Dry Soil Layer Research Articles

Determination of dry soil layer and its soil heat flux distribution using actively heated distributed temperature sensing

A dry soil layer (DSL) of certain thickness is usually formed on the sandy soil surface during the soil water evaporation process, with the bottom boundary of DSL corresponding to the evaporating surface. As the water transfer mechanism in the DSL is different from that beneath the DSL, in the numerical analysis, it is essential to know such DSL in particular in terms of initial conditions. Moreover, the change in water transfer mechanism further leads to the change in energy transfer mechanism in the DSL, rendering the whole soil drying process more complex. To master the DSL distribution and its accompanying energy changes, a new method for determining the DSL and its soil heat flux (G) distribution based on the actively heated distributed temperature sensing (AH-DTS) technology is proposed. Through in-situ tests on the Loess Plateau in Yan'an, China, the DSL thickness and the G profiles in sandy soil are determined successfully. The results show that there is a DSL with a thickness of 0.25 m in the test site. The G in the DSL fluctuates with depth, and its value is significantly higher than that of the soil below the DSL, indicating that the G in the DSL provides energy for water evaporation at the evaporating surface. Note that the AH-DTS technology can realize multi-parameter measurement at the same time with high-resolution, which is effective and reliable for determining the DSL thickness and the G distribution. In practical applications, it is recommended that the DSL thickness be determined according to both soil temperature (T0) profile and volumetric water content (θ) profile. Moreover, it is suggested that a reasonable heating time-interval of at least two hours and the close contact between the AH-DTS sensor and the surrounding soil be guaranteed to ensure the accuracy of G profile determination.

Lysimeter and In Situ Field Experiments to Study Soil Evaporation Through a Dry Soil Layer Under Semi‐Arid Climate

AbstractDuring droughts, soil evaporation is often constrained by water vapor transport through an air‐dry soil layer (DSL). Fick's water vapor diffusion is widely regarded as the only process for such transport; however, field studies conducted in arid and semi‐arid conditions showed measured evaporation rates higher than those predicted by diffusion. Therefore, transport processes other than diffusion could be relevant. To study the evaporation through a DSL, the same lysimeter column with 70 cm thick DSL as earlier applied in laboratory in Balugani et al. (2021), was installed in the field in Spain applying an original weighing setup to measure evaporation. The correlation between the measured evaporation and possible drivers of the water vapor transport were evaluated. With the DSL thickness of 70 cm in 2012 and 12 cm in 2015, the lysimeter recorded similar groundwater evaporation rates: 1.25 and 1.05 mm days−1, respectively; these rates were much larger than the laboratory recorded rates (0.3 mm days−1) and those estimated in this study using Hydrus1D accounting for non‐isothermal liquid water fluxes and water vapor diffusion. The main forcing driver of the field lysimeter evaporation was the soil profile temperature fluctuation, which concealed other less important forcing factors, that is, atmospheric pressure fluctuations and diffusion. A multivariate regression model to estimate evaporation was proposed, based on the profile temperature fluctuation, that, when added to the atmospheric pressure fluctuations, yielded reliable estimates of the cumulative evaporation measured in both 2012 and 2015.

Water-use characteristics of two dominant plant species in different community types in the Mu Us Desert

Shrubs are pioneer species for ecological restoration in drylands and can promote the growth of herbaceous plants. Knowledge of the water-use characteristics of these revegetated species in different community types is crucial for understanding interspecific and intraspecific interactions and guiding ecological restoration; however, few studies have focused on this process. In this study, we explored the water sources and root distribution of plants in three community types in the Mu Us Desert: a mixed community including a shrub species Artemisia ordosica and a grass species Leymus secalinus, a pure A. ordosica community, and a pure L. secalinus community. We measured the δ18O of the plant xylem and soil water within 100 cm of the soil surface and surveyed the root biomass distribution. The main water source for both A. ordosica and L. secalinus in mixed communities was from the shallow soil layer (0–20 cm) and it overlapped most of the time. When soil moisture in the shallow soil layer was low, L. secalinus still used water from the shallow soil layer, whereas A. ordosica mainly used water from the dry deep soil layer (60–100 cm). The water sources for A. ordosica were significantly different among different communities. In contrast, L. secalinus had similar water sources in different communities. We also found that the root biomass of shrubs was significantly lower in mixed communities than in pure communities. Moreover, when available soil water increased, the root biomass of grasses increased significantly, whereas that of shrubs did not significantly change. These results suggest grasses have an advantage over shrubs in competing for water and this may lead to the exclusion of shrubs from the community, and grasses dominated community. Our study can be used as a reference for assessing the persistence of shrub species to guide future ecological restoration.

