Soil Matrix Research Articles

Influence of Arsenate Competition on Tungstate Sorption by Soil

The green and digital transitions toward sustainable development will drive an increased demand for critical raw materials, among which tungsten plays a crucial role in emerging sustainable technologies. Understanding the sorption processes of tungsten in soils is essential for assessing its bioavailability and potential toxicity to living organisms. In many soils, tungsten may co-exist with other contaminants, such as arsenic. Investigating the competitive sorption between these two anions helps clarify how they interact within the soil matrix. Batch experiments were conducted on three Mediterranean soils to evaluate the sorption behavior of tungstate and arsenate, both individually and in combination, using a “Langmuir-type” model. Both anions exhibited the highest sorption in acidic soils and the lowest in alkaline soils. While the shapes of the isotherms were similar in both single and binary systems, the maximum sorption values decreased when a co-occurring anion was present. These reductions can be attributed to competition for soil sorption sites, which have a high affinity for both anions. In all tested soils, the percentage decrease in arsenate sorption in the presence of tungstate was greater than the decrease observed for tungstate in the presence of arsenate. Gaining a deeper understanding of tungsten’s sorption mechanisms is critical, not only for advancing environmental research but also for informing regulations that currently give limited attention to the presence of tungsten in soils.

Arsenic, cadmium, and chromium concentrations in contrasting phosphate fertilizers and their bioaccumulation by crops: Towards a green label?

Potentially toxic elements such as arsenic (As), cadmium (Cd), and chromium (Cr) are severely regulated in fertilizers and deserve continuous investigation. Phosphate-derived Cd has been a stepping-stone toward achieving sustainable and safe worldwide food production, especially after a new regulation aiming for reduced limits of Cd in P fertilizers (EU, 2019/1009). Three pot experiments were conducted to assess the variability of As, Cd, and Cr concentrations - with a particular focus on Cd - from monoammonium phosphates (MAP 1, MAP 2, and MAP 3 from different geographic origins) and their accumulation in limed and unlimed soils, and contrasting crops, representing staple food and significant sources of these elements for humans (i.e., potato, tobacco, and rice). A diverse array of sensitive techniques for trace elements determination were used to reveal the highest level of Cd of MAP 3 (20.71 mg kg−1 MAP), which loaded the highest amounts of this element to the soil matrix and solution, plant shoots, and xylem sap, contrasting with results for MAP 1 (0.87 mg kg−1 MAP), which has almost ten times less Cd than that required for low-Cd labeling of P fertilizers (≤20 mg Cd kg−1 P2O5). MAP 3 also had the highest Cr concentration (139.3 mg kg−1 MAP). Among crops, rice accumulated 16-fold less Cd than potato plants. Liming decreased Cd in tobacco and potato shoots up to 35%. Moreover, reductions of about 20% were also observed for Cd accumulation in tubers and sap. Conversely, Cd from MAP 3 was always much more accumulated in soil solution, achieving up to 20 μg L−1, while values < 5 μg L−1 (i.e., a groundwater limit) were obtained from MAP 1. Our findings may be used as a reference in developing green labels for fertilizers in scenarios where Cd accumulation represents a potential risk for soil and human health.

Recovery and Restructuring of Fine and Coarse Soil Fractions as Earthen Construction Materials

