Temporal and spatial distribution of soil water repellency in grassland soils and its relation to soil moisture, hydrophobic matter, and particle size

In grassland soils, soil water repellency (SWR) may be one of the triggers of soil erosion and degradation as it can reduce water infiltration and penetration into the soil. Few studies were focusing on the evaluation of soil hydro-physical properties, such as hydrophobicity, and their relation to soil moisture, hydrophobic matter, and particle size in grassland soils. In this study, 800 soil samples were collected from the Xilingol grassland in Inner Mongolia, China, using the water droplet penetration time (WDPT) test to evaluate water repellency and we aimed to investigate the temporal and spatial distribution of SWR in grassland soils using the Kriging and Inverse Distance Weighting (IDW) interpolation methods and determine the physical-chemical properties that trigger the SWR. The results showed that the grassland soils in the studied area were slightly water-repellent and a few portions of the area exhibited strong water-repellency. In April, areas of soils with a depth of 0-5 cm and slight to strong SWR accounted for 80 % of the total studied area, of which 5 % had strong water repellency. Moreover, in August, 90 % of the studied area consisted of soils with slight to strong SWR, of which 60 % accounted for soils with strong SWR. With a soil water content of 10.95 %, the SWR reached its peak, with an average value of 60.32 s. The SWR was positively correlated with total N, available N, and soil organic matter (SOM) contents, and therein the hydrophobic acid matter and the hydrophobic basic matter content had a positive contribution to SWR, and the hydrophilic basic matter and the hydrophilic acidic matter had a negative contribution on SWR. In addition, SWR was found to be negatively related to the soil particle size (r = -0.672). A slight SWR was also observed in the majority of the studied area, particularly in the topsoil and fine soils, especially during the monsoon period; hence, SWR must be also considered to reduce the risk of occurrence of soil erosion and degradation in grasslands.

Hydrophobicity of soils affected by fires: An assessment using molecular markers from ultra-high resolution mass spectrometry

Soil water repellency (SWR) is a physical property due to a complex interaction of factors (e.g., fire, soil organic matter, soil texture) that reduces the soil water infiltration capacity. Traditionally, SWR is attributed to the accumulation and redistribution of hydrophobic compounds within soil profile. To obtain further insight into chemical compounds, which could be associated with SWR, a study was done on coarse (1-2 mm) and fine (< 0.05 mm) granulometric fractions of burned and unburned sandy soils under two Mediterranean vegetation biomes from Doñana National Park (Spain). The water drop penetration time (WDPT) test was used to assess the SWR. The molecular composition of extracted humic substances from the soil organic matter (SOM) was determined by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS). Partial least squares (PLS) regressions showed that the SWR can be predicted (P = 0.006) solely based on the abundances of approximately 1200 common compounds determined by FT-ICR/MS. This model confirmed the significant correlation between a specific SOM molecular composition and the SWR. The comparative analysis revealed that the SWR in the burned samples was significantly (P < 0.05) related to the abundance of aromatic and condensed compounds, while in the unburned samples there was a significant influence of aromatic hydrocarbons and lignin compounds. In the fine fraction, lipid compounds were significantly associated with the SWR. Contrastingly, the coarse fraction did not show any correlation. Alternatively, soils with a high SWR were significantly related to the presence of lipids and lignin. This analysis showed that combining FT-ICR/MS molecular characterizations with statistical treatments is a powerful approach for exploratory analysis suggesting that the structural features associated with SWR in the studied soils are different depending on the types of vegetation or the soil physical fractions with different particle size.

Altitudinal differentiation in the leaf wax-mediated flowering bud protection against frost in a perennial Arabidopsis

An altitudinal gradient of leaf water repellency is often observed between and within species. In a previous study of Arabidopsis halleri, cauline leaves (stem leaves that wrap flowering buds) showed higher water repellency in exposed semi-alpine plants than in understory low-elevation plants. Here, we examined altitudinal variations in the cuticular wax content of the leaf surface and experimentally evaluated the role of high water repellency of cauline leaves. Leaf cuticular wax was analysed using comprehensive two-dimensional gas chromatography (GC)-mass spectrometry and a GC-flame ionisation detector. Young flowering buds wrapped by cauline leaves were exposed to freezing temperatures with or without water, and frost damage to the flowering buds was compared between plants from semi-alpine and low-elevation habitats. Higher amounts of C29, C31, and C33 alkanes were observed in the cauline leaves of semi-alpine plants than in those of low-elevation plants. In the freezing experiment, water application increased damage to the flowering buds of low-elevation plants, and the extent of damage to the flowering buds was lower in semi-alpine plants than in low-elevation plants when water was applied to the plant surface. Genetic variations in the amounts of alkanes on the leaf surface depending on the altitude occurred specifically in cauline leaves. Our results indicate that the water repellency of cauline leaves presumably minimises frost damage to flowering buds at high altitudes.

