Subsoil Layers Research Articles

Spatial distribution, chemical fractionation and risk assessment of Cr in soil from a typical industry smelting site in Hunan Province, China.

The closure or relocation of many industrial enterprises has resulted in a significant number of abandoned polluted sites enriched in heavy metals to various degrees, causing a slew of environmental problems. Therefore, it is essential to conduct research on heavy metal contamination in the soil of industrial abandoned sites. In this study, soils at different depths were collected in a smelting site located in Hunan Province, China, to understand the Cr distribution, speciation and possible risks. The results revealed that the high-content Cr and Cr(VI) contamination centers were mainly concentrated near S1 (Sample site 1) and S5. The longitudinal migration law of chromium was relatively complex, not showing a simply uniform trend of decreasing gradually with depth but presenting a certain volatility. The vertical distribution characteristics of chromium and Cr(VI) pollution suggest the need for attention to the pollution from chromium slag in groundwater and deep soil layers. The results of different speciation of Cr extracted by the modified European Community Bureau of Reference (BCR) method showed that Cr existed primarily in the residual state (F4), with a relatively low content in the weak acid extraction state (F1). The correlation analysis indicated that Cr was affected by total Cr, pH, organic matter and total carbon during the longitudinal migration process. The RSP results revealed that the smelting site as a whole had a moderate level of pollution. Soil at depths of 2-5m was more polluted than other soil layers. Consequently, it is necessary to treat the site soil as a whole, especially the subsoil layer (2-5m). Health risk assessment demonstrated that the soil chromium pollution was hazardous to both adults and children, and the probability of carcinogenic and non-carcinogenic risk was relatively high in the latter group. As a result, children should be a group of special concern regarding the assessment and remediation of soil contaminated with Cr. This study can provide some insight into the contamination characteristics, ecological and health risks of chromium in contaminated soils and offer a scientific basis for the prevention and control of chromium pollution at abandoned smelting sites.

The effects of pastoral hill country natural landscape features and land management practices on nitrate losses and its potential attenuation for improved water quality

AbstractPastoral farming on hill country landscapes influences nitrogen (N) dynamics and its losses to freshwater. This study reviewed the current literature identifying key effects of pastoral hill country landscape features and land management practices on nitrate losses to receiving waters. The review also highlighted the potential effects of inherent landscape features on nitrate attenuation pathways for better water quality outcomes. Intensive land use activities involving high rates of fertiliser application, higher stocking rates and cattle grazing, relative to sheep grazing, are more likely to increase nitrate loss, especially on lower slopes. However, soils with a high carbon (C) storage capacity such as allophanic soils potentially limit nitrate loss via denitrification in subsoil layers. Hill country seepage wetlands also offer an opportunity to attenuate nitrate loss, though their efficacy is largely impacted by hydrological variations in their inflows and outflows. By enhancing the natural nitrate attenuation capacity of seepage wetlands, mapping and strategic use of high subsoil denitrification potential, effective riparian management, efficient fertiliser and grazing practices and the incorporation of these farm management strategies into Freshwater Farm Plans (FWFPs), wider environmental and farm productivity/profitability goals, including improved water quality, would be achieved on pastoral hill country landscapes.

Vertical bearing capacity of spun precast concrete pile in liquefiable soil: a case study

Spun precast prestressed concrete (SPC) pile has been used in many parts of the world as a viable foundation alternative. This study assesses the load-carrying capacity of SPC piles passing through a deep soft subsoil layer and resting on a dense sand layer. The vertical bearing capacity of the SPC pile has been estimated using various analytical methods and static pile load tests for the Jolshiri area of Dhaka. The liquefaction potential of the site has been assessed using local seismic site conditions, field and laboratory test data. The liquefaction analysis suggests that the topsoil layer is liquefiable to a depth of 4.5 m. The capacity obtained from the load test is compared with those obtained from the different methods, ultimate push-in load, and local design guidelines. Finite Element Analysis (FEA) was performed considering the hardening soil (HS) model using PLAXIS 3D to simulate field conditions. The load settlement response obtained from FEA shows a good agreement with the test results. The capacity examinations primarily suggest that the SPC pile can be a viable foundation solution for the subsoil conditions of Jolshiri Abasion area of Dhaka.

