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The Loess Plateau is one of the key areas for soil and water erosion control in China. Planting vegetation, such as Robinia pseudoacacia, is one of the mainstream methods to prevent soil and water erosion. However, the combination of abundant calcium ions and phosphate in the soil of the Loess Plateau limits the phosphorus nutrition of plants. In the present study, soil samples were collected under the R. pseudoacacia forest, from which two PSB strains with efficient phosphate solubilization capacities, named PSB2 and PSB7, were isolated and screened. The dissolved phosphate concentrations of their culture media were 9.68-fold and 11.61-fold higher, respectively, than that of the control group. After identification, PSB2 was classified as Pseudomonas and PSB7 as Inquilinus. This is the first time that Inquilinus has been isolated as a PSB from calcareous soil in the Loess Plateau. We then investigated the effects of different growth conditions on their phosphate solubilization capacities. Both strains effectively utilized glucose and ammonium nitrogen while maintaining high phosphate solubilization efficiency. In addition, PSB2 preferred to survive under neutral conditions and PSB7 under acidic conditions. Pot experiments indicated that the inoculation with PSB7 significantly increased the phosphorus content in the roots of R. pseudoacacia. These results imply the potential of this PSB as a phosphorus biofertilizer for R. pseudoacacia, which may be beneficial for soil and water management on the Loess Plateau.

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Landslide risk mapping can be an effective reference for disaster mitigation and land use planning, but the modelling process involves multidisciplinary knowledge which leads to its complexity. In this study, Jiaxian County in Shaanxi Province on the Loess Plateau of China, served as the study area, primarily characterized by Quaternary loess-covered geomorphology, with an average rainfall of about 400 mm annually. Soil erosion and human engineering activities have contributed to significant slope failures, posing threats to local residents and infrastructure. A reasonable inventory of landslides in the region was established by field survey combined with aerial imagery, allowing for characterization of their development and spatial distribution. Nine thematic maps related to landslide occurring and three vulnerability maps were prepared as influencing factors for landslide risk assessment. Subsequently, landslide susceptibility and hazard were evaluated using a kernel extreme learning machine (KELM) and information value (IV) model, followed by map validation. A decision table was then employed to generate the landslide risk map. The results of landslide hazard mapping showed that the historical landslide events were mainly developed in the central part of the study area, particularly concentrated near the developed river network. Integration of overall risk elements suggested that landslide risks in the study area were generally at a low level. Besides, a total of 0.25 % and 2.05 % of the areas were classified as having very high and high landslide risk levels, respectively, where 65.11 % of inventory landslides occurred. Therefore, the proposed procedure is a valuable tool for assessing landslide risk in Jiaxian Country.

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Soil moisture is a key factor for vegetation restoration in arid and semi-arid regions. Clarifying the vertical characteristics of soil moisture in artificial forests on a regional scale and its response mechanisms can benefit for land use management in water-deficient areas such as the Loess Plateau. The study targets Robinia pseudoacacia on the Loess Plateau with a meta-analysis based on 790 soil moisture data points abstracted from 35 published papers. The results show that extensive cultivation of R pseudoacacia on the Loess Plateau leads to a significant reduction in soil moisture (P < 0.05). Soil moisture decreases significantly with growth of trees, especially between 400 and 500 cm soil layers. Soil moisture increases with the hydrothermal gradient. The results indicate that intensive afforestation activities in high temperature and rainy areas still significantly consume deep soil moisture. The main reason is that the impact of hydrothermal factors on soil moisture is significant between 0 and 200 cm soil layers and decreases with increasing soil depth. However, the continuous depletion of deep soil moisture leads to insignificant differences in soil moisture responses under different topographical conditions in the region. Therefore, neglecting the impact of forest age and hydrothermal factors on soil moisture in afforestation activities, the excessive water consumption by R pseudoacacia during growth poses potential risks to the ecological environment of the Loess Plateau. This study provides references for knowledge on water relating problems and sustainable management of artificial forests in arid and semi-arid areas.