Excessive consumption of natural resources to meet the growing demands of building and infrastructure projects has put enormous stress on these resources. On the other hand, a significant quantity of soil is excavated for development activities across the globe and is usually treated as waste material. This study explores the potential of excavated soils in the Brittany region of France for its reuse as earthen construction materials. Characterization of soil recovered from building sites was carried out to classify the soils and observe their suitability for earthen construction materials. These characteristics include mainly Atterberg limits, granulometry, organic matter and optimum moisture content. Soil samples were separated into fine and coarse particles through wet sieving. The percentage of fines (particles smaller than 0.063 mm) in studied soil samples range from 28% to 65%. The methylene blue value (MBV) for Lorient, Bruz and Polama soils is 1, 1.2 and 1.2 g/100 g, and French classification (Guide de terrassements des remblais et des couches de forme; GTR) of soil samples is A1, B5 and A1, respectively. The washing of soils with lower fine content helps to recover excellent-quality sand and gravel, which are a useful and precious resource. However, residual fine particles are a waste material. In this study, three soil formulations were used for manufacturing earth blocks. These formulations include raw soil, fines and restructured soil. In restructured soil, a fine fraction of soil smaller than 0.063 mm was mixed with 15% recycled sand. Restructuring of soil fine particles helps to improve soil matrix composition and suitability for earth bricks. Compressed-earth blocks of 4 × 4 × 16 cm were manufactured at a laboratory scale for flexural strength testing by using optimum molding moisture content and compaction through Proctor normal energy. Compressive strength tests were performed on cubic blocks of size 4 × 4 × 4 cm. Mechanical testing of bricks showed that bricks with raw soil had higher resistance with a maximum of 3.4 MPa for Lorient soil. Removal of coarse particles from soil decreased the strength of bricks considerably. Restructuring of fines with recycled sand improves their granular skeleton and increases the compressive strength and durability of bricks.

Undrained cyclic responses of sandy soils improved by soybean-induced calcium carbonate precipitation

ABSTRACT This study investigates the behaviour of sandy soils under cyclic conditions, emphasising the efficacy of soybean-induced calcium carbonate precipitation (SICP) treatment in mitigating liquefaction. Analysis of shear wave velocity and undrained cyclic responses was conducted, with a focus on understanding deformation response and development of excess pore pressure. Results revealed CaCO3 content as a definitive indicator for improvements in soil stiffness, with its distribution influenced by soil’s relative density and number of SICP injections. Enhanced cementation efficacy was observed in denser or deeper layers of sand column, attributed to prolonged SICP retention. A marked correlation between shear wave velocity and CaCO3 content highlighted SICP’s role in enhancing the liquefaction resistance of loose sandy soils. Dynamic triaxial testing showed the nuanced strain behaviours across varying relative densities and cyclic stress ratio (CSR) values, emphasising the strengthening influence of CaCO3 formations in the soil matrix. The empirical equation served as a testament to the potential of SICP in soil stabilisation endeavours. The intervention of SICP treatment presents a potent tool for bolstering soil stability, especially in regions susceptible to seismic activities. The insights garnered advocate for further research aimed at harnessing and optimising this soil improvement technique on a global scale.

The divergent role of straw return in soil O2 dynamics elucidates its confounding effect on soil N2O emission

The divergent effects of straw returns on nitrous oxide (N2O) emissions from soil require elucidation of the underlying mechanisms and factors that explain the inconsistency in in-situ conditions. We conducted a field experiment based on a long-term trial under different regimes of nitrogen (N) fertilization and straw management, complemented by laboratory incubation experiments involving visualized O2 dynamics imaging. In the field trial, we performed hourly basis high-time-resolution measurements of soil matrix oxygen (O2), N2O concentrations and fluxes during N2O “hot moment” events. We found that straw return increased cumulative N2O emissions by 32.7% under conventional high N input (Ncon), but showed no effect on N2O emission under optimized N input (Nopt). In situ O2 content and further microcosm experiments with visualized O2 spatiotemporal distribution suggested that long-term straw return increases porosity and soil O2 content, which reduced N2O emission under low N substrate conditions by improving soil pore structure and aeration during “hot moment” events. By contrast, straw return increased N2O emission via creating short-term O2 depletion zone and triggering denitrification in anoxic microsites when excess N substrate was available. Although straw return showed inconsistent effects on N2O emission under different N application rates, it consistently decreased N2O concentration in the soil matrix during the "hot moment" events, suggesting that straw return increases the transport of the produced N2O in soil matrix to the soil surface. Our study underscores the multifaceted role of straw return in soil O2 dynamics, i.e., stimulating O2 consumption in a short-term microscale of soil, but increasing soil porosity in a long-term mesoscale of soil. This explains the confounding effects of straw management on the production and transportation of soil N2O in situ and emphasizes the importance of optimized N fertilization for reducing the “hot moment” N2O emissions when straw is incorporated.