Straw return influences the structure and functioning of arbuscular mycorrhizal fungal community in a rice-wheat rotation system

Straw incorporation is a sustainable soil fertility-building practice, which can affect soil microbial communities. However, how straw incorporation affects arbuscular mycorrhizal fungi (AMF) is not well explored. Here, we studied the impacts of different straw management practices over eight years on the structure and function of AMF communities in a rice-wheat rotation system. The straw managements included no tillage with no straw (NTNS), rotary tillage straw return (RTSR) and ditch-buried straw return (DBSR). The community structure of AMF was characterized using high-throughput sequencing and the mycorrhizal functioning was quantified using an in situ mycorrhizal-suppression treatment. Different straw management practices formed unique AMF community structure, which was closely related to changes in soil total organic carbon, available phosphorus, total nitrogen, ammonium and nitrate. RTSR significantly increased Shannon diversity in 0-10 cm soils while DBSR increased it in 10–20 cm soils when compared with NTNS. DBSR significantly increased hyphal length density in the whole (0–20 cm) ploughing layer, but RTSR only increased it in the subsoil layer (10–20 cm). The mycorrhizal responses of shoot biomass and nutrient (N and P) uptake were positive under both straw incorporation treatments (RTSR and DBSR), but negative under NTNS. AMF community composition was significantly correlated to hyphal length density, and the latter was further a positive predictor for the mycorrhizal responses of plant growth and nutrient uptake. These findings suggest that straw return can affect AMF community structure and functioning, and farmers should manage mycorrhizas to strengthen their beneficial effects on crop production.

Nitrate leaching characteristics of red soils from different parent materials in subtropical China

Globally, nitrate (NO3−) leaching from agroecosystems has been of major concern. There is evidence that NO3− leaching exhibits intense seasonal variation in subtropical regions. However, influencing factors to the seasonal dynamics remain unclear. In this study, a two-year field lysimeters experiment was conducted with three red soils derived from different parent materials (Quaternary red clay (QR), red sandstone (RS), and basalt (BA)). An N fertilizer (15N-enriched urea, 10 atom% excess) of 200 kg N ha−1 yr−1 was applied for maize. The effect of parent material on NO3− leaching characteristics was examined in surface (0–20 cm) and subsoil (20–100 cm) layers. The results showed due to the weakening of abundant drainage, there was no significant effect of parent materials on NO3− leaching characteristics in surface layers. Environmental factors (precipitation and temperature) and fertilization together led to obvious seasonal characteristics, i.e. abundant NO3− leaching during both crop growth and fallow periods. In subsoil layers, NO3− leaching characteristics were completely different among three soils. The concentrations and δ15N of NO3− in QR and RS soils showed a continuous increase after first year's fertilization, while those in BA soil remained relatively stable after reaching peak levels around harvest in first year. Meanwhile, the NO3− leaching amount in BA soil was significantly lower than in the other two soils. These might be explained by different NO3− adsorption capacities caused by the differences in mineral composition and free iron and aluminium contents. These elucidated in subsoil layers, NO3− leaching characteristics highly depended on parent materials. Meanwhile, adsorption capacity was limited and cannot slow NO3− leaching in the long run. Our results suggest that seasonal variation of NO3− leaching in surface layers and temporary retardant effect from NO3− adsorption capacity in subsoil layers should receive much attention when calculating and predicting NO3− leaching in subtropical regions.

Calibration and Testing of Parameters for the Discrete Element Simulation of Soil Particles in Paddy Fields

The parameters of the discrete element simulation model for rice field soils serve as valuable data references for investigating the dynamic characteristics of the walking wheel of high-speed precision seeding machinery in paddy fields. The research specifically targets clay loam soil from a paddy field in South China. Calibration of essential soil parameters was achieved using EDEM_2022 software (and subsequent versions) discrete element simulation software, employing the Edinburgh Elasto-Plastic Adhesion (EEPA) nonlinear elastic-plastic contact model. The tillage layer and plough sub-base layer underwent calibration through slump and uniaxial compression tests, respectively. Influential contact parameters affecting slump and axial pressure were identified through a Plackett–Burman test. The optimal contact parameter combinations for the discrete element model of the tillage layer and plough sub-base layer were determined via a quadratic rotational orthogonal test. The accuracy of the discrete element simulation model’s parameters for paddy field soils was further validated through a comparative analysis of the simulation test’s cone penetration and the field soil trench test. Results indicate that the Coefficient of Restitution, surface energy, Contact Plasticity Ratio, and Tensile Exp significantly influence slump (p < 0.05). Additionally, the Coefficient of Restitution, Contact Plasticity Ratio, coefficient of rolling friction, and Tangential Stiff Multiplier significantly impact axial pressure (p < 0.05). Optimal contact parameters for the plough layer were achieved with a particle recovery coefficient of 0.49, a surface energy of 18.52 J/m2, a plastic deformation ratio of 0.45, and a tensile strength of 3.74. For the plough subsoil layer, optimal contact parameters were a particle recovery coefficient of 0.47, a coefficient of interparticle kinetic friction of 0.32, a plastic deformation ratio of 0.49, and a tangential stiffness factor of 0.31. Results from the cone penetration test reveal no significant disparity in compactness between the actual experiment and the simulation test. The calibrated discrete element model’s contact parameters have been verified as accurate and reliable. The findings of this study offer valuable data references for understanding the dynamic characteristics of the walking wheel of the entire machinery in high-speed precision seeding in paddy fields.