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Vegetation restoration is an effective measure to cope with global climate change and promote soil carbon sequestration. However, during vegetation restoration, the turnover and properties of carbon within various aggregates change. The effects of plant source carbon input on surface soil and subsurface soil may be different. Thus, the characteristics of carbon components in aggregates are affected. Therefore, the research object of this study is the Robinia pseudoacacia forest located in 16-47a of the Loess Plateau, and compared with farmland. The change characteristics of organic carbon functional groups in 0-20 cm, 20-40 cm, and 40-60 cm soil layers were analyzed by Fourier near infrared spectroscopy, and the relationship between the chemical structure of organic carbon and the content of organic carbon components in soil aggregates was clarified, and the mechanism affecting the distribution of organic carbon components in soil aggregates was revealed in the process of vegetation restoration. The results show the following: (1) The stability of surface aggregates is sensitive, while that of deep aggregates is weak. Vegetation restoration increased the surface soil organic carbon content by 1.97~3.78 g·kg-1. (2) After vegetation restoration, the relative contents of polysaccharide functional groups in >0.25 mm aggregates were significantly reduced, while the relative contents of aromatic and aliphatic functional groups of organic carbon were significantly increased. The opposite is true for aggregates smaller than 0.25 mm. (3) With the increase in soil depth, the effect of litter on organic carbon gradually decreased, while the effect of root input on the accumulation of inert carbon in deep soil was more lasting.

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The northwestern half of the Chinese Loess Plateau (i.e., the examined area) is reported to have been sensitive to the East Asian Summer Monsoon (EASM) and might have also been exposed to the influence of the Indian Summer Monsoon (ISM) during the Holocene. This study utilizes the already reported pollen data from four high-resolution fossil pollen sequences to quantitatively reconstruct the Holocene mean annual precipitation (Pann) in the examined area. It also incorporates those quantitative precipitation reconstructions from the same area reported by others to delineate the regional Pann patterns. It finally brings the regional Pann patterns into the perceived forcing contexts to explore the underlying mechanisms. Our delineation shows that the Holocene Pann exhibits different temporal trends between the western part and the northern part of the Chinese Loess Plateau. That is, the "higher-than-average" Pann occurred in the early mid-Holocene from ~10.0 to ~5.5 cal. kyr BP in the western part and the "higher-than-average" Pann occurred in the late mid-Holocene from ~8.0 to ~2.5 cal. kyr BP in the northern part. We propose that the Pann differences between the western part and the northern part might have been associated with two mechanisms: (1) differences in the thermal sensitivity to the solar insolation between the Indian Ocean and the Pacific Ocean, and (2) differences in terms of the relative importance of precipitable water vapor transports either from the ISM or from the EASM between the western part and the northern part.

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Vegetation restoration is an effective and important measure for controlling soil erosion in arid and -arid regions. Both its aboveground and underground parts play a crucial role in controlling surface runoff and soil detachment on slopes. But how much the parts of vegetation contribute to the runoff and sediment reducing benefits of rill erosion on slopes is unclear. We used grassland slopes at four successional stages for simulated scouring experiments to observe how successional vegetation community structures, root characteristics, and soil structures contribute to erosion and sand production. Initial flow production time increased, and total runoff decreased. Under the scour intensities, the 11-year slope had the lowest flood peak and volume and the greatest runoff reduction benefit. The 25-year slope had the lowest sand peak and volume and the greatest sediment reduction benefit. As scour intensity increased, runoff reduction effect of vegetation at the successional stages decreased; the sediment reduction benefit remained high. PLS-PM analysis showed that the indirect effects of the aboveground and underground parts of vegetation on sand production were -0.364 and -0.439, respectively. Aboveground parts mainly embodied the regulation of runoff, in which stem count, humus mass, and biomass were the main factors affecting runoff and sand production. Underground parts mainly reflected their soil structure improvement, in which root volume density, root surface area density, and root mass density are the main explanatory variables. The direct effects of runoff and soil structure on slope rill erosion were 0.330 and -0.616, respectively, suggesting the stability of soil structure is the primary factor affecting the sand production, not erosion energy. The results provide a reference for scientific assessment of the key role of natural vegetation restoration in regional soil erosion control and the development of biological measures for soil and water conservation on the slopes of the Loess Plateau.