Attraction of pitfall trap preservation fluids complicates the estimation of Collembola density

Collembola (springtail) communities consist of three eco-morphologically defined life forms: the euedaphics dwell inside the soil matrix, the epedaphics (including atmobiotics) live on the ground and in vegetation, and the hemiedaphics are intermediate. The vast majority of springtail community studies focus on the belowground (eu- and hemiedaphic) forms that are generally collected by taking and extracting soil cores. Few investigations have dealt with epedaphic Collembola that are usually captured with pitfall traps, and only very few studies so far covered all three life forms. When epedaphic and belowground species are sampled using both methods simultaneously, core data (true densities, [individuals m-2]) and pitfall data (activity abundances, [individuals trap-1 length of trapping period-1]) may be analyzed independently, but are incompatible in a common statistical framework. As a remedy, two competing numerical approaches to estimate true densities from activity abundances have been described in literature: the nested-cross array and the two-circle method. Attraction or deterrence effects of trap preservation fluids bias the density estimation of the nested-cross array, but not of the two-circle method. To determine whether this bias may be expected for Collembola, and thus which of the two methods should be used in future studies, we experimentally tested potential effects of preservation fluids on trap catch rates. Three preservation fluids (sodium benzoate, propylene glycol, formaldehyde) and a detergent (Tween80) significantly increased the number of captured springtails, thus demonstrating an attraction effect and the deficiency of the nested-cross array. In future studies of collembolan communities, we therefore suggest complementing the traditional focus on the eu- and hemiedaphic life forms by sampling epedaphic species using pitfalls, and subsequently remodelling the trapping data with the two-circle method.

In vitro cluster buds regeneration and control of shoot tip necrosis in tissue cultures of Trichosanthes cucumerina L.

Trichosanthes cucumerina L. is a medicinal vine with great potential that can be used as a vegetable. In this study, an efficient in vitro regeneration system of T. cucumerina was established by inducing adventitious buds and controlling Shoot Tip Necrosis (STN) in the in vitro culture. The results of hormone single factor and orthogonal experiments showed that 6-BA had a significant effect on inducing adventitious buds. The best medium for bud proliferation was MS + 6-BA 3.0 mg·L− 1 + KIN 2.0 mg·L− 1 + NAA 0.5 mg·L− 1, and the proliferation capacity was 14.68. However, after three consecutive subcultures, a higher incidence of STN (78.86%) was observed. The results of the STN control experiment showed that reducing the concentration of inorganic salt, shortening the subculture interval, and increasing the Ca2+ in the medium could effectively control the occurrence of STN. However, reducing the concentration of inorganic salt and shortening the subculture interval excessively reduces the proliferation capacity significantly. Considering both the incidence of STN and proliferation capacity, the measures proposed in this study to alleviate STN were to keep the concentration of inorganic salt unchanged, adjust the subculture interval from 45 days to 40 days, and increase the Ca2+ concentration in the medium to 480 mg·L− 1. When the subculture interval was 40 d, the stem segments obtained the lowest STN incidence (8.3%) and higher shoot proliferation capacity (10.02) on the medium of MS + 3.0 mg·L− 1 6-BA + 2.0 mg·L− 1 KIN + 0.5 mg·L− 1 NAA + 360.36 mg·L− 1 Ca2+ (360 mg·L− 1 is the concentration of additional calcium to be added, and after this addition, the total calcium concentration in the MS medium was 480.48 mg·L− 1). The best rooting medium for stem segments was MS + 0.50 mg·L− 1 NAA; the rooting rate was 100%, and no STN occurred. After acclimation, the survival rate of the rooted plantlet reached 95% after 40 days in the soil matrix of coconut husk: humus soil: perlite = 2: 2: 1 (v/v). The results of this study not only reveal the mechanism of STN in T. cucumerina, but also provide technical support and a theoretical basis for its further breeding and cultivation.