Topsoil microstructure changes after a shrubland prescribed burn (Central Pyrenees, NE Spain)

The dense thicket Echinospartum horridum (Vahl, Rothm) is expanded in secondary pastures of the Central Pyrenees (NE-Spain). The control of this grassland encroachment is attempted through prescribed burnings, trying to minimize its direct effects on the soil. But the structural changes on the new soil surface, burned and bare, are unknown in the medium-term. To check it, soil aggregate stability (SAS), mean weight diameter of the aggregates (MWD), water repellency (WR), unsaturated hydraulic conductivity (k), and soil organic carbon (SOC) were measured in the surface (at 0-1, 1-2, 2-3, and 3-5 cm) in both unburned and 1-yr burned soils, after verifying that it suffered no direct damage. We also used the digital images of thin sections, obtained from undisturbed and oriented topsoil samples, to detect potential changes in soil microstructure. No significant changes were found in SAS, MWD and SOC for any thickness of soil studied. Nevertheless the WR, which was high before and just after burning, decreased significantly in the upper soil cm after 1-yr burning. WR decrease coincides with the 6-fold increase of the unsaturated hydraulic conductivity (k) and the presence of cappings on the burned topsoil. Cappings are coatings poor in organic matter and composed by fine sand-sized particles of angular quartz, mixed with charcoal, covering irregularly the original topsoil. The formation of cappings seems to derive from the impact of raindrops on the bare soil surface, hence its irregular spatial distribution. Summarizing, removing bushes by means of a low-intensity fast-moving prescribed burning caused the formation of discontinuous cappings without worsening significantly the rest of the measured properties.

Interfacial phenomena of water striders on water surfaces: a review from biology to biomechanics

Water striders have intrigued researchers for centuries from the viewpoints of biology to biomechanics. In this review, we introduce the basic theories and techniques of physics and force measurement for biomechanical research into water striders. Morphological and behavioral traits of water striders are summarized and discussed from biomechanical perspectives, along with comparative study. This integrated review also highlights potential directions for studies on water-walking arthropods, which might inspire future biological and biomechanical research.

Post-fire management treatment effects on soil properties and burned area restoration in a wildland-urban interface, Haifa Fire case study

In November 2016, the urban dry streams (wadis) of the city of Haifa in Northern Israel were on fire. However, it was not just the fire that was threatening urban areas. Post-fire precipitation could turn into urban floods, further aggravating the fire damages. Several months after the fire a considerable restoration effort was initiated to restore the burned areas and mitigate future events. For urban forests the rehabilitation strategy was planned and implemented according to the topographic structure of the burned site and anticipated soil erosion. Accordingly, various post-fire management techniques were used: salvage-logging, afforestation, log erosion barriers and coconut fibre-webs. This study aimed to look at the effects of these methods on soil properties, namely, gravimetrical soil moisture, soil organic matter content, pH, electrical conductivity, hydraulic conductivity and soil water repellency. Results indicate that the control (burned, non-managed) site was the highest in soil moisture, organic matter and electrical conductivity compared to all other sites, however, the existence of ash cover made the response to precipitation unpredictable. The hydraulic conductivity (K) of the black ash (24.1 ± 8.6 mm/h), the white ash (19.0 ± 10.7 mm/h) and the disturbed (mixed) ash (11.7 ± 3.7 mm/h) were significantly higher than the underlying soil (3.3 ± 0.7 mm/h). As a result of these differences in K value, precipitation only infiltrates through the ash layers and then flows along the interface of the ash and the soil, triggering soil erosion. Most of the sites that were salvage logged showed signs of erosion. The log barriers were only effective for downstream areas. The afforestation could help to homogenise the soil, but the vegetation cover would be less dense and stable than after natural reforestation. Furthermore, the coconut fibre webs helped to improve the soil water retention and decreased the direct impact of rainfall.