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Exploring nutrient limitation in forest soil holds significant implications for forest tending and management. However, current research on nutrient limitation status of microorganisms in Robinia pseudoacacia plantations within the Loess Plateau remains insufficient. To investigate soil microbial nutrient limitation of R. pseu-doacacia plantations on the Loess Plateau, we selected R. pseudoacacia plantations with different afforestation time series (15, 25, 35, and 45 years) and a pile of barren slope cropland (control) in Yongshou County, Shaanxi Province as the research objects. We analyzed the contents of soil organic matter, total nitrogen, and total phosphorus, and the activities of ß-1,4-glucosidase (BG), cellobiose hydrolase (CBH), leucine aminopeptidase (LAP), ß-1,4-N-acetylglucoside (NAG) and phosphatase (AP). We analyzed the soil nutrient limitation by stoichiometry and enzyme metrology. The results showed a shift in soil pH from alkaline to acidic during vegetation restoration process, and that total phosphorus exhibited a gradual decrease over the course of 0 to 25 years. Soil orga-nic matter, total nitrogen and enzyme activities exhibited an increasing trend during the same time frame. However, between 25 and 45 years of age, soil total phosphorus, soil organic matter, total nitrogen, AP and LAP gradually declined while NAG, BG, and CBH initially increased and then decreased. Notably, the values of (BG+CBH)/(LAP+NAG), (BG+CBH)/AP and (LAP+NAG)/AP in R. pseudoacacia plantations were higher than the global average throughout the process of vegetation restoration. In the study area, the vector length was less than 1 and gradually increased, indicating that a progressive increase in microbial carbon limitation during the process of vegetation restoration. The vector angle exceeded 45° and exhibited an overall decreasing trend, suggesting that soil microorganisms were constrained by phosphorus (P) with a gradual deceleration of P limitation, without any nitrogen (N) limitation. The restoration of R. pseudoacacia plantation resulted in significant change in soil physical and chemical properties, while the time series of afforestation also influenced nutrient limitation of soil microorganisms.

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Ecological security pattern is an important spatial way to maintain ecological processes and ensure the stability of ecosystem functions. As the implementation of landscape planning and decision-making, it is critically needed to consider the consistency of differentiated methods and their spatial outputs in the construction of ecological security patterns and the matching and applicability of research objects. From the perspective of integration, we combined the regional topography and landscape characteristics, integrated the morphological spatial pattern analysis and the importance evaluation results of ecosystem services to identify the ecological source, and constructed the ecological security pattern of the Ansai District of Yan'an City, the main implementation area of the Grain-for-Green Project on the Loess Plateau. The results showed that the structural and functional construction methods had low consistency in the identification of spatial protection priority. The integration-oriented method could complement each other and achieve the dual goals of structural connectivity and functional improvement. There were 202 ecological sources in the study area, with a total area of 391.58 km2, accounting for 13.3% of the total area of the study area. There were 110 ecological corridors in the study area, with a total length of 599 km, which were mainly distributed around the river channel, showing a distribution pattern of 'short and narrow dense in the north and south, long and wide in the middle'. The structure-function integration method provides new insights for ecological restoration planning of land space and promotes the research of landscape pattern, process and service.