Chemical and mineralogical constitution of redoximorphic features and mechanism of formation of Plinthosols from the Araguaia River plain, Brazil

ABSTRACT Currently in Brazil, large grain cultivation projects on Plinthosols are a reality, however, there is little or no knowledge of the real mechanism of formation of the plinthite feature, in addition to what is reported in the literature as being a product of oxidation-reduction processes of iron element. This study evaluates iron redoximorphic features and investigates their chemical and mineralogical composition in two profiles of Plinthosols from the Araguaia River plain (P1 and P2). The study strengthens the understanding of the pedogenetic processes involved in the formation of mottles and plinthite. In this sense, it assesses whether the formation mechanisms corroborate the literature. Soil features were sampled in the upper right and left position at the initial plinthic horizon, upper right and left position at the main plinthic horizon, and lower right position at the base horizon of the plinthite zone in the profile. Separated samples comprising the soil matrix, mottles, and plinthite under natural moisture conditions were ground into powder form for chemical determinations by X-ray fluorescence (XRF), sulfuric acid attack (H 2 SO 4 ), sodium dithionite-citrate-bicarbonate (DCB), and ammonium acid oxalate; and mineralogical determinations by X-ray diffraction. Iron contents in all determined forms were always higher in the plinthite feature, intermediate in the mottle feature, and lower in the soil matrix feature. Most of the Fe in all redoximorphic features is included in the structure of primary minerals and their derivatives (vermiculite, illite, and VHEs). Only part of the iron present (about 35.40 % in P1 and 41.98 % in P2) is detected in the form of oxides such as goethite and hematite, which could be formed in redox processes. The mottle and plinthite features under study are not the product of the classic process of segregation, mobilization, and accumulation of iron as a consequence of redox processes. These features were formed or emerged as a result of a relatively slow and constant weathering process of their source material, which is gradually decomposed in an aqueous medium, releasing most of its components. These components include iron and more mobile elements such as bases and silicon, which leave the system through drainage water, and of which a small part may eventually recombine to form new less complex minerals such as kaolinite and oxides.

Biosurfactant-assisted bio-electrokinetic enhanced remediation of heavy metal-contaminated soil.

Environmental soil contamination is a serious problem for humans worldwide, as it causes many diseases. The present study focuses on utilizing biosurfactants produced by Pseudomonas stutzeri (P. stutzeri) NA3 and Bacillus cereus (B. cereus) EN6, as an electrolyte for removing chromium (Cr) from contaminated soil using the electrokinetic (EK) process. As a result, biosurfactants produced by P. stutzeri NA3 and B. cereus EN6, being lipopeptides, increase heavy metal mobility in the EK process. The Cr removal efficiency of a novel electrolyte (biosurfactants) in the EK process was compared with that of NA3 and EN6 biosurfactants. The EK results revealed a maximum Cr removal of 75 and 70% by NA3 and EN6, respectively, at the end of 7 days. The biosurfactant aids in the breaking down of the heavy metals that are present deeper into the soil matrix. From the metagenomics analysis, it was identified that biosurfactant changes the microbial community with an enhanced ability to remove heavy metals. The phytotoxicity assay confirms that NA3 biosurfactant solution showed 95% seed germination and can lower hazardous pollutants in the soil. The application of biosurfactants as a potent electrolyte for the remediation of hazardous pollutants is an integrated process. Overall, the results of this study suggest that biosurfactants can serve as an economic and efficient electrolyte in the EK process to remove Cr from polluted soil.

Exploring the Effects of Fissures on Hydraulic Parameters in Subsurface Flows from the Perspective of Energy Changes

Reynolds number (Re), pore water pressure (P), and water flow shear force (τ) are primary indicators reflecting the characteristics of subsurface flow. Exploring the calculation of these parameters will facilitate the understanding of the hydrodynamic characteristics in different subsurface flows and quantify their differences. Hence, we conducted a study to monitor soil water content, matrix potential, and pore water pressure in two typical soil profiles (with and without fissures). The distribution of Re, P, and τ in both matrix flow (MF) and preferential flow (PF) were calculated with an improved calculation method, focusing on their energy changes. Results showed that these hydrologic parameters are quite different between MF and PF. Re values in MF remained below 0.1, indicating lower water flow velocities, while the Re values ranged from 0.8 to 2 in PF, indicating higher flow velocities. The P values in PF was tens to hundreds of times higher than that in MF, which is mainly due to the rapid accumulation and leakage of water within soil fissures. Additionally, the larger hydraulic radius and gradient in PF also resulted in higher τ values in PF (2~6 N m−2) than in MF (0~1.5 N m−2). In PF, the pressure potential was the significant factor for τ, while τ in MF was dominated by the matrix potential and varies with the magnitude of the matrix potential gradient. This study suggests that Re, P, and τ could be considered as the major indexes to reflect dynamic characteristics of subsurface flow.