Self-healing capacity of GCLs under simulated field conditions

The self-healing capacities of a GCL with natural bentonite (NB) as core (NB-GCL) and a GCL with a polymerized bentonite (PB) as core (PB-GCL) were investigated under simulated field conditions, i.e. geomembrane-GCL-a clayey subsoil layer composite liner system with a damage hole on the geomembrane and the GCL. The clayey subsoils tested had initial water contents of 22.2% and 26.8%. The liquids used were deionized (DI) water and 0.3 M NaCl solution. The test results indicate that for both the PB-GCL and NB-GCL only hydrated on the subsoils for 1–3 months, a damage hole of 15 mm in diameter was almost not self-healed. For cases of applying a constant liquid head on the top of the geomembrane of 100 mm, the sizes of damage holes were self-healed in term of diameter is about half of the values reported in the literature for tests with plenty liquid supply to the damaged GCLs. Further, for the conditions tested and for the cases self-healed, DI water leaked into the subsoil was less than about 60 g, and 0.3 M NaCl solution leaked was less than 150 g.

Maintenance of Balanites aegyptiaca scattered trees for improving sesame (Sesamum oriental) grain yield and soil properties in lowlands of Tigray, Ethiopia

A species of dryland tree called Balanites aegyptiaca provides numerous environmental and socioeconomic benefits. Those trees are cultivated by agriculturalists in their fields in lowlands of Tigray, but there was insufficient scientific data on how Balanites aegyptiaca trees affected the study area's soil characteristics and sesame yields. The purpose of this study was to look into the behavior of Balanites aegyptiaca to improve sesame yields and soil properties in Kafta-Humera District. Two variables were utilized to think about soil physicochemical properties and collect 48 pooled tests: separate from the tree trunk at four levels and soil profundity at two levels, reproducing the RCBD calculate framework on six trees. To study sesame yield, four-level log spacings were used, replicated six times.In addition, ANOVA was used to determine sesame seed yield and soil physicochemical properties. The outcomes demonstrated that the sesame seed yield was significantly (using p<0.05) from the Balanites aegyptiaca tree. Under the canopy as opposed to outside of it, and in the topsoil as opposed to the subsoil layer, the BD was significantly (p<0.05) lower. Findings for additional soil chemical parameters, including pH, OC, total N, available P, K, Ca, and CEC, showed a significant (p<0.05) increase in the crown area relative to the field and surface area over the subsurface layer. Under its canopy, the multipurpose tree Balanites aegyptiaca generally improves soil fertility; when grown on agricultural land, it can also boost soil and plant productivity in drought-prone areas when managed properly.

The impact of the grinding angle of the soil dredging tool on its performance

This investigation explores the optimization of the backhoe blade's entry angle, addressing its crucial role in enhancing energy efficiency and operational effectiveness in subsoil manipulation - a fundamental aspect of agricultural tillage technologies. Utilizing an integrated approach that combines detailed field experiments with robust theoretical simulations, the study methodically quantifies the effects of angular variations on energy demands and mechanical performance during soil dredging. Our results reveal precise angular configurations that offer optimal reductions in energy use while significantly improving the disruption and aeration of the subsoil layer. This optimization contributes directly to the development of advanced, precision-engineered agricultural implements aimed at boosting sustainability and productivity in farming practices. Furthermore, the outcomes of this research provide pivotal insights into soil health management strategies, potentially influencing crop yield through improved root penetration and water absorption. These findings are expected to serve as a benchmark for future innovations in agricultural machinery design, aligning with global trends towards more sustainable agricultural practices and enhanced food security. This study sets a new standard for agricultural tool engineering, paving the way for transformative changes in the sector.