The effect of biochar on severity of soil water repellency of crude oil-contaminated soil

Crude oil contamination adversely affects soil water repellency. In this study the effect of biochar on this soil characteristic has been investigated in the laboratory. Soil sample was collected from a field located near Pars Oil Company, at the top depth of 0-15 cm below surface. After air-drying and passing through a 2-mm sieve, the soil was artificially contaminated with four levels of crude oil (1:0, 1:25, 1:16.6, and 1:12.5 ratios). Biochars used in this research were generated from beechwood and maize residues at three different pyrolysis temperatures (350 °C, 550 °C, and 750 °C). Chemical functionality of all biochar samples was determined using Fourier-transform infrared spectrometry. Sufficient amounts of beechwood and maize biochars, passed through a 0.053-mm and 2-mm sieves, were mixed into crude oil-contaminated soil at the rate of 0, 0.5, 1, and 2% of total dry soil weight. The mixed samples were then laboratory incubated for 90 days at 24 °C and 10% soil moisture. Water repellency was measured using water drop penetration time (WDPT). The experimental results showed that functional groups on the biochars' surfaces produced at the studied temperatures were distinct. Beechwood and fine size of biochar showed more ability in reducing the hydrophobicity. The produced biochars, at higher temperature, had more potential to alleviate water repellency due to the strong interactions between functional groups of biochars and crude oil. The highest amount of biochar used (2%) significantly alleviated water repellency.

Effect of crude oil-induced water repellency on transport of Escherichia coli and bromide through repacked and physically-weathered soil columns

Knowledge of the transport and fate of pathogenic Escherichia coli, especially in the areas contaminated with crude oil, is required to assess contamination of shallow groundwater resources. The present study aims to investigate the effect of crude oil-mediated water repellency on the movement of nalidixic acid-resistant Escherichia coli strain (E. coli NAR) and bromide (Br) as an inert tracer in two soil types. The soils were contaminated at three levels of 0, 0.5 and 1% w/w of total petroleum hydrocarbons (TPHs) using crude oil. Steady-state saturated flow in the soil columns was controlled using a tension infiltrometer. Leaching experiments were conducted through the columns of repacked (un-weathered) and physically-weathered clay loam (CL) and sandy loam (SL) soils. The columns leachate was sampled at specific times for 4 pore volumes. The shape of breakthrough curves for the E. coli NAR and Br depended on soil texture and structure and the TPHs level. Preferential flow in the crude oil-mediated water-repellent soils facilitated the transport of contaminants especially E. coli NAR. Filtration coefficient and relative adsorption index of bacteria were greatest in the repacked CL soils and were lowest in the weathered SL soils. Discontinuity of soil pores and lower flow velocity resulted in greater filtration of E. coli NAR in the repacked CL soil than other treatments. Physical weathering induced the formation of aggregates which reduced soil particle surfaces available for retention of water-repellent oil and contaminants. Movement of both bacteria and Br tracer in the weathered SL soil with 1% TPHs was higher than other treatments. This finding was attributed to low specific surface area, continuity of the pores and water repellency-mediated preferential pathways in the weathered SL soil columns. Our findings implied that shallow groundwater resources could be very sensitive to microbial contamination particularly in the oil-mediated water-repellent soils.

Transport, retention, and release of Escherichia coli and Rhodococcus erythropolis through dry natural soils as affected by water repellency