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Fluoroquinolone antibiotics have been extensively used in clinical treatments for human and animal diseases. However, their long-term presence in the environment increases the risk of producing resistance genes and creates a potential threat to ecosystems and the health of humans and animals. Batch equilibrium experiments were utilized to investigate the adsorption and retention behavior and mechanism of the quinolone antibiotic enrofloxacin (ENR) in farmland soil in North China. The adsorption and desorption kinetics of ENR in soil were best fitted by pseudo-second-order model (R2 > 0.999). Both the adsorption and desorption processes of ENR in soil reached equilibrium in 1 h. The desorption amounts of ENR were significantly lower than the adsorption amounts, with the hysteresis coefficient (HI) being less than 0.7. The adsorption thermodynamic process of ENR followed the Linear and Freundlich models (0.965 < R2 < 0.985). Hydrophobic distribution and heterogeneous multimolecular layer adsorption were identified as critical factors in the adsorption process. The adsorption amount of ENR gradually decreased with increasing temperature and the initial concentration of ENR. The adsorption rate of ENR was above 80%, while the desorption rate remained below 15%, indicating strong retention ability. The adsorption rate of ENR in soil decreased with increasing pH, the adsorption rate reached 98.3% at pH 3.0 but only 31.5% at pH 11. The influence of coexisting ions on adsorption primarily depended on their properties, such as ion radius, ionic strength, and hydrolysis properties, and the inhibition of adsorption increased with increasing ionic strength. These findings contribute to understanding the fate and risk of veterinary antibiotics in loess soil in North China.

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In this study, magnesium oxysulfate cement (MOS) was used as a binder for curing loess. The changes in bulk density, porosity, mineral structure and microstructure of the consolidated loess were systematically studied and verified. The porosity decreased from 40.97 % in pure loess to 28.75 % in 13 % MOS solidified sample. Scanning electron microscopy, energy spectrum analysis and thermogravimetric analysis revealed that the addition of MOS binder resulted in the formation of hydrated products, including Mg(OH)2, MgO·mSiO2·nH2O (M-S-H), and 3Mg(OH)2·MgSO4·8H2O (3·1·8 phase), which effectively filled the voids between the grains and facilitated strong bonding among them. After a curing period of 28 days, the compressive strength of loess stabilized with 13 % MOS exhibited an increase to 7.9 MPa. Moreover, following immersion in water for 24 h, the softening coefficient K remained at 0.66. Furthermore, after undergoing five cycles of freeze-thaw cycling, the rate of change in compressive strength RP was only 6.3 %. All the results indicate that MOS exhibits promising potential as a binder for soil stabilization applications.

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Long-term trend forecast of chlorophyll-a concentration (Chla) holds significant implications for eutrophication management and pollution control planning on lakes, especially under the background of climate change. However, it is a challenging task due to the mixture of trend, seasonal and residual components in time series and the nonlinear relationships between Chla and the hydro-environmental factors. Here we developed a hybrid approach for long-term trend forecast of Chla in lakes, taking the Lake Taihu as an instantiation case, by the integration of Seasonal and Trend decomposition using Loess (STL), wavelet coherence, and Convolutional Neural Network with Bidirectional Long Short-Term Memory (CNN-BiLSTM). The results showed that long-term trends of Chla and the hydro-environmental factors could be effectively separated from the seasonal and residual terms by STL method, thereby enhancing the characterization of long-term variation. The resonance pattern and time lag between Chla and the hydro-environmental factors in the time-frequency domain were accurately identified by wavelet coherence. Chla responded quickly to variations in TP, but showed a time lag response to variations in WT in Lake Taihu. The forecasting method using multivariate and CNN-BiLSTM largely outperformed the other methods for Lake Taihu with regards to R2, RMSE, IOA and peak capture capability, owning to the combination of CNN for extracting local features and the integration of bidirectional propagation mechanism for the acquisition of higher-level features. The proposed hybrid deep learning approach offers an effective solution for the long-term trend forecast of algal blooms in eutrophic lakes and is capable of addressing the complex attributes of hydro-environmental data.