Characteristics of soil pore structure response to electric field strength and their effects on Cr(VI) removal from a historically chromium-contaminated soil

The characteristics of soil pore structure response to electric field strength and their effects on the migration of hexavalent chromium during electrokinetic remediation remain unclear. This paper investigates the effects of soil actual voltage and pore redistribution on hexavalent chromium removal under various voltage gradients during electrokinetic remediation. The electric field across the soil was the dominant factor that affected the soil properties, pore structure, and transportation of hexavalent chromium. The removal efficiency of dissolved hexavalent chromium was significantly enhanced from 36.1 % to 80.5 % as the theoretical voltage gradients increased from 0.5 V·cm−1 to 3 V·cm−1 due to the highest voltage proportion (52.7 %) of the soil chamber at 3 V·cm−1, while it only increased to 82.0 % at 4 V·cm−1. The optimal actual electric field strength of 1.00–1.50 V·cm−1 across the soil matrix facilitated the efficient electromigration of hexavalent chromium. The electric field force exerted a direct (0.70) or indirect action intensity (1.55) on soil pore structure by altering soil chemical properties. Soil mesopores and ultra-micropores transformed into micropores through collapsing, shrinking, ion migration, weakening electrostatic repulsion, and enhancing adhesive bonding among particles with increasing electric field. The distribution and structure of soil pores were more homogenous under optimal electric field conditions, thereby improving the migration path of hexavalent chromium. Soil mesopores facilitated hexavalent chromium removal and the formed micropores were opposite. These findings have significant scientific and practical implications for applying the in-situ electrokinetic techniques in the remediation of metal-contaminated soils.

Thermal desorption technique to speciate mercury in carbonate, silicate, and organic-rich soils

Thermal desorption is a well-assessed technique to speciate mercury (Hg) in soils and sediments. However, the effects related to the different matrices are still not properly assessed. In this study, thermal desorption was applied to Hg-free calcite mixed with Hg standard and soils rich in carbonate and silicate minerals, as well as organic matter. Hg0, HgCl2, HgO, α-HgS, β-HgS and organo-mercuric compounds were recognized, pointing out that the soil matrix operates notable differences in terms of breakdown temperatures of the Hg-compounds and suggesting that the mineralogical composition of soil has to be investigated before applying the thermal desorption technique. Furthermore, the presence of Hg0 was carefully evaluated since, as already observed, it forms Hg2+, which increases mercury mobility in the pedological cover with important consequences for those soils contaminated and located close to decommissioned or active mining areas and/or industrial sites (e.g. chloro-alkali industries). Experimental runs were thus carried out by using carbonate-, silicate- and organic-rich soils doped with liquid Hg. It was observed that Hg0 tends to be oxidized to form Hg+ and then Hg2+ as a function of soil matrix and reaction time. Surprisingly, the oxidation rate is rather fast, since after 42 days the initial content of Hg0 is halved, thus following an exponential decay. This implies that in Hg0-polluted areas, the fate of the resulting Hg2+ can be that to: i) be adsorbed by organic matter and/or Fe–Mn–Al oxides and/or ii) feed shallow aquifers. This study is a further step ahead to understand the behavior of Hg in contaminated soils from industrial and mining areas where liquid Hg is occurring in different soil matrices and may provide useful indications for remediation operations.