Continuous maize cultivation with high nitrogen fertilizers associated with the formation of dried soil layers in the semiarid farmland on the Loess Plateau

• Different land use types were chosen to determine soil desiccation. • Long-term maize cultivation with high yield caused a DSLs in the deep soil layers. • Crop rotations are recommended for maize production in the semiarid environment. Sustainable agriculture ensures food security and combats climate change by minimizing reliance on water resources. Maize ( Zea mays L.) is commonly grown with plastic mulching practice on the Loess Plateau of China, but its impact on soil desiccation is not well understood. We compared the effects of different land-use, including 17-year continuous wheat cultivation, 19-year pea-wheat rotations, 12-year continuous maize cultivation (high yield), 30-year multi-crop rotations, and 9-year continuous maize cultivation (varying yield) under four nitrogen (N) fertilization rates (i.e., 0, 100, 200, and 300 kg N ha −1 ). Average crop evapotranspiration during 2009–2020 for long-term continuous wheat, pea-wheat rotation, high-yielding maize, and multi-crop rotations was 222, 201, 277, and 393 mm, respectively, while average effective precipitation from March to September was 329 mm for all treatments. Continuous high-yielding maize cultivation formed a dry soil layer (DSL) in the deep soil profile (>400 cm), which was not found in other cultivation modes. After 9-year continuous high-yielding maize cultivation, 300 kg N ha −1 applying (20.9 Mg ha −1 biomass yield) formed the DSLs in the 360–960 cm soil profiles, while under 200 kg N ha −1 (20.1 Mg ha −1 biomass yield), the soil water content was almost close to the threshold of DSL in the 600–740 cm soil depth; while under 0 and 100 kg N ha −1 (<15 Mg ha −1 biomass yield), the DSL was not found along the soil profile, indicating that DSL formation under continuous maize cultivation was is driven by high N rates (or high yield level). These results suggest that long-term high-yielding maize cultivation with plastic mulching causes severe soil water depletion, resulting in the formation of a DSL in the deep soil profile. To maintain productivity, farmers should not be encouraged to carry out long-term continuous cropping of high-yielding maize in the semiarid environment on the Loess Plateau.

New physiological thresholds improve soil desiccation prediction rationality in apple orchards converted from farmland on the Chinese Loess Plateau

AbstractThe dry soil layer (DSL), as a typical indicator for degradation in soil water supply capacity and soil drought, could provide important information for land use and reconstruction in semiarid and arid areas. However, the traditional index of DSL assessment neglected the water absorption characteristics and drought resistance of different crops and lacked uniformity and comparability. These may misjudge the occurrence and severity of the DSL and bring uncertainty to land development and vegetation selection. In this study, the DSL severity of the main production zone of apples in the Chinese Loess Plateau was reassessed using new plant physiological indices covering the next 60 years (2020–2080) under four general circulation models. Physiological indices were established based on the response of leaf net photosynthetic (TPN) and transpiration rate (TTR) to soil available water which is the difference between soil moisture and permanent wilting point (n = 113). The environment policy integrated climate (EPIC) model was used to predict future soil water dynamics and drought yield loss (YL). The results showed that TPN and TTR significantly slowed down the accumulation of DSL severity quantitative index (QI, based on thickness moisture and formation depth of DSL) and enhanced the correlation between DSL and YL, which correlation coefficient increased from 0.51 to 0.73 and 0.64. Forming serious DSL (QI &gt;0.5) has slowed from 2040 to 2055 years. Moreover, future climate change accumulatively reduced 9.95%–14.18% of the YL. These results indicated that the traditional method overestimated the environmental contradiction between economic benefits and eco‐hydrology of apple orchards, which could send unreliable messages to policymakers to restrict further development of apple industries. This study emphasized that evaluating DSL based on plant physiological threshold reflected better soil desiccation level and YL, which will contribute to further study of the sustainable development of fragile ecosystems.