Simultaneous identification of the solely time‐dependent potential and source control terms from known moisture moments

Pseudo‐parabolic equations are commonly used as mathematical models in mechanics, biology, and physics to address various applied problems. One particular application involves describing moisture transfer dynamics in subsoil layers using pseudo‐parabolic equations. This manuscript examines the inverse problem (IP) of identifying the moisture transfer function, along with the time‐varying potential and source control terms, in a linear pseudo‐parabolic equation from known moisture moments. We prove the existence of a unique solution to the inverse problem for sufficiently small times by employing the contraction principle. The inverse problem is reformulated as a nonlinear least‐squares minimization problem, with the unknowns subjected to simple bounds. To guarantee stability, the Tikhonov regularization technique is utilized. For the numerical discretization, we develop the Crank–Nicolson finite‐difference method to solve the direct problem. To solve the nonlinear least‐squares minimization problem, we utilize the built‐in subroutine lsqnonlin from the MATLAB optimization toolbox. We present and thoroughly discuss numerical outcomes for a benchmark test example, providing insights into the performance and effectiveness of the proposed methodology.

Experimental study of natural convection and cooling capacity of closed crushed-rock layers affected by summer rainfall in cold regions

Climate-induced rises in air temperature and humidity on the Qinghai-Tibet Plateau modify the hydrothermal conditions of closed crushed-rock embankments (CRE). This alternation arises from the infiltration of summer rainfall, which is closely correlated with the cooling performance of the crushed-rock layers (CRL). Nevertheless, the cooling performance of the closed CRL during the summer rainfall season remains unexplored. In these present, physical models were employed to examine the convective properties and hydrothermal response processes. The results reveal that summer rainfall greatly diminishes the natural convection intensity within the closed CRL. Conversely, cycle-average temperatures showed an upward trend under these conditions. Moreover, based on fluctuations in the thawing/freezing index, the cooling capacity of the CRL weakens post-summer rainfall, with a maximum decline of 27.3% compared with the initial cycle. Additionally, the humidity in the subsoil layer experiences a stepwise growth followed by a gradual decrease owing to concentrated rainwater percolation. After enduring four rainfall events, the volumetric water content in the subsoil layer presents an upward trajectory. Overall, infiltrated water introduces additional heat, modifying the hydrothermal and convective properties of closed CRE, consequently impairing the cooling efficacy of the CRL.

Geochemical baselines of potentially toxic elements in Brunei-Muara soils, Brunei Darussalam: Towards environmental assessment and risk management

We present a comprehensive study on the presence of potentially toxic elements (PTEs) in the topsoil and subsoil of Brunei-Muara District in Brunei Darussalam. Descriptive statistics were employed to analyze the data, and the threshold values for PTEs in both soil layers were determined. Adopting the Tukey's inner fence method, we recommend baselines for PTEs. The study delved into the distribution patterns and potential sources of PTEs in Brunei-Muara, and we identified traffic and agricultural activities as significant contributors to the presence of As, Cr, Cu, and Pb. Industrial and agricultural areas exhibit spatial associations with Mn, Ni and Zn. Cadmium (Cd) was determined to be negligible in Brunei-Muara's soils. Furthermore, geochemical mapping unveiled highly acidic soil conditions in the region, characterized by variations in pH between the topsoil (2.13–8.00) and subsoil (1.89–6.76) layers that generally facilitate mobility of heavy metals. We employed Principal Component Analysis on the log10-transformed data to examine the behavior and mechanisms governing the mobility and retention of PTEs in the soil. This helped us to better characterize the element distribution within the baseline and the recognition of three additional processes enriching the topsoil and subsoil in PTEs. Overall, this study offers valuable insights into the distribution, sources, and behavior of PTEs in the soils of the Brunei-Muara District, which can greatly aid environmental monitoring and management endeavors.