Interaction of Te(IV) and Te(VI) with the soil matrix – Sorption and fractionation as a function of soil composition

Tellurium is a technology-critical element (TCE), with relatively limited data on its behavior in the environment, especially the pedosphere. As with other TCEs, its more widespread use, especially in new energy sources, might lead to Te spillage during production or in the eventual waste. Investigation of tellurium's interaction with soil is a necessary step in the research into the physiochemical transformation and determining the mobility of different tellurium species. To broaden the hitherto scarce knowledge of tellurium behavior in the soil environment, selected soluble tellurium compounds were introduced into different types of well-characterized soil (content of fertilizers, organic matrix, clay minerals, Mn, Fe, pH). The study of Te(IV) and Te(VI) sorption indicated that after 7 days the sorption is quantitative and close to 100%. Addition of Fe2O3 to a soil deficient in Mn and Fe increases its sorption potential by about 10 percentage points. Based on fractionation study (0.11 mol L−1 CH3COOH, 0.1 mol L−1 ascorbic acid in oxalate buffer (pH 3), 30% H2O2 at 85 °C followed by 0.5 mol L−1 CH3COONH4), it was shown that the presence of Mn/Fe (oxyhydr)oxides plays an essential role in the mobility of Te, especially Te(VI), regardless of the soil type. In the soil poor in reducible fraction and rich in organic matrix (peat), the organic fraction was responsible for the immobilization of Te, especially Te(IV). Extraction of the mobile fraction after incubation in the presence of DI water (Te extraction: 7–8%), oxalic acid (5–7%) or citric acid (6%) (mimicking rhizosphere activity) indicated that these did not play a significant role in Te retention. Nevertheless, soil modification with biocarbon limited the effect of citrates on Te mobilization. This knowledge is fundamental, i.e. in the context of soil remediation processes and counteracting the migration of Te in the environment from anthropogenic sources (e.g. solar farms).

Artificial intelligence-driven enhanced CBR modeling of sandy soils considering broad grain size variability

The soil packing, influenced by variations in grain size and the gradation pattern within the soil matrix, plays a crucial role in constituting the mechanical properties of sandy soils. However, previous modeling approaches have overlooked incorporating the full range of representative parameters to accurately predict the soaked California bearing ratio (CBRs) of sandy soils by precisely articulating soil packing in the modeling framework. This study presents an innovative artificial intelligence (AI)-based approach for modeling the CBRs of sandy soils, considering grain size variability meticulously. By synthesizing extensive data from multiple sources, i.e. extensive tailored testing program undertaking multiple tests and extant literature, various modeling techniques including genetic expression programming (GEP), multi-expression programming (MEP), support vector machine (SVM), and multi-linear regression (MLR) are utilized to develop models. The research explores two modeling strategies, namely simplified and composite, with the former incorporating only sieve analysis test parameters, while the latter includes compaction test parameters alongside sieve analysis data. The models' performance is assessed using statistical key performance indicators (KPIs). Results indicate that genetic AI-based algorithms, particularly GEP, outperform SVM and conventional regression techniques, effectively capturing complex relationships between input parameters and CBRs. Additionally, the study reveals insights into model performance concerning the number of input parameters, with GEP consistently outperforming other models. External validation and Taylor diagram analysis demonstrate the GEP models' superiority over existing literature models on an independent dataset from the literature. Parametric and sensitivity analyses highlight the intricate relationships between grain sizes and CBRs, further emphasizing GEP's efficacy in modeling such complexities. This study contributes to enhancing CBRs modeling accuracy for sandy soils, crucial for pertinent infrastructure design and construction rapidly and cost-effectively.

Cement slurry penetration behavior of swirl grouting technology

Traditional pressure grouting technology operates under steady pressure conditions, causing the grout to easily flow along preferential pathways. This results in uneven grout penetration and increased economic costs. This study proposes swirl grouting technology, which effectively improves this problem. To verify the effectiveness of swirl grouting, a fan-shaped blade tool was also proposed. The grout penetration performance was investigated through experimental studies. The length, width, height, weight, and uniformity of the grouted bodies produced by the swirl grouting method were compared with those produced by the steady pressure grouting method. Then, the mechanisms of swirl grouting were analyzed through transparent disc visualization experiments. The results demonstrated that, at different water–cement ratios, the swirl device increased the penetration length in the X, Y, and Z directions by 43.3%, 27.8%, and 45.8%, respectively, compared to the conventional straight device, and by 57.3%, 39.4%, and 55.6%, respectively, compared to the fan blade device. Moreover, the swirl device increased the weight of the grouted stone body by 54.9% compared to the conventional straight device and by 91.0% compared to the fan blade device, significantly enhancing filling efficiency. The uniformity coefficient of the swirl device permeation decreased by 56.6% and 51.0%, respectively, compared to the conventional straight device and the fan blade device, resulting in a more uniform grout distribution. The transparent disc visualization experiment further revealed the advantage of the swirl device in promoting the migration of fine particles, with a significant increase in average penetration distance and a penetration shape closer to a regular circle. The rotating flow path of the swirl device imparts additional rotational momentum and multidirectional penetration capabilities. The resulting turbulence accelerates the mixing of grout with the soil matrix, facilitating the migration of fine particles, expanding flow channels, and reducing flow resistance. This combination of effects enhances penetration efficiency and reduces energy loss. This study offers significant practical application value for improving engineering quality, construction efficiency, and reducing costs.