Microbial transport in soil affects pathogen retention, colonization, and innoculant delivery in bioremediating agricultural soils. Various bacteria strains residing in the fluid phases of soils are potential contaminants affecting human health. We measured the transport of hydrophilic Escherichia coli (E. coli) and hydrophobic Rhodococcus erythropolis (R. erythropolis) bacteria through initially air-dried wettable or water-repellent soil columns to understand the effect of water repellency and the hydrophobicity of the organism on its retention, release, and transport properties. Bacteria suspensions infiltrated the top of the columns under saturated (0 cm) and unsaturated (-5 cm) flows in the air-dried (pulse 1) and rewetting (pulse 2) conditions. Cells were recovered from the leachates and the soil extracts by the viable counts. Wettable soil efficiently retained both hydrophobic and hydrophilic bacteria (>80%) in initial air-dried conditions (pulse 1). Even after rewetting, and the formation and expansion of water films and corresponding reduction of the air-water interfacial area (pulse 2), few bacteria were released (maximum 31.5% and 10.1% for saturated and unsaturated flows, respectively), whereas more cells were released from the water-repellent counterpart (more that 72%). The smaller size of hydrophobic R. erythropolis made cell transport possible within the thinner water films of both soils compared to hydrophilic E. coli through pulses 1 and 2. The shape of each strain's retention profiles was uniform and exponential as influenced by soil, strain, and water flow conditions. The results suggest that hydrophobic bacteria will disperse readily when leached into initially dry soil, while hydrophilic bacteria are more susceptible to leaching, posing a risk of pathogen contamination. Clearly the wettability of soil and organisms affects fate and transport.

The impact of wildland fires on calcareous Mediterranean pedosystems (Sardinia, Italy) - An integrated multiple approach

Sardinia (Italy), the second largest island of the Mediterranean Sea, is a fire-prone land. Most Sardinian environments over time were shaped by fire, but some of them are too intrinsically fragile to withstand the currently increasing fire frequency. Calcareous pedoenvironments represent a significant part of Mediterranean areas, and require important efforts to prevent long-lasting degradation from fire. The aim of this study was to assess through an integrated multiple approach the impact of a single and highly severe wildland fire on limestone-derived soils. For this purpose, we selected two recently burned sites, Sant'Antioco and Laconi. Soil was sampled from 80 points on a 100×100m grid - 40 in the burned area and 40 in unburned one - and analyzed for particle size fractions, pH, electrical conductivity, organic carbon, total N, total P, and water repellency (WR). Fire behavior (surface rate of spread (ROS), fireline intensity (FLI), flame length (FL)) was simulated by BehavePlus 5.0.5 software. Comparisons between burned and unburned areas were done through ANOVA as well as deterministic and stochastic interpolation techniques; multiple correlations among parameters were evaluated by principal factor analysis (PFA) and differences/similarities between areas by principal component analysis (PCA). In both sites, fires were characterized by high severity and determined significant changes to some soil properties. The PFA confirmed the key ecological role played by fire in both sites, with the variability of a four-modeled components mainly explained by fire parameters, although the induced changes on soils were mainly site-specific. The PCA revealed the presence of two main "driving factors": slope (in Sant'Antioco), which increased the magnitude of ROS and FLI; and soil properties (in Laconi), which mostly affected FL. In both sites, such factors played a direct role in differentiating fire behavior and sites, while they played an indirect role in determining some effects on soil.

Facing the rain after the phase out: Performance evaluation of alternative fluorinated and non-fluorinated durable water repellents for outdoor fabrics

Fluorinated durable water repellent (DWR) agents are used to obtain water and stain repellent textiles. Due to the on-going phase-out of DWRs based on side-chain fluorinated polymers (SFP) with "long" perfluoroalkyl chains, the textile industry lacks suitable alternatives with comparable material characteristics. The constant development and optimization of SFPs for textile applications initiated more than half a century ago has resulted in a robust and very efficient DWR-technology and textiles with exceptional hydro- and oleo-phobic properties. The industry is now in the predicament that the long-chain SFPs with the best technical performance have undesirable toxicological and environmental behaviour. This study provides a comprehensive overview of the technical performance of presently available fluorinated and non-fluorinated DWRs as part of a chemical alternatives assessment (CAA). The results are based on a study with synthetic outdoor fabrics treated with alternative DWRs and tested for repellency using industrial standard and complementary methods. Using this approach, the complex structure-property relationships of DWR-polymers could be explained on a molecular level. Both short-chain SFPs and non-fluorinated DWRs showed excellent water repellency and durability in some cases while short-chain SFPs were the more robust of the alternatives to long-chain SFPs. A strong decline in oil repellency and durability with perfluoroalkyl chain length was shown for SFP DWRs. Non-fluorinated alternatives were unable to repel oil, which might limit their potential for substitution in textile application that require repellency towards non-polar liquids.