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Recent advancements in the application of lipid biomarkers as paleoenvironmental indicators have provided invaluable insights into the dynamics of climatic variations, vegetative histories, and anthropogenic impacts. However, our current understanding of nitrogen-containing lipid biomarkers (NCLBs) in sedimentary deposits remains limited, notwithstanding their potential significance in global nitrogen cycling. To bridge this research gap, a comprehensive study was conducted to characterize the distribution patterns of n-alkyl amides (NAAs) and n-alkyl nitriles (NANs) in representative paleo-lake and loess sedimentary profiles from the arid region of northwestern China (NWC). The widespread occurrence of these NCLBs across late Quaternary strata, with distinct distribution patterns observed in various settings, suggests their formation under diverse environmental conditions. The prevalence of NAAs in arid sedimentary deposits can primarily be attributed to the diverse array of local biota, including vascular plants, algae, and fungi, rather than being solely associated with biomass burning, as commonly assumed. Furthermore, the distribution patterns of NANs closely align with those of NAAs, indicating their formation through thermally induced dehydration of precursor NAAs. Both groups of NCLBs exhibit significant preservation potential in sediments within NWC, which is believed to be intimately linked to the region's arid and cold climate as well as its neutral or weakly alkaline depositional setting. These findings underscore the prospective use of NAAs as indicators of environmental changes and NANs as potential markers of past fire-related activities, making them valuable tools for paleoenvironmental reconstructions of late Quaternary strata, especially in arid, cold, and weakly alkaline regions.

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Soil degradation, characterized by the deterioration of soil physical and chemical properties, nutrient loss, and an increase in toxic substances, is a key ecological concern in mining activities. This study explores the use of waste black shale from mining development as an additive to loess to enhance soil properties for reclamation in mining areas. The research includes resistivity and organic carbon content tests on modified reclaimed loess with varying black shale and water contents. Additionally, the electrical properties of these modified soils are investigated across different AC frequencies. The results highlight the significance of soil plasticity and a 1.5% black shale content in influencing reclaimed loess's electrical properties. Moisture content and black shale influence changes in soil conductive paths and resistivity. The abundance of clay minerals in black shale plays a crucial role in altering soil electrical resistivity due to the adsorption of cations in water and the directional transport under an electric field. Considering soil's three-phase composition and diffuse bilayer structure, the study elucidates the mechanism behind changes in the electrical properties of improved reclaimed loess, accounting for water and black shale content. This research demonstrates the feasibility of using black shale as a soil additive and emphasizes the non-destructive assessment potential of electrical resistivity test (ERT) measurements for modified reclaimed soils.

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Accurately assessing the changes in soil organic carbon storage （SOCS） before and after the Grain for Green Project （GFG） in the Loess Plateau （LP） and exploring the relationship between its spatial and temporal distribution and the influencing factors were important references for the development of regional recycling as well as the formulation of ecological protection policies. Based on the data of climate, human activities, and SOCD in the surface （0-20 cm） and deep （0-100 cm） soil before and after GFG in the LP from 2001 to 2020, we investigated the changes in SOCD at different spatial and temporal scales by using the methods of trend analysis, the kriging method, and variance partitioning analysis. The results showed that： ① Before and after the GFG, the surface SOCS of the whole region increased by 8 338.7×104 t； the deep SOCS increased by 1 160.02×104 t. ② In each bioclimatic subregion, the whole-region average SOCD of Ⅰ （Semi-Humid Forest Region）, Ⅱ （Semi-Humid Semi-Arid Forest and Grassland Region）, and Ⅲ （Semi-Arid Typical Grassland Region） showed a significant increasing trend, with a decreasing trend in Ⅳ （arid semi-arid desert grassland area） and Ⅴ （arid desert area）. ③ The average surface SOCS increase in different ecosystems was ranked as follows： cropland > grassland > woodland > shrubs > bare land and sparse vegetation. The deep soil increase was ranked as follows： grassland > cropland > woodland > shrubs > bare land and sparse vegetation. ④ Climate factors were the most important driving factors for changes in SOCD； the annual average temperature and precipitation were significantly positively correlated with changes in SOCD. The results of the study could provide data support for regional ecological management and land use policy formulation to promote high quality development of the ecological environment in the LP.