Effect of salt concentration on osmotic potential in drying soils—Measurement and models

AbstractThe water potential in drying soils, comprising both matric potential and osmotic potential components, can be measured using the dew point method (DPM). By combining DPM data with retention curve data acquired from techniques such as the suction plate method or the simplified evaporation method (SEM), it becomes possible to determine the soil water retention curve across the entire moisture spectrum. However, as the latter methods only determine the matric potential, the osmotic potential component in DPM data must either be negligible or known so that osmotic and matric potential components can be separated. This study aims to critically analyse common approaches for calculating the osmotic potential. To achieve this, we measured the water retention properties of a silt loam, a sandy loam and a sand across the entire moisture range by combining SEM and DPM. By using almost salt‐free soil material, we characterized reference water retention curves with negligible osmotic potential components. The impact of salt on water potential was analysed by conditioning soils with MgCl2 solutions of different concentrations, drying them, and measuring the water potential at different water contents using the DPM. The resulting water potentials were compared to the reference potentials and differences were interpreted as the osmotic potential component. The DPM‐measured water potentials in drying soils can be significantly affected by osmotic potential, especially at higher matric potentials (low suctions). Two models accounting for ideal and one model accounting for non‐ideal electrolyte behaviour were used to compare osmotic potential predictions with measurements. At low to medium salt concentrations, all models performed fairly well. At high concentrations, only the model accounting for non‐ideal behaviour predicted the osmotic potential satisfactorily, whereas at very high concentrations, all models underestimated the impact of osmotic potential on water potential. This suggests that the surface properties of the soil matrix, such as the specific surface area and surface charges, may lead to a decrease in osmotic potential beyond what is expected in pure solutions.

Links between soil pore structure, water flow and solute transport in the topsoil of an arable field: Does soil organic carbon matter?

An improved understanding of preferential solute transport in soil macropores would enable more reliable predictions of the fate of agrochemicals and the protection of water quality in agricultural landscapes. The objective of this study was to investigate how soil organic carbon (SOC) and soil texture shape soil pore structure and thereby determine the susceptibility to preferential transport under steady-state near-saturated flow conditions. To do so, we took intact topsoil samples from an arable field that has large variations in SOC content (1.1–2.7%) and clay content (8–42%). Soil pore structure was quantified by X-ray tomography and soil water retention measurements. Non-reactive solute transport experiments under steady-state near-saturated conditions were carried out at irrigation rates of 2 and 5 mm h−1 to quantify the degree of preferential transport. Near-saturated hydraulic conductivities at pressure heads of −1.3 and −6 cm were also measured using a tension disc infiltrometer. The results showed that larger abundances of small macropores (240–720 µm diameter) and mesopores (5–100 µm diameter) resulted in weaker preferential transport, due to larger hydraulic conductivities in the soil matrix that prevented the activation of water flow and solute transport in large macropores. In particular, the degree of preferential transport was most strongly and negatively correlated with the mesoporosity in the 30–100 µm diameter class. In contrast, the degree of preferential transport was not correlated with connectivity measures (e.g. the percolating fraction and critical pore diameter for the macropore network), probably because i.) the pore space of almost all samples was highly connected, being dominated by one percolating cluster, and ii.) only a part of this percolating macroporosity was active under the near-saturated conditions of the experiment. We also found that the degree of preferential transport was strongly and negatively correlated with clay content, whilst the effects of SOC were not significant. Nevertheless, macroporosity in the 240–720 µm diameter class and mesoporosity were positively correlated with SOC content in our soils and in some previous studies. Therefore, SOC sequestration in arable soils may potentially reduce the risk of preferential transport under near-saturated flow conditions through better developed networks of small macropores and mesopores.

