal stress of circular arc after stress deflection. The soil layers at the top and bottom part of the retaining wall are restrained when the drum deformation occurs, and the soil layers are in a non-limit state. The linear relationship between the wall movement’s magnitude and the mobilization of the internal friction angle and the wall friction anger is presented. The level layer analysis method is modified to propose the resultant force of active earth pressure, the action point’s height, and the pressure distribution. Model tests are carried out to emulate the process of drum deformation and soil rupture with limited width. Through image analysis, it is found that the failure angle of soil within the limited width is larger than that of infinite soil. With the increase of the aspect ratio, the failure angle gradually reduces and tends to be constant. Compared with the test results, it is shown that the horizontal earth pressure reduces with the reduction of the aspect ratio within critical width, and the resultant force decreases with the increase of the limit state region under the same ratio. The middle part of the distribution curve is concave. The active earth pressure strength decreases less than Coulomb’s value, the upper and lower soil layers are in the non-limit state, and the active earth pressure strength is more than Coulomb’s value.

The Stress-Strain State of the Tunnel Lining that Crosses the Fault Zone of Soil Blocks during an Earthquake

The study examines the question of the tunnel behavior under seismic or geophysical load in the zone of changes in the hardness of the surrounding soil mass. In the course of the study, the internal forces and displacements arising in the structure of a tunnel in the zone of intersection of the boundaries of soil layers with different properties, in the case when these layers move relative to each other, were determined by analytical and numerical solutions. The data obtained by the analytical method was compared to numerical models using practical examples.

Interpretation of Construction Procedure and Physicochemical Characteristics for Soil Layers from Sowangneung (Small Royal Tomb) of Ssangneung (Twin Tombs) in Iksan, Korea

The Iksan Ssangneung (twin tombs), a pair of tombs comprising the Daewangneung (large royal tomb) and the Sowangneung (small royal tomb), were constructed in the typical style of stone tunnel and chamber tombs in the Baekje Kingdom during the Sabi period (538 to 660 AD) of ancient Korea. Soil layers exposed during excavation of Sowangneung in a trench east of the tomb are: the bottommost layer, the ground level layer, the Panchuk (rammed earth) layer of the Baekje, the layer created by a grave robbery, and soil recovered during the Japanese colonial period. Soil samples were obtained by segmenting an easy stratigraphic horizon into sub categorized soil layers, and their material properties were analyzed; they are composed mainly of sandy loam based on the particle size distributions. In the site foundation, loamy sand is packed in the bottommost layer, and sandy loam with high sand and silty sand fills most of the overlying layer. The central and topmost portion of the Baekje layer is composed of loam with high clay content. All soil layers show geochemical behaviors similar to those of the bottommost layer. X-ray diffraction analysis verified kaolinite in all layers, also observed in soil layers displaying high crystallinity. Kaolinite and halloysite were identified by scanning electron microscopy. Thus, we conclude that the Baekje layer of the Sowangneung is composed of sandy loam containing kaolin procured from near the site. An impermeable middle to upper layer was created using viscous loam. The top of the tomb was closed tightly.

Numerical Evaluation of The Nonlinear Behavior of Soil-Pile Interaction Under The Effect of Coupled Static-Dynamic Loads

Liquefaction of saturated soil layers is one of the most common causes of structural failure during earthquakes. Liquefaction occurs as a result of increasing pore water pressure, whereby the rise in water pressure occurs due to unexpected change in stress state under short-term loading, i.e., shaking during an earthquake. Thus, general failure occurs when the soil softens and eliminates its stiffness against the uplift pressure from the stability of the subsurface structure. In this case, the condition of soil strata is considered undrained because there is not enough time for the excess pore water pressure to dissipate when a sudden load is applied. To represent the non-linear characteristics of saturated sand under seismic motions in Kobe and Ali Algharbi earthquakes, the computational model was simulated using the UBCSAND model. The current study was carried out by adopting three-dimensional-based finite element models that were evaluated by shaking table tests of a single pile model erected in the saturated soil layers. The experimental data were utilized to estimate the liquefaction and seismicity of soil deposits. According to the results obtained from the physical models and simulations, this proposed model accurately simulates the liquefaction phenomenon and soil-pile response. However, there are some differences between the experiment and the computational analyses. Nonetheless, the results showed good agreement with the general trend in terms of deformation, acceleration, and liquefaction ratio. Moreover, the displacement of liquefied soil around the pile was captured by the directions of vectors generated by numerical analysis, which resembled a worldwide circular flow pattern. The results revealed that during the dynamic excitation, increased pore water pressure and subsequent liquefaction caused a significant reduction in pile frictional resistance. Despite this, positive frictional resistance was noticed through the loose sand layer (near the ground surface) until the soil softened completely. It is worth mentioning that the pile exhibited excessive settlement which may attribute to the considerable reduction, in the end, bearing forces which in turn mobilizing extra end resistance.