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Numerous manually excavated loess caves are present within a cultural relic protection zone in the northwestern region of China. The collapse of these caves frequently leads to the cracking, tilting, and even collapse of ancient buildings above, posing a severe threat to the safety of cultural architectural relics. Investigating the stability and characteristics of deformation and failure in loess caves is essential for effectively reinforcing and protecting cultural relics. A two-dimensional model of a loess underground cavern was developed using OptumG2. The stability and modes of deformation and failure in the underground cavern were analyzed through the augmentation of soil gravity and the strength reduction method. This analysis determined the cavern's safety factor, force, deformation and damage mode, and the plastic zone's progression. Numerical simulations analyzed the force characteristics of the support structure under different stress release ratios. The findings revealed that, with the implementation of an anchor rod concrete lining support scheme, the most probable failure mode is a shear failure, initiating at the arch foot. The ground's stress release rate does not influence the safety factor of the cavern but rather the material, design, and strength of the support structure. However, the magnitude of the internal forces acting on the supporting structure by the soil in the cavern is related to the degree of ground stress release. When applied during significant stress release, support structures may experience reduced internal forces, albeit with more substantial stratum displacement; opting for an appropriate stress release when applying support structures is crucial for achieving optimal stratum displacement and lining internal forces.

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The Ili River Valley in Xinjiang, China, is a typical seasonal frozen area where loess landslide disasters have become increasingly common during the freeze-thaw periods in recent years. This study analyzed the macroscopic mechanical strength and microstructure changes of the Ili loess under different freeze-thaw cycles (FTCs) through the post-freeze-thaw triaxial compression test on the unsaturated soil in laboratory. Apart from the scanning electron microscopy (SEM), and the nuclear magnetic resonance (NMR), the macro-micro correlation analysis and the cluster-principal component analysis were applied for the theoretical discussion. The results indicated that the cohesive force of the loess exhibits an initial decreases, followed by the increases, and eventually keep stable after various FTCs, while the internal friction angle showed the opposite developing trend before the final constant. Similar to the strong correlation between the cohesive force and the particle abundance, the internal friction angle is also closely related to the abundance and orientation fractal dimension of the loess particles. However, the principal component analysis results showed that cohesive force strongly correlates with the average maximum pore size and the pore size fractal dimension, for which the internal friction angle most strongly affected by the average maximum particle size. The possible reason is that the extracted principal components represent a class of microscopic parameters with the same or similar change trend, although there may be a certain offset between them. The mechanical deterioration of loess is attributed to the repeated frost heaving force and the migration potential caused by FTCs. The alterations of the microstructure accelerated the deterioration of the macroscopic mechanical properties of the loess, which further widens the understanding of the mechanism behind the deterioration of loess mechanical strength in the Ili River Valley under FTCs, and contributes to the prevention and management of the local landslide disasters.

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In the Loess Plateau, the impact of abandoned farmland on soil properties and enzyme activity, along with its temporal variations and potential driving factors, remains a mystery. This study was designed to systematically and comprehensively examine the variations in soil enzyme activities, particle size distribution, and stability of soil aggregates at different stages of ecological recovery in the Loess Plateau. Our findings reveal a nuanced temporal pattern: with the progression of cropland abandonment, there is a notable decrease in soil bulk density. Concurrently, a dynamic trend in enzyme activities is observed-initially exhibiting a decline, followed by an increase over extended periods of recovery. Notably, prolonged abandonment leads to marked enhancements in soil structure. Parameters such as the mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates show an overall increasing trend. In terms of the Relative Dissipation Index (RSI), our data indicate a sequence of control > 2 years of abandonment > 4 years > 6 years > 14 years. From this, it can be seen that fallowing may be an effective natural restoration strategy for improving the physical structure of soils in the Loess Plateau and restoring soil nutrients. However, positive changes may take a long time to become evident.