Numerical modeling of single closed and open-ended pipe pile embedded in dry soil layers under coupled static and dynamic loadings

Abstract For the design of a deep foundation, piles are presumed to transfer the axial and lateral loads into the ground. However, the effects of the combined loads are generally ignored in engineering practice since there are uncertainties to the precise definition of soil–pile interactions. Hence, for technical discussions of the soil–pile interactions due to dynamic loads, a three-dimensional finite element model was developed to evaluate the soil pile performance based on the 1 g shaking table test. The static loads consisted of 50% of the allowable vertical pile capacity and 50% of the allowable lateral pile capacity. The dynamic loads were taken from the recorded data of the Kobe earthquake. The current numerical model takes into account the material non-linearity and the non-linearity of pile-to-surrounded soil contact surfaces. A lateral ground acceleration was adapted to simulate the seismic effects. This research emphasizes modeling the 1 g model by adapting MIDAS GTS NX software. This will, in turn, present the main findings from a single pile model under a combined static and dynamic load. Consequently, the main results were first validated and then used for further deep investigations. The numerical results predicted a slightly higher displacement in the horizontal and vertical directions than the 1 g shaking table. The shear stress–shear strain relationship was predicted. Positive frictional resistance for the closed-ended pile was captured during the first 5 s when low values of acceleration were applied and, consequently, the pile resistance decreased and became negative. Internal and external frictional resistance was captured for the open-ended pipe pile. Overall, frictional resistance values were decreased with time until they reached the last time step with a minimum value. As a result, the evaluation of the current study can be used as a guide for analysis and preliminary design in engineering practice.

Land Use Affects Soil Water Balance and Soil Desiccation within the Soil Profile: Evidence from the Western Loess Plateau Case

This study evaluated the properties of soil water dynamics and desiccation to a depth of 500 cm and tested the idea that land use affects soil drying in deep profiles. Four land use types were chosen: farmland, artificial forest and grassland, and abandoned land. Soil water content was most outstanding under long-term wheat fields, but average soil water content under artificial vegetation of Caragana korshinskii Kom. and alfalfa dropped to 6–8% within the 160–500 cm soil profile, very near to the 7.0% wilting point. Long-term continuous maize cultivation in a fully mulched ridge–furrow system significantly depleted soil water and resulted in a dried soil layer with a thickness of 240 cm. Short-term or long-term land abandonment enhanced soil reservoir and reduced soil water storage deficit degree compared to long-term maize and artificial vegetation. Soil water storage in the 160–500 cm soil profile was depleted by 240 and 464 mm under long-term maize and Caragana korshinskii Kom., respectively, by 267, 319, 381, and 463 mm under 5-, 10-, 20-, and 30-year alfalfa, and by −58, 278, 234, and 93 mm under 5-, 10-, 20-, and 30-year abandonment land, respectively, compared to long-term wheat. Based on the analysis of long-term experimental results, this study shows that the phenomenon of soil drying caused by long-term intensive maize production cannot be ignored in semi-arid areas and that natural re-vegetation under long-term abandonment, rather than artificial vegetation, may be the best type of vegetation reconstruction for this region based on soil water balances.

Deep soil water deficit causes Populus simonii Carr degradation in the three north shelterbelt region of China

Large-scale afforestation exacerbates soil water scarcity in a semiarid region of northern China, causing the formation of a dried soil layer (DSL) and ecosystem degradation, and overmature Populus simonii Carr (>30 years) degradation in the Three North Shelterbelt Program (TNSP) area has recently been observed. Determining the relationship between degraded Populus simonii Carr and soil water deficit is essential for afforestation in semiarid regions. We monitored soil water content (SWC) to a 160 cm depth in an area with five degradation degrees (healthy, light degradation, moderate degradation, severe degradation, and dead) to investigate soil water dynamics and DSL characteristics from May 2018 to December 2021. The results showed that: compared with that of healthy Populus simonii Carr (94.7 ± 26.2 mm), the shallow layer (0–80 cm) soil water storage of severely degraded (114.1 ± 29.9 mm) and dead Populus simonii Carr (128.0 ± 35.7 mm) was restored by 19.4 mm and 33.3 mm, respectively. However, the deep-layer (80–160 cm) SWC was difficult to recover and a DSL gradually developed, the DSL − SWC was only 9.3–11.95 %. The deep-layer relative extractable soil water (REW) of healthy Populus simonii Carr was significantly higher than that of moderately degraded, severely degraded, and dead Populus simonii Carr (P < 0.05). The deep-layer SWC decreased with increasing degradation due to coarser soils and increased BD. A greater bulk density (BD) and higher sand proportion reduced the soil water holding capacity. Poor site conditions reduced the resistance of shelterbelts, causing the degradation and mortality of Populus simonii Carr during drought. Therefore, our results improve the understanding of the soil water dynamics and degradation mechanism of Populus simonii Carr, providing a theoretical basis for managing and maintaining the TNSP.