Soil Salinity Mapping of Croplands in Arid Areas Based on the Soil–Land Inference Model

Soil salinization can decrease soil productivity and is a significant factor in causing land degradation. Precision mapping of salinization in agricultural fields would improve farmland management. This study focuses on the cropland in the Manas River Basin, located in the arid region of northwest China. It explores the potential of a soil mapping method, the Soil–Land Inference Model (SoLIM), which only requires a small number of soil samples to infer soil salinization of farmlands in arid areas. The model was utilized to create spatial distribution maps of soil salinity for the years 2009 and 2017, and changes in the distribution were analyzed. The research results indicate: (1) Through the analysis of sample point data, it was observed that soil salinity in the study area tends to accumulate in the surface layer (0–30 cm) in spring and in the subsoil layer (60–90 cm) during the crop growing season, with significant spatial variability. Therefore, it is necessary to conduct detailed salinity mapping. (2) Using field measurements as validation data, the simulation results of the SoLIM were compared with spatial interpolation methods and regression models. The SoLIM showed higher inference accuracy, with R2 values for the simulation results of the three soil layers all exceeding 0.5. (3) The SoLIM spatial inference showed salt accumulation in the northern part and desalination in the southern part. The findings of this study suggest that the SoLIM has the potential to effectively map soil salinization of croplands in arid areas, offering an efficient solution for monitoring soil salinity in arid oasis croplands.

Opening up the subsoil: Subsoiling and bio-subsoilers to remediate subsoil compaction in three fodder crop rotations on a sandy loam soil

The intensification of modern arable farming has brought on a steady increase of the risk of soil compaction, while simultaneously pushing the impact deeper into the soil profile. In the subsoil, outside the reach of regular tillage operation, it can become exceedingly difficult to deal with compacted layers. Commonly, farmers turn towards mechanical remediation strategies, like subsoiling, to deal with this issue. However, the numerous drawbacks that come with these operations have prompted the search for more sustainable alternatives, like bio-subsoilers, which are deep-rooting plant species with the capacity to penetrate compacted subsoil layers. This study aimed at investigating both mechanical and biological remediation strategies in a side-by-side comparison. The effectiveness of subsoiling and the use of two deep rooting (cover) crops (i.e. alfalfa and fodder radish) as remediation measures to alleviate subsoil compaction was assessed in terms of soil quality indicators, like penetration resistance, bulk density, porosity, plant available water capacity, air capacity and air permeability at 35 and 45 cm depth, and the above- and belowground growth of the main crop. In addition, a combination of both subsoiling and use of deep rooting (cover) crops was included to evaluate the potential role of the latter in protecting the mechanically loosened subsoil from recompaction. This complex comparison was done in three distinct maize-based fodder crop rotations (rotation 1: maize monoculture; rotation 2: maize/winter cereal rotation with fodder radish as cover crop after winter cereal; rotation 3: maize/winter cereal/alfalfa rotation). The study field (sandy loam soil) was selected based on the presence of a highly compacted layer at the interface between topsoil and subsoil (30–50 cm). The experiment demonstrated that subsoiling is an effective remediation measure to disrupt heavily compacted layers in the upper-subsoil, at least in the short-term. The crop response was however highly variable and only positive when the opportunity of reaching the subsoil proved to be advantageous for the crop (i.e. dry season). Besides its high degree of variability between years, the effects also proved to be very short-lived under a standard arable fodder crop rotation. Including deep rooting (cover) crops, like fodder radish and alfalfa, and limiting the initial subsoil disruption helped to partially stabilize the disrupted subsoil and reduce recompaction. Fodder radish and alfalfa penetrated the compacted subsoil without previous disruption by subsoiling. However, this did not lead to a significant improvement of the measured soil physical parameters.

Seismic microzonation for Lembang fault hazard area in West Bandung Regency based on microtremor measurement

Lembang district in Bandung Regency is prone to seismic risk because it is in the vicinity of a very well-known active fault called the Lembang Fault. The effort to reduce the seismic risk requires knowledge of the dynamic properties of the soils, mainly derived from young volcanic rocks. This paper presents the results of a seismic microzonation study based on microtremor measurements. In total 195 microtremor data from single-station measurements were processed using the HVSR method to estimate the dominant frequency and amplification factors. Furthermore, the Rayleigh wave ellipticity inversion has also been used to estimate the corresponding Vs profiles. The HVSR results show that the dominant frequency for the Lembang District is 0.2 Hz or higher, and the amplification factors are dominantly higher than 3. Some areas also have a lower dominant frequency than 2 Hz, especially the area with thick soft sediments. Results of the inversion show that the shear wave velocities of subsoil layers for the upper 30 m vary from 250 m/s to 300 m/s, indicating site class D. Meanwhile, the areas with Vs30 less than 250 m/s are especially located north of the Lembang Fault. Thus, the area is likely to have a stronger ground response to earthquake shaking than the other areas in Lembang District.