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Mechanical soil parameters are not constants and can be defined in various ways. Therefore, determination of their values for engineering practice is difficult. This problem is discussed based on results of piezoceramic element tests and triaxial tests (unconfined and confined) on loess specimens improved by compaction and sand admixture (20% by weight). The study indicated also the effectiveness of this simple method of loess stabilization. The influence of specimen size, draining conditions, stress and strain state, and different calculation methods on the evaluation of basic mechanical parameters were analyzed. The initial shear and Young's moduli, the degradation of secant moduli with strain, tangent moduli, and Poisson' ratio were determined. The results showed that the shear strength parameters are much less sensitive to the test variables than the stiffness parameters are. In triaxial tests, the strength criterion adopted, the sample size, and the drainage conditions influenced the measured value of cohesion, with a much smaller impact on the angle of internal friction. On the other hand, the adopted definition of the parameter and the range of strains had the greatest influence on the value of the stiffness modulus. Moreover, larger specimens were usually found to be stiffer.

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In order to reduce the consumption of sand and gravel resources, the use of loess can reduce transportation costs and realize the in-situ construction of spongy in areas with rich loess resources. But the collapsibility and low permeability of loess make it unable to be directly used as the filler of bioretention cells. In this study, sulfoaluminate cement (SAC) mixed with a small amount of basalt fiber was considered to be used for loess modification, and the physicochemical properties and nutrient removal effect of SAC-modified loess as filler in bioretention cells were comprehensively evaluated. The results showed that when the SAC dosage was 15% and the basalt fiber addition was 0% (S15B0) and 0.6% (S15B6) and the curing time was 14 days, the stability and appropriate permeability can be exhibited, which can preliminarily satisfy the requirements of bioretention cell. SAC made the maximum adsorption capacity of S15B0 and S15B6 for ammonia nitrogen (NH4+-N) and phosphate higher than that of sand by 10.96%-31.51% and 45.92%-76.72%, respectively. The hydration products in SAC modified loess can fill the internal pores of loess particles and provide structural support, and ultimately reduce the accumulated pores, mesoporous pore size (20%) and surface homogeneity. Both S15B0 and S15B6 showed good removal effects of NH4+-N and COD. The TP removal efficiency was stable at 95.43%∼99.95%. Both the antecedent drying days and the submerged zone have an effect on the nitrogen removal in the bioretention cells, where a longer antecedent drying days is detrimental to the nitrogen removal, and the installation of a submerged zone improves the nitrogen removal. The basalt fiber can enhance the transformation process from nitrate-nitrogen to nitrite-nitrogen in the bioretention cell. Therefore, the modification of SAC can provide a certain idea for the in-situ use of loess as the filler of the bioretention cell.

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A thorough comprehension of nitrogen biogeochemical processes in the vadose zone is crucial for the effective prevention and remediation of soil-groundwater system contamination. Despite the growing research on this subject, the full scope of nitrogen biogeochemical characterization in different geological environments remains poorly understood. This study addresses this knowledge gap by integrating geochemical, microbiological and numerical simulation approaches to gain a deeper insight into nitrogen biogeochemistry in agriculture. Our findings indicate the biogeochemical behavior of nitrogen in the vadose zone is mediated by microorganisms, driven by hydraulics, influenced by geological conditions and environmental factors. Along the groundwater flow, NH4+-N was found to be heavily accumulated in the topsoil of 0-40 cm, while NO3--N was transported and driven by hydrodynamics from both vertical and horizontal directions. Microbial diversity, species composition and functional microorganisms were significantly influenced by soil depth, rather than geomorphological types. Oxidation-reduction potential (ORP), total organic carbon (TOC), soil moisture (MOI), bicarbonate (HCO3-), and ferrous (Fe2+) were identified as the principal environmental factors that regulate nitrogen metabolism and the dominant biochemical processes, encompassing nitrogen fixation, nitrification, and denitrification. Driven by hydrodynamics, NH4+-N, NO2--N and NO3--N tend to form distinct biochemical reaction zones in the vertical vadose zone. These areas are dynamic and subject to geomorphologies. It should be noted that NO3--N can migrate towards groundwater from the clayey sand in the Alluvial Plain, which presents a potential risk of groundwater contamination. The fissure structure of loess may serve as the major transport pathway for groundwater nitrogen contamination in the Loess Tableland. This finding highlights the importance of integrating microbiology, geochemistry and hydraulics to elucidate the biogeochemical processes of nitrogen in the vadose zone with a dynamic mindset.

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