Spatial–temporal dynamics and recovery mechanisms of dried soil layers under Robinia pseudoacacia forest based on in‐situ field data from 2017 to 2020

AbstractPlanting Robinia pseudoacacia in water‐limited regions can promote soil and water conservation and ecological service functions. However, it can cause the formation of below ground dried soil layers (DSLs), causing land degradation and tree mortality. To ascertain the spatial–temporal dynamics and recovery processes of DSLs, we monitored the deep soil water content (SWC) to a depth of 500 cm at 27 sites on a typical R. pseudoacacia forest from 2017 to 2020 and calculated the evaluation indices of DSLs based on plant‐available SWC (PASWC) and stable field capacity (SFC). Compared with PASWC‐identified DSLs, the degree of SFC‐identified DSLs was more severe, although the spatial–temporal characteristics were similar. Severe soil desiccation was identified as 79% of the 500 cm profile dried out below 101 cm. The mean thickness of DSL and mean SWC within a DSL were 396 cm and 9.0%, respectively, and the quantitative index reached Grade III (severe) DSLs. All DSL indices demonstrated weak or moderate variability in space and strong variability in time. The heterogeneity of rainfall events and microtopographic positions explain the spatial–temporal variation of DSL indices well. A heavy rainfall event (&gt;50 mm) triggered the disappearance of DSLs at some sites, indicating the possibility of DSLs recovery based on field observations under natural condition. Microtopography (such as the swales) provides advantageous conditions to the recovery of DSLs by redistributing rainfall flow at the local scale. These results provide insights into DSLs recovery, which is useful for land development and rehabilitation in planted forests.

Soil water deficit was effectively alleviated by higher water infiltration after the short-term forestland-to-farmland conversion in semi-arid area

The increasing global vegetation aggravated the consumption of soil water and caused soil desiccation, which may result in a threat to the survival and sustainability of vegetation, especially in semi-arid and arid areas. How to solve soil water deficit caused by afforestation is an important practical problem. Here, a field experiment of soil water infiltration and replenishment was conducted before and after forestland-to-farmland conversion. Results showed that the cumulative infiltration of each stage (the initial stage, the transitional stage, and the stable stage) was the highest in farmland, followed by forest interspace, and the lowest in forest understory. The non-capillary porosity at 0–20 cm soil layer in farmland was higher than that in forest understory and interspace, which positively improved the infiltration accumulation in the initial stage. Thus, the cumulative infiltration in initial stage of farmland (28.93 mm) was significantly higher than that of forest understory (19.45 mm). Moreover, soil water at the depth of 0–160 cm soil layer was replenished with short term after forestland-to-farmland conversion. It is concluded that forestland-to-farmland conversion could improve soil water infiltration rate and cumulative infiltration by changing porosity, and effectively replenished soil water storage in the short term. These findings have potential implications for effective mitigation of soil water deficit caused by afforestation, and provide a new idea or method for the solution to dry soil layers caused by artificial forestland in semi-arid areas.

Restoring soil water content after alfalfa and Korshinsk peashrub conversion to cropland and grassland