Soil Quality Dynamics under Isolated Stands of Irvingia gaboneensis (Bush Mango) and Within the Rainforest in Isoko Region, Nigeria

This study investigated soil quality dynamics under isolated stands of Irvingia gaboneensis within the rainforest of Isoko South region in Nigeria. Both experimental and quasi experimental designs were adopted; while the study area was stratified into 10 from which 2 sampling sites (I. gaboneensis and rainforest respectively) were selected, making 20 sampling sites examined. Samples of soil were derived from topsoil (0-15 cm) and subsoil (15-30 cm) layers of soil under I. gaboneesis and rainforest using core sampler. Each selected site of 30 m x 30 m was divided into quadrant of 10 m x 10 m from which samples were taken. Laboratory analyses of the samples were carried out following standard procedures. All data generated were statistically analysed using descriptive and paired t-test. Results revealed that soil attributes varied under I. gaboneensis and rainforest. With t-value = 0.114 and P (0.9110) > 0.05, mean differences in the topsoil are not significant. For the subsoil, t-value = 0.706 with P (0.4980) > 0.05 thus, mean differences in the subsoils are not significant. Also, with t-value = 1.0250 and P (0.3320) > 0.05, mean differences between the topsoil and subsoil under I. gaboneensis is not significant. For the rainforest, the t-value = 1.1520 and P (0.2790) > 0.05, mean differences between the topsoil and subsoil under adjoining rainforest trees is not significant. The observed insignificant difference in properties of soils under I. gaboneensis and rainforest shows the capacity of I. gaboneensis in soil management. Therefore, its conservation should be encouraged to manage the degraded rainforest ecosystem, while their incorporation into agro-forestry farming should be encouraged

Soil Organic Carbon Dynamics and Influencing Factors in the Zoige Alpine Wetland from the 1980s to 2020 Based on a Random Forest Model

Wetlands provide important ecosystem services, such as water conservation, biodiversity protection, and carbon sequestration. The Zoige alpine wetland is the largest high-altitude swamp in the world and plays a critical role in regional ecological balance and climate change. However, little is known about the fate of its soil organic carbon (SOC) storage. In this study, we estimated the degradation status of the wetland over the past 35 years and used machine learning to investigate the dynamics and driving factors of SOC at different soil depths of the Zoige wetland in 1985, 2000, and 2020. We also simulated the future SOC balance under different scenarios. The results showed that the area of Zoige wetland has degraded by 378.71 km2 in the past 35 years. Increased precipitation and solar radiation offset the adverse effects of global warming, making the soil act as a carbon sink in the past 35 years. The total SOC storage of the wetland soils in 1985, 2000, and 2020 was estimated to be 2.03 Pg, 2.05 Pg, and 2.21 Pg, respectively, with 46.95% of SOC distributed in the subsoil layers. Climate change was the most important driving factor controlling the SOC storage of the Zoige wetland, explaining 51.33% of the SOC changes in the soil. Temperature change was always the most important factor controlling wetland SOC, and precipitation had a greater impact on the topsoil. Under the temperature control targets of 1.5 °C and 2 °C, the SOC pool of the Zoige wetland will decrease by 60.21 Tg C and 69.19 Tg C, respectively. Under scenarios of a 10% and 20% increase in precipitation, the wetland soil will accumulate an additional 46.53 Tg C and 118.89 Tg C, respectively. The study results provide important references for the sustainable management of the Zoige wetland under the background of global climate change.

Fungal necromass is reduced by intensive drought in subsoil but not in topsoil.

The frequency and intensity of droughts worldwide are challenging the conservation of soil organic carbon (SOC) pool. Microbial necromass is a key component of SOC, but how it responds to drought at specific soil depths remains largely unknown. Here, we conducted a 3-year field experiment in a forest plantation to investigate the impacts of drought intensities under three treatments (ambient control [CK], moderate drought [30% throughfall removal], and intensive drought [50% throughfall removal]) on soil microbial necromass pools (i.e., bacterial necromass carbon, fungal necromass carbon, and total microbial necromass carbon). We showed that the effects of drought on microbial necromass depended on microbial groups, soil depth, and drought intensity. While moderate drought increased total (+9.1% ± 3.3%) and fungal (+13.5% ± 4.9%) necromass carbon in the topsoil layer (0-15 cm), intensive drought reduced total (-31.6% ± 3.7%) and fungal (-43.6% ± 4.0%) necromass in the subsoil layer (15-30 cm). In contrast, both drought treatments significantly increased the BNC in the topsoil and subsoil. Our results suggested that the effects of drought on the microbial necromass of the subsoil were more pronounced than those of the topsoil. This study highlights the complex responses of microbial necromass to drought events depending on microbial community structure, drought intensity and soil depth with global implications when forecasting carbon cycling under climate change.

