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Vermicompost is an organic material that is abundant in humic acids and nutrients. It is obtained through the bio-oxidation and stabilization processes carried out by earthworms. It has been proven to bring several benefits to different soil properties, including bulk density, soil structure, and plant available water capacity (PAWC). This investigation was conducted to fill the knowledge gap in some critical factors related to vermicompost application, specifically the short-term influence of a single vermicompost application with increasing doses on soil wettability and physical quality of differently textured soils. Water repellency of vermicompost and soil/vermicompost mixtures was investigated at different moisture contents by the water drop penetration time test, whereas physical quality was assessed by 35 soil indicators related to bulk density, soil water retention curve, and pore size distribution function. Despite vermicompost showed from strong to severe hydrophobicity at moisture content lower than the field capacity, amended soils were at the most slightly water repellent thus indicating that, under field conditions, the hydrophobicity attributable to soil amendment with vermicompost could be considered negligible. Soil physical quality was effectively affected by vermicompost addiction with different outcomes depending on soil texture. Indicators linked to PAWC generally increased at increasing the vermicompost rate in the coarse soils whereas no significant effect was observed for intermediate and fine soils. For example, plant available water capacity of coarse-textured soils increased from an average initial value of 0.056 cm3 cm-3 to an optimal value of 0.15 cm3 cm-3 when a vermicompost addition dose of about one-third by volume (34 %) was applied. In the finest soil, drainable porosity significantly increased from an initial value of 0.09 cm3 cm-3 to 0.23 cm3 cm-3 when the maximum vermicompost dose (43 %) was applied thus indicating that amendment could be effective in enhancing water and air circulation.

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Soil water repellency (SWR) is a physical phenomenon whereby water cannot penetrate or has difficulty penetrating the soil surface. There are many factors involved in its occurrence, but the main factors controlling its emergence in loess remain unclear. In this work, we have studied numerous physicochemical and biological factors functioning in different dominant vegetations (Pinus tabulaeformis Carr., Robinia pseudoacacia L., and Hippophae rhamnoides L.) in a loess hilly region by gas chromatography-mass spectrometry (GC-MS) and high-throughput sequencing techniques. We observed that more than 75% of the soils under Robinia and Hippophae are categorized as slightly or strongly water repellent, while nearly 50% of the soils under Pinus are categorized as severely to extremely water repellent. The relative concentrations of total free lipids in the soil in the same water-repellency class were Pinus > Robinia > Hippophae, where fatty acids, alkanols, and sterols were positively correlated with SWR, whereas alkanes were not. For the abundance and diversity index of bacterial and fungal communities, the three species ranked in the following order: Robinia ≈ Hippophae > Pinus. Thus, solvent-extractable polar waxes were indicated to be better preserved in water-repellent soils under Pinus due to lower microbial diversity than Robinia and Hippophae. Here, we demonstrate polar waxes to be the principal factor controlling SWR. Moreover, the dominant phyla of fungi varied greatly than those of bacteria under three vegetation types. Correlation analysis showed that the abundance of Actinobacteria in dominant bacteria increased with SWR. Nonmetric multidimensional scaling suggested the fungal community in different water-repellent soils under Pinus to vary more than those under Robinia and Hippophae. The indicator species mainly belonged to Actinobacteria in bacteria and Basidiomycota in fungi at the phylum level; this finding was further supported by the linear discriminant analysis (LDA) effect size (LEfSe). Additionally, GC-MS identified a small amount of ergosterol, a specific biomarker of fungi under Pinus. These pieces of evidence collectively reveal that severe to extreme SWR occurs under Pinus and appears to be the most influenced by fungi and actinomycetes when the topsoil is close to air drying. However, there is a need for further testing on different plant species or land use.