AbstractThe introduction of Korshinsk peashrub (Caragana korshinskii) and alfalfa (Medicago sativa) as restorative vegetation in the Loess Plateau, China has resulted in the widespread occurrence of a dried soil layer (DSL). Soil water recovery with respect to the soil water content (SWC) following the occurrence of a DSL (i.e., DSL‐SWC recovery time) and the effect of the number of planting years of restorative vegetation on soil water recovery have received little attention. The aim of this study was to investigate soil water dynamics after vegetation conversion (include three fields of converting cropland with soybean or millet into alfalfa land, three fields of converting alfalfa land into soybean land or four fields of converting peashrub land into natural grassland) in the Liudaogou watershed in a water‐wind erosion crisscross region in the Loess Plateau, China. With this aim in mind, we conducted in‐situ field experiments for 3 or 5 years and continuously monitored the SWC in a 0–5 m or 0–8 m profile. The results showed that the SWC increased drastically after alfalfa was converted to soybean, especially after consecutive wet year when the SWC of 0–800 cm profile was recharged up to 0.16 cm3 cm−3, which means the DSL‐SWC in the 0–800 cm profile recovered within 5 years. The DSL‐SWC of the profile improved obviously within 3 years after peashrub was converted to natural grass. The SWC of the two fields was 0.16 cm3 cm−3 and 0.15 cm3 cm−3, respectively. The SWC depletion time was 6.1, 7.5, and 8.4 years in the three fields converted from cropland to alfalfa. Based on our findings, the number of alfalfa planting years should not exceed 8 years. However, planting years of peashrub needs further study because it has excellent drought resistance and a long root system. Taking the advantages and disadvantages of two restoration management methods into account, alfalfa‐crop rotations and natural grasslands are the two best choices for ecological restoration in the Loess Plateau, China.

Deep root information “hidden in the dark”: A case study on the 21-m soil profile of Robinia pseudoacacia in the critical zone of the Chinese loess Plateau

Deep roots can uptake deep soil water to support plant growth and mitigate climate change in water-limited ecosystems. Few studies have been conducted on the deep root characteristics in the soil profile as limited field data are available. To explore the relationship between the deep root characteristics, depth, and soil water status, we obtained the distribution pattern of deep roots and soil water content along a 21-m soil profile (n = 22) in a mature Robinia pseudoacacia plantation. The maximum rooting depth, mean root biomass, and mean depths corresponding to 50% and 90% of the total root biomass of 17-year-old R. pseudoacacia trees were >20 m, 0.083 ± 0.029 g·cm−2, 1.17 ± 1.74, and 6.15 ± 3.98 m, respectively. We established an exponential power model to fit the relationship between the deep root biomass and soil depth, which had a higher goodness-of-fit than the typical exponential model, logistic model, and piecewise linear model. The root morphological characteristics along the 21-m vertical profile rapidly decreased with depth in the 0–1-m layer, and then stabilized, while the specific root length (SRL) and specific root area (SRA) increased with depth. The relationships between the SRL, SRA, and soil water content in the dried soil layers with roots (SWC-DSL) were significantly positive (both p < 0.01), while those between average root diameter density and SWC-DSL were significantly negative (p < 0.001), suggesting that roots increased their length rather than diameter to acquire water resources with relatively low carbon investment. The soil depth and soil water availability co-controlled root biomass and root morphological characteristics. Our findings highlight the importance and complexity of deep roots and their morphological characteristics, which are fundamental for forest models and management to maintain sustainable development between soil water and root water uptake.

Modeling long-term water use and economic returns to optimize alfalfa-corn rotation in the corn belt of northeast China