Burn effects on soil properties associated to heat transfer under contrasting moisture content

The aim of this work is to investigate the topsoil thickness affected by burning under contrasting soil moisture content (field capacity versus air-dried conditions). A mollic horizon of an Aleppo pine forest was sampled and burned in the laboratory, recording the temperature continuously at the topsoil surface and at soil depths of 1, 2, and 3cm. Changes in soil properties were measured at 0-1, 1-2, 2-3, and 3-4cm. Both the maximum temperature and the charring intensities were significantly lower in wet soils than in air-dried soils up to 3cm in depth. Moreover, soil heating was slower and cooling faster in wet soils as compared to dry soils. Therefore, the heat capacity increase of the soil moistened at field capacity plays a more important role than the thermal conductivity increase on heat transfer on burned soils. Burning did not significantly modify the pH, the carbonate content and the chroma, for either wet or dry soil. Fire caused an immediate and significant decrease in water repellency in the air-dried soil, even at 3cm depth, whereas the wet soil remained hydrophilic throughout its thickness, without being affected by burning. Burning depleted 50% of the soil organic C (OC) content in the air-dried soil and 25% in the wet soil at the upper centimeter, which was blackened. Burning significantly decreased the total N (TN) content only in the dry soil (to one-third of the original value) through the first centimeter of soil depth. Soluble ions, measured by electrical conductivity (EC), increased after burning, although only significantly in the first centimeter of air-dried soils. Below 2cm, burning had no significant effects on the brightness, OC, TN, or EC, for either wet or dry soil.

Thermal destruction of organic waste hydrophobicity for agricultural soils application

Use of organic amendments is a good strategy for combating the growing problem of soil degradation due to deterioration of organic matter content, particularly severe in semi-arid European Mediterranean regions, while at the same time providing an opportunity for recycling organic wastes. Olive mill pomace (OMP), the main by-product of the olive oil industry, is being used increasingly in olive grove soils for this purpose. Although the positive effects of OMP amendments have been widely studied, they also have some negative effects on soil. One of the most critical is that they increase water repellency (WR) due to the presence of poorly evolved, strongly aliphatic compounds. This detrimental effect has received very little attention, although it may impair plant water availability and infiltration rates, increase erosion and lower long-term soil quality. This study proposed, for the first time, thermal treatment as an effective way of reducing WR in organic amendments (i.e. mixtures of OMP, olive tree pruning, chicken manure and spent coffee grounds) prior to their application to soil. Thermal treatment at 275 °C proved effective in removing WR, while lower temperatures (175 or 225 °C) can even increase it. Changes by thermal treatment in the characteristics of the organic amendments studied with FTIR and UV-Vis spectroscopy and thermogravimetric analysis showed that it strongly reduced the aliphatic compounds mainly responsible for their hydrophobicity, concentrated aromatic compounds and increased thermostability. Heating also reduced phytotoxicity, making all of the organic amendments usable in the field (germination index over 100%). Therefore, heating at 275 °C could be an acceptable option for removing WR from organic amendments, enhancing their quality with more stable evolved characteristics.

Morphology and functions of astrocytes cultured on water-repellent fractal tripalmitin surfaces

In the brain, astrocytes play an essential role with their multiple functions and sophisticated structure, as surrounded by a fractal environment which has not been available in our traditional cell culture. Water-repellent fractal tripalmitin (PPP) surfaces can imitate the fractal environment in vivo, so the morphology and biochemical characterization of astrocytes on these surfaces are examined. Water-repellent fractal PPP surface can induce astrocytes to display sophisticated morphology with smaller size of cell area, longer and finer filopodium-like processes, and higher morphological complexity. The super water-repellent fractal PPP surface with water contact angle of 150°∼160° produces the maximal effects compared with other surfaces at lower water contact angles. The trends of characteristic protein expression, including that of nestin, vimentin, GFAP and glutamine synthetase, for astrocytes cultured on super water-repellent fractal PPP surfaces approximate more to in vivo pattern. The super water-repellent PPP surface also render astrocytes to perform more pronounced promotion of neurogenesis by increasing the release of nerve growth factor in a co-culture system. Altogether, our results suggest that the super water-repellent fractal PPP surface facilitates the astrocytes to mimic their in vivo performance, thus provides a closer-to-natural culture environment for experimental assessment of glial structure and functions.