Geotechnical characterization and modelling of the “Fallen Lands” phenomenon in the amazon environment

In many rivers of the Amazon, mainly along the Amazon River, the phenomenon of "fallen land" occurs at the time of the rivers' ebb, causing the erosion of river banks, resulting in major social and economic problems. The influence of saturation/percolation of soil layers on the shear resistance of the affected massifs is unknown. Thus, the main objective of the characterization and geotechnical modelling of these slopes is to determine the shear strength of the affected massifs. For this purpose, deformed and undeformed samples were collected from the various layers of the soil massif in a location where the phenomenon was under development. These samples were characterized physically, chemically, and mechanically. Subsequently, the massifs were recreated in reduced scale models, observing the critical conditions in which the disaster occurred, and qualitatively comparing them with slope stability computational models. The results showed that the layers are classified as silty and sandy soils, with a small fraction of clay, and that there are three types of movements associated with the fallen soils: the fall, overturning, and rotational landslide, all actively influenced by the effects of the river's ebb, saturation/percolation, and texture.

Past rainfall and trace element variations in a tropical speleothem from India

An actively growing stalagmite (cave deposit) was collected in 1996 from the Dandak cave in Madhya Pradesh. Trace elements Mg, Sr and Ba were measured in order to asses their palaeoclimatic significance. More than 100% variations in the concentrations of trace elements have been observed during the growth period. The chemical and physical properties of the trace elements along with the data of stable isotope ratios of exygen and carbon from our earlier work has been discussed in this paper. Ratio of Ba to Sr concentrations has remained constant suggesting that the soil layers and bedrock dissolution have contributed the same proporation of trace elements to the cave seepage water. Slowly varying components in the Mg, Sr and Ba concentrations and profiles show high degrees of correlations. It is found that changes in the trace elements are mainly driven by the varying intensity of past rainfall. Trace element variations of speleothems in tropical regions can be used as indicators of the past rainfall intensities. Different climatic intervals are characterised based on trace elements and stable isotoes. Period 3700 to 3350 yr BP was found to be arid. High rainfall was observed between 3350 to 3200 yr BP. After a hiatus period of 2000 yr due to non-climate dependent proceses, again a high rainfall phase between 1200 to 400 yr BP was observed with a peak level at ~200 yr BP and a short arid phase at ~90 yr BP was also observed.

Water Use Characteristics of Two Dominant Species in the Mega-Dunes of the Badain Jaran Desert

The sparse natural vegetation develops special water use characteristics to adapt to inhospitable desert areas. The water use characteristics of such plants in desert areas are not yet completely understood. In this study, we compare the differences in water use characteristics between two dominant species of the Badain Jaran Desert mega-dunes—Zygophyllum xanthoxylum and Artemisia ordosica—by investigating δ2H and δ18O in plant xylem (the organization that transports water and inorganic salts in plant stems) and soil water, and δ13C in plant leaves. The results indicate that Z. xanthoxylum absorbed 86.5% of its water from soil layers below 90 cm during growing seasons, while A. ordosica derived 79.90% of its water from the 0–120 cm soil layers during growing seasons. Furthermore, the long-term leaf-level water use efficiency of A. ordosica (123.17 ± 2.13 μmol/mol) was higher than that of Z. xanthoxylum (97.36 ± 1.16 μmol/mol). The differences in water use between the two studied species were mainly found to relate to their root distribution characteristics. A better understanding of the water use characteristics of plants in desert habitats can provide a theoretical basis to assist in the selection of species for artificial vegetation restoration in arid areas.