The long-term effects of alfalfa (Medicago sativa L.)-corn (Zea mays L.) rotation on soil water use and economic returns have not been evaluated and optimized in the corn belt of northeast China (CBNC). Crop modeling based on long-term weather data and short-term field data helps evaluate agricultural practices’ long-term effect. The Agricultural Production Systems sIMulator (APSIM) was calibrated and validated based on the data sets on soil water storage (SWS), aboveground biomass (biomass), and yield under continuous corn (CC), five years of alfalfa (5A), and three years of alfalfa followed by two years of corn (3A2C) cropping systems from 2014 to 2018 at Quanyangou-Lishu in Jilin province, China. The APSIM could simulate well SWS, evapotranspiration (ET), and annual biomass of alfalfa (5A) with normalized root mean square error (NRMSE) of 9.6%, 5.0% and 13.2%, respectively. This model was also confirmed to simulate well the SWS, ET, yield, and biomass of corn (CC) with the NRMSE of 9.6%, 3.4%, 6.5% and 9.8%, respectively and that of alfalfa-corn rotation (3A2C) with the NRMSE of 8.4%, 4.5%, 8.7% and 12.2%, respectively. The validated APSIM was used to optimize the alfalfa-corn rotation systems by simulating water use and economic returns under five scenarios, including CC, 3A2C, four years of alfalfa followed by two years of corn (4A2C), five years of alfalfa followed by two years of corn (5A2C) and five years of alfalfa followed by five years of alfalfa (5A5A) based on the weather data from 1984 to 2018 (35 years) at Quanyangou-Lishu (black soil with losing mollic epipedon, relatively warm and moist climate, in Jilin province, the center of CBNC), Fujia-Lishu (aeolian sandy soil, relatively warm and moist climate, in Jilin province), Chaoyang (cinnamon soil, warm and dry climate, in Liaoning province, the south of CBNC) and Hailun (black soil with moderately thick mollic epipedon, cold and wet climate, in Heilongjiang province, the north of CBNC). Dry soil layers were only found in 250–400 cm soil layer in 18-yr 5A5A or 20-yr 5A2C treatment at Chaoyang and in 350–450 cm soil layer in 20-yr 5A5A or 26-yr 5A2C treatment at Quanyangou-Lishu. At four sites, the highest and lowest mean annual biomass water use efficiency (WUEB) could be found in CC (30.91–38.93 kg ha−1 mm−1) and in 5A5A (20.53–26.31 kg ha−1 mm−1), respectively. The alfalfa-corn rotation treatments had similar WUEB ranged from 22.91 to 30.17 kg ha−1 mm−1. The NR of alfalfa-corn rotation treatments at four sites were 38–169% higher than that of CC and 6–35% lower than that of 5A5A. Alfalfa-corn rotation treatments improved the stability of NR at Fujia-Lishu and Chaoyang by the CV of 40–75% and 65–95%, compared to that of CC treatment by the CV of 79% and 101%, respectively. Among alfalfa-corn rotation treatments, the 5A2C had higher and more stable NR than 4A2C and 3A2C. Therefore, we suggested 5A2C at Fujia-Lishu and Hailun and 4A2C at Quanyangou-Lishu and Chaoyang to achieve dry soil layer’s prevention, relatively high WUEB and high and stable NR.

Impact of Environmental Factors and System Structure on Bioretention Evaporation Efficiency

Bioretention is an important low impact technology that has prominent stormwater detention and purification capacity. Current study focused on analyzing the impact of environmental factors and system structure on bioretention evaporation efficiency. In operational phase, the moisture content in bioretention packing changes constantly, directly affecting the stagnation efficiency of the bioretention. Therefore, it is very important to study the evaporation efficiency of the bioretention for objective evaluation of hydrologic effects. In this study, an artificial climate chamber was used to investigate the effect of environmental factors and bioretention structure on the evaporation efficiency of bioretention. The evaporation capacity of bioretention was analyzed under different temperature and relative humidity conditions in a laboratory-scale artificial climate chamber. The result showed that evaporation rate at the initial stage was close to the maximum evaporation capacity under an environmentally controlled rapid decrease. Results revealed that after 15 h, the evaporation rate decreased more than 60%, and the evaporation rate decreased rapidly at the higher temperature, whereas the evaporation rate in the third stage was low and stable. It was about 1 mm/d (0.82~1.1 mm/d) and formed a dry soil layer. The results revealed that cumulative evaporation of the bioretention with a submerged zone was notably higher than that without the submerged zone, and the cumulative evaporation after 50 h was 16.48% higher. In the second stage of evaporation, the decreasing amplitude of the evaporation capacity of bioretention with the submerged zone was also relatively slow. Moisture content in upper layers in bioretention packing was recharged from the bottom submerged zone by capillary action and water vapor diffusion. These research findings can be used to evaluate the hydrologic effect of bioretention and can also be used to guide its design.

Assessing soil water recovery after converting planted shrubs and grass to natural grass in the northern Loess Plateau of China