The distribution of particulate organic matter in the heterogeneous soil matrix - Balancing between aerobic respiration and denitrification

Denitrification, a key process in soil nitrogen cycling, occurs predominantly within microbial hotspots, such as those around particulate organic matter (POM), where denitrifiers use nitrate as an alternative electron acceptor. For accurate prediction of dinitrogen (N2) and nitrous oxide (N2O) emissions from denitrification, a precise quantification of these microscale hotspots is required. The distribution of POM is of crucial importance in this context, as the local oxygen (O2) balance is governed not only by its high O2 demand but also by the local O2 availability.Employing a unique combination of X-ray CT imaging, microscale O2 measurements, and 15N labeling, we were able to quantify hotspots of aerobic respiration and denitrification. We analyzed greenhouse gas (GHG) fluxes, soil oxygen supply, and the distribution of POM in intact soil samples from grassland and cropland under different moisture conditions. Our findings reveal that both proximal and distal POM, identified through X-ray CT imaging, contribute to GHG emissions. The distal POM, i.e. POM at distant locations to air-filled pores, emerged as a primary driver of denitrification within structured soils of both land uses. Thus, the higher denitrification rates in the grassland could be attributed to the higher content of distal POM. Conversely, despite possessing compacted areas that could favor denitrification, the cropland had only small amounts of distal POM to stimulate denitrification in it. This underlines the complex interaction between soil structural heterogeneity, organic carbon supply, and microbial hotspot formation and thus contributes to a better understanding of soil-related GHG emissions.In summary, our study provides a holistic understanding of soil-borne greenhouse gas emissions and emphasizes the need to refine predictive models for soil denitrification and N2O emissions by incorporating the microscale distribution of POM.

Risk quick sketch: Soil captured most anatoxin-a and microcystin-RR rather than cylindrospermopsin and microcystin-LA/-LY

Cyanobacteria proliferate in warm, nutrient-rich environments and release toxic secondary metabolites into natural waters. Using cyanotoxin-contaminated water to irrigate crops could expose humans and biota, but the risk may be low if agricultural soils can sorb and retain cyanotoxins. In this report, we compared the sorption and desorption capacities of multi-class cyanotoxins/anabaenopeptins in soils of variable properties with a batch sorption procedure. The target compounds were anabaenopeptin-A, anabaenopeptin-B, anatoxin-a, cylindrospermopsin, and microcystins -LR, -RR, -LA, -LY, -LW, and -LF. Based on solid-liquid distribution coefficients (Kd), we classified cylindrospermopsin and microcystin-LA/-LY as “very low sorptivity”, anabaenopeptin-A, -B and microcystin-LR, -LF, and -LW as “low sorptivity”, and anatoxin-a and microcystin-RR as “medium sorptivity”. We remain concerned about irrigating agricultural land with water contaminated with high levels of CYN and MC-LA/-LY because of their relatively low affinity and high desorption proportion in soils. The results also suggest that soil sorption can be an effective immobilization pathway for anatoxin-a and microcystin-RR. The generated data will be useful for prioritizing research on the most bioavailable cyanotoxins/anabaenopeptins that are likely to be released by the soil matrix, for environmental risk assessment.

Flying microbes-survival in the extreme conditions of the stratosphere during a stratospheric balloon flight experiment.

Earth's stratosphere is a hostile environment that has challenged microbial survival. We set out to test the effect of stratosphere exposure on survival of single species (Bacillus subtilis) and complex microbial communities from soils and sediment. B. subtilis survival was strongly impacted by sun exposure, i.e., ultraviolet (UV) radiation, with only 1% survival at full sun exposure. Complex microbial communities had high survival rates, and the soil or sediment matrix may have provided protection against radiation and desiccation, supporting the survival of environmental microorganisms.