Conversion of forest to cinnamon plantation depletes soil carbon stocks in the top metre of the tropical highlands of Kerinci Regency, Jambi Province, Indonesia

AbstractThis study aimed to investigate the effect of conversion from natural forest to cinnamon plantation on the top 1 m soil carbon stocks and soil characteristics. The project was conducted on Andosols of Kerinci Regency, Sumatera, Indonesia, sampling the soil profile under natural forests and a chronosequence of cinnamon plantations of different ages (1, 5 and 10 years). SOC stocks were quantified alongside physical properties (bulk density) and chemical properties (carbon, nitrogen, C/N ratio) to investigate the impact of land conversion. SOC stocks increased 1 year after conversion to cinnamon plantations, but then tended to decrease as the plantations got older. The initial increase was observed alongside decreasing bulk density 1 year after forest conversion to cinnamon plantation, likely as a result of the fresh input of (less dense) pyrogenic soil organic matter as a result of slash and burn practices and transport down the soil profile owing to leaching. In older plantations SOC stocks were lower, probably because organic matter had been decomposed or leached out of the profile. The free particulate organic matter (fPOM) was isolated from selected topsoil and subsoil layers and analysed for carbon, nitrogen, and FTIR analysis. FTIR analysis revealed that topsoil fPOM contained more aromatic functional groups than subsoils and had a higher degree of decomposition. Aromatic and carbohydrate functional groups were initially lower in recently converted cinnamon plantation, but the trend was reversed 10 years after conversion. The initial flush of fresh organic matter into soils after slash and burn provides fPOM with a lower degree of decomposition but is short‐lived and fPOM becomes more microbially processed as the cinnamon plantation ages. We conclude that, after a short term increase brought about by slash and burn, forest conversion to cinnamon plantation in Kerinci Regency depletes SOC stocks both in topsoils and subsoils.

Effects of the age of raised beds on the physicochemical characteristics of fruit orchard soil in the Vietnamese Mekong Delta.

To grow fruit plants, farmers in the Vietnamese Mekong Delta (VMD) must use raised bed constructions to avoid waterlogging during the rainy season. This study aimed to evaluate the effects of the age of the raised beds on the soil physicochemical properties of longan orchards located in the VMD. Two raised bed systems were evaluated: a young bed constructed 10 years ago and an old bed constructed 42 years ago. Soil samples were collected from five different soil layers (0-20, 20-40, 40-60, 60-80, and 80-100 cm) with four replicates per layer. Soil samples were tested for pH, electrical conductivity (EC), available phosphorus (AP), total nitrogen (TN), soil organic matter (SOM), exchangeable cations (Ca2+, K+, Mg2+, and Na+), cation exchange capacity (CEC), bulk density (BD), soil porosity, available water-holding capacity (AWC), particle composition (sand, silt, and clay), and size. The soil pH was approximately 1.0 units lower in the old bed compared to the young bed at depths of 0-20 and 20-40 cm. The BD was higher in the old bed (0.15 g cm-3) than in the young bed at a soil depth of 0.4 m. SOM, AP, exchangeable cations (Ca2+, Na+, and Mg2+), AWC, and soil porosity were significantly lower in both the topsoil (0-20 cm) and subsoil (20-40 cm) layers in the old bed than in the young bed. In particular, the SOM, AP, AWC, and soil porosity contents in the old bed decreased by 18%, 20%, 15%, and 17%, respectively, compared with those in the young bed at soil depths of 0-40 cm. Therefore, cultivating raised bed soil for a longer period significantly reduced the soil exchangeable cations, porosity, and fertility of the surface and subsurface soils. Based on these results, farmers should use soil conservation practices, such as cover crops, rice straw mulching, and soil amendments in their orchards to mitigate topsoil degradation.