The Chinese Loess Plateau (CLP) is prone to adverse effects from drought, especially the widespread creation of a dried soil layer (DSL), a problem intensified by revegetation under the Grain for Green Program. Using 13-year soil moisture (SM) data, we compared soil water consumption by planted shrubs (Korshinsk peashrub, KOP), planted grass (purple alfalfa, ALF), and natural grass (NAG) from 2004 to 2016 in the CLP. To assess the soil water recovery processes, long-term (30 years) SM dynamics were simulated using the simultaneous heat and water (SHAW) model based on field data and local meteorological data under two scenarios (A: converting KOP to NAG and B: converting ALF to NAG). The results showed that the decline rates of SM in 1–4 m profiles for NAG (24.0–29.8%) were much lower than those for KOP (47.6–51.4%) and ALF (48.8–50.2%) during the 13-year growth period. Modelling SM dynamics at depths of 1–4 m for 30 years showed that SM gradually increased and that the DSL prevalence could be reduced under scenarios A and B. The complete elimination of DSL requires at least 6 years at 1–4 m under scenario A, 13 years at 2–4 m, and 22 years at 1–2 m under scenario B. Soil water restored to local stable soil water levels requires approximately 19, 13, and 15 years in the 1–2 m, 2–3 m, and 3–4 m profiles, respectively, under scenario A. Soil water recovery will take approximately 28 years in the 2–3 m profile and 27 years in the 3–4 m profile under scenario B. Our results enhance the understanding of the soil water depletion and recovery processes under different vegetation types and can could be used to provide scientific guidance for sustainable ecological restoration in the CLP.

The Process of Soil Desiccation under Haloxylon ammodendron Plantations: A Case Study of the Alxa Legue Desert, China.

Haloxylon ammodendron is a desert shrub widely used as a windbreak and for sand fixation, and it has achieved remarkable results in China. However, in desert areas, large-scale afforestation increases soil water consumption and forms a dried soil layer (DSL), the development of which seriously threatens the sustainable development of the ecosystem. In this study, soil moisture in the 0–400 cm soil profile was measured in selected 5-, 11-, 22-, 34-, and 46-year-old plantations of Haloxylon ammodendron plantations in Alxa Legue, China, and three soil desiccation evaluation indices were calculated—the soil desiccation index (SDI), DSL thickness (DSLT), and DSL soil water content (DSL-SWC)—to analyze the change pattern of the soil water content for different stand ages. The results showed that the shallow water layer (0–200 cm) was depleted sharply in the first five years of Haloxylon ammodendron plantation growth, but no DSL developed; the inflection point of soil water content change appeared after 10 years of growth, after which the shallow soil water was depleted and the drying process of the deep soil water content was significantly faster than that in the early growth period. The deep soil layer (200–400 cm) was depleted seriously after 22 years of afforestation, the soil drying phenomenon was obvious, and the DSL developed from the 172 cm soil layer. After 46 years of afforestation, the DSL was fully developed and the DSL-SWC was only 0.034 cm3 cm−3. Priority should thus be given to the use of less water-consuming shrub species; alternatively, after 5 years of growth of Haloxylon ammodendron plantations, certain water control measures should be taken to maintain the soil water balance.

Changes in vegetation and soil properties across 12 years after afforestation in the hilly-gully region of the Loess Plateau

Vegetation restoration is the primary method used to prevent soil erosion and improve the ecological environment. The objective of this study was to explore how vegetation and soil properties changed over time in natural and artificial vegetation types. We conducted vegetation surveys in four permanent plots over 12 years (2007–2018) and measured soil properties in 2007, 2013, and 2016. The results showed that the change in species composition in natural vegetation was a continuous and progressive process. After approximately ten years of natural herb restoration, trees began to appear and formed sparse forests. In artificial vegetation, the species composition of the undergrowth varied under Robinia pseudoacacia forests during restoration years, and was dominated by Poaceae and annual plants that could tolerate shade and drought. Species composition was stable under Caragana korshinskii shrubs, where the undergrowth was dominated by perennial plants that were similar to natural vegetation. In different vegetation types, species richness, diversity, and herb community cover fluctuated, while the succession rate decreased with the number of restoration years. The soil moisture condition was better in natural vegetation than in artificial vegetation, and could be maintained near the stable field capacity. The soil moisture was supplemented to 280 cm in R. pseudoacacia forests and 320 cm in C. korshinskii shrubs in a year with extreme rainfall, but the dry soil layer returned in the following years. Soil nutrients improved over time, especially in artificial vegetation, which appears to be better than natural vegetation. In general, natural vegetation improved plant community and soil properties. Artificial vegetation only improved soil nutrients, but caused soil desiccation which is difficult to recover. Therefore, the natural vegetation restoration model of sparse forest grassland was better than artificial vegetation model.

The slenderness ratio effect on the response of closed-end pipe piles in liquefied and non-liquefied soil layers under coupled static-seismic loading

AbstractThis study presents the findings of a 3D finite element modeling on the performance of a single pile under various slenderness ratios (25, 50, 75, 100). These percentages were assigned to cover the most commonly configuration used in such kind of piles. The effect of the soil condition (dry and saturated) on the pile response was also investigated. The pile was modeled as a linear elastic, the surrounded dry soil layers were simulated by adopting a modified Mohr-Coulomb model, and the saturated soil layers were simulated by the modified UBCSAND model. The soil-pile interaction was represented by interface elements with a reduction factor (R) of 0.6 in the loose sand layer and 0.7 in the dense sand layer. The study was compared with the findings of 1g shaking table tests which were performed with a slenderness ratio of 25. In the validation case, there was a clear correlation between the laboratory findings and the numerical analyses. It was observed that the failure mechanism is influenced by the soil condition and the slenderness ratio to some extent. Under the dry soil condition, no base pile deformation was observed; However, tip pile movement was observed under the saturated soil condition with pile slenderness ratios of 25 and 50. The findings of this study are also aimed to include an approximation of the long-term deformations at the ground surface which has experienced shaking.

Soil water dynamics and water balance on a tropical coral island

AbstractUnderstanding soil water dynamics and the water balance of tropical coral islands is important for the utilization and management of their limited freshwater resources, which is only from rainfall. However, there is a significant knowledge gap in the influence of soil water on the water cycle of coral islands. Soil water dynamics and the water balance of Zhaoshu Island, Xisha Archipelago were thus investigated using soil moisture measurements and the Hydrus‐1D model from October 2018 to September 2019. Over the study period, vegetation transpiration, soil evaporation, groundwater recharge and storage in the vadose zone were approximately 196, 330, 365 and 20 mm, occupying 22%, 36%, 40% and 2% of annual rainfall total (911 mm), respectively. For the wet season (from May to October) these values became 75, 202, 455 and 40 mm, occupying 10%, 26% and 59% and 5% of the seasonal rainfall total (772 mm), respectively. During the dry season (from November to April), a dry soil layer between 40 and 120 cm depth of the soil profile was identified that prevented water exchange between the upper soil layers and the groundwater resulting in the development of deep roots so that vegetation could extract groundwater to supplement their water requirements. Vegetation not only consumes all dry season rainfall (140 mm) but extracts water deeply from groundwater (90 mm) as well as from the vadose layer (20 mm). As such, the vegetation appears to be groundwater‐dependent ecosystems. The research results aid us to better understand the process of water dynamics on coral islands and to protect coral island ecosystems.

Natural compensation mechanism of soil water infiltration through decayed roots in semi-arid vegetation species

In arid and semi-arid areas, unsuitable afforestation programs may cause soil water deficit and even, in some cases, deep-soil desiccation and the subsequent death of part of the vegetation. However, the benefits of preferential flow through decayed root channels for supplying water in the dried soil layers are rarely evaluated. This study examines the effects of root channels on soil water content (SWC) and infiltration (initial and steady-state) in two planted species with high drought tolerance, namely: Korshinsk peashrub (Caragana korshinskii Kom.), and red plum apricot (Armeniaca vulgaris Lam.), using bare land as control site and published data from alfalfa (Medicago sativa L.) plots to refine results analysis. The infiltration rates were measured using a double-ring infiltrometer in areas with alive and decayed roots; and methylene blue was used to trace the pathways of water flow. The highest SWC appeared in the Korshinsk peashrub land, whereas the alfalfa grassland had the lowest SWC; and the differences of SWC among treatments were significant at all soil layers. Korshinsk peashrub had the highest steady infiltration rates, which were about 23% and 83% higher than those rates measured in the fruit tree plantation and alfalfa grasslands, respectively. The steady infiltration rates were significantly and positively correlated with the average root diameter and area. Within the same species, the decayed root plots significantly increased the soil water infiltration capacity compared to the alive root plots. Considering the water scarcity conditions of the study area, the preferential flow formed through decomposed roots was conducive to soil water supplement, appearing as a natural compensation mechanism caused by dead vegetation. Our findings contribute to a better understanding of soil water infiltration during root decay of the plants in water-limited ecosystems, and how this dynamic replenishes soil moisture and alleviates soil desiccation.