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Soil crust is a normal natural phenomenon with different water hydrophilicity and repellency due to different formation mechanism, thus affecting soil hydraulic characteristics and hydrological cycle. In this study, we measured water repellent characteristics of physical and biological crusts under different vegetations in the field using water drop penetration time (WDPT). The surface morphology of crusts was observed using scanning electron microscopy, and the infiltration characteristics of crusts and their non-crust soils (control) was evaluated with micro-infiltration device. The results showed that: 1) The average WDPT of physical crusts and the control soils was 3.3 s and 0.9 s, respectively, indicating that both were hydrophilic. The average WDPT of biological crusts ranged from 20.9 s to 140.9 s, which was 2.8 to 19 times that of control, and that under Diospyros lotus and Robinia pseudoacacia was 134.5 s and 140.9 s, respectively. 2) Compared with the control, the cumulative infiltration amount, average infiltration rate and moisture absorption force of physical crusts decreased by 0-4.3%, 3.5%-5.1%, and 27.2%-90.1%, respectively, while those of biological crusts decreased by 0-25%, 1.4%-28.2% and 36.0%-84.9%, respectively. 3) Regardless of the presence of crusts or not, there were "hockey-stick-like" curves by using Philip model to fit infiltration data. Before the WRCT point in the "hockey-stick-like" curve, the point source infiltration was mainly horizontal diffusion. After the WRCT point, the infiltration was mainly vertical diffusion. The presence of soil crust prolonged the formation time of the turning point. In all, physical crusts formed by inorganic mineral particles blocking the surface soil did not affect water repellency, while biological crusts that reflected the effects of hydrophobic organic compounds on soil structure enhanced its water repellency. Both physical crusts and biological crusts decreased the cumulative infiltration amount and average infiltration rate of soil. Compared with the control, physical crusts mainly affected soil hygroscopicity, but with limited effects on the steady infiltration rate. Biological crust decreased soil hygroscopicity and increased steady infiltration rate.

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Repeated application of soil surfactants, or wetting agents, is a common practice for alleviating soil water repellency associated with soil organic coatings. However, wetting agents are organic compounds that may also coat soil particle surfaces and reduce wettability. For this experiment, hydrophobic sands from the field and fresh, wettable sands were collected and treated with either a polyoxyalkylene polymer (PoAP) or alkyl block polymer (ABP) wetting agent, or water only treatments served as a control. Following repeated treatment application and sequential washings, dissolved and particulate organic carbon (OC) were detected in the leachates of both sand systems. The total amount of OC recovered in leachates was 88% or less than the OC introduced by the wetting agents, indicating sorption of wetting agent monomers to soil particle surfaces regardless of soil hydrophobicity status. While ABP treatment did not alter solid phase organic carbon (SOC) in the sands studied, PoAP application increased SOC by 16% and 45% which was visible in scanning electronic microscopy images, for hydrophobic and wettable sands, respectively. PoAP application also increased the hydrophobicity of both sands that were studied. In contrast, ABP treatment increased the wettability of hydrophobic sand. Our results provide strong evidence that certain wetting agents may increase soil hydrophobicity and exacerbate wettability challenges if used repeatedly over time.

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Soil water repellency (SWR) is a widespread phenomenon that influences patterns of soil wetting, runoff, evapotranspiration and availability of water for plants. In natural ecosystems there is emerging evidence that some plants can take advantage of non-uniform wetting patterns, leading to the emergence of co-evolutionary behaviour. In this review, SWR is considered in terms of five spheres of influence. Given the presence of hydrophobic organic material in the biosphere, the strength, severity and persistence of SWR is influenced by properties at the surface of the lithosphere and prevailing conditions in the atmosphere and hydrosphere. These in turn, can be modified by activities in the anthroposphere. This review thus examines the strength, severity and persistence of non-wetting behaviour with reference to these five spheres of influence and also the interactions between the spheres. It is focused on (i) how SWR is characterised to provide insight into how different measurement techniques have specific operational ranges, (ii) how SWR has developed as an indirect consequence of evolution in natural ecosystems and (iii) how feedbacks across the different spheres have emerged. It demonstrates that management and restoration of natural ecosystems with water repellent soils is very different from management of productive crops in monocultural agricultural systems, controlled in the anthroposphere.

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The hydrological response of forest soil in the Mediterranean environment is characterised by high runoff and erosion rates, mainly due to low infiltration and high repellency of soils. However, little literature exists about the effects of forest ages on soil water repellency (SWR) and hydraulic conductivity (SHC). This study evaluates these hydrological parameters in five Pinus nigra Arn ssp. Salzmannii stands of different ages in Central-Eastern Spain; one of these stands, unmanaged, was chosen as reference system. SWR (measured in terms of water drop penetration time, WDPT) and SHC as well as the main physico-chemical properties and surface characteristics of soils were surveyed in forty-five plots. Water infiltration was higher in the older stands (including the older and unmanaged forest) and lower (by over 60%) in the more recent pine forests. Four of the studied stands did not show water repellency; only the more recent plantation showed a slight SWR. The differences in SHC among the forest ages were mainly driven by the organic matter (OM) and nutrient contents of the soils as well as by the bulk density and quantity of dead wood. SWR was similar among the plots (despite significantly differences in WDPTs), although having variable OM contents. Considering these differences in soil properties, SHC and SWR were simply predicted for each forest stand using on dbRDA models and the following soil properties: (i) OM and total nitrogen contents of soil (for SHC and SWR); (ii) dead wood and bulk density (for SHC); and (iii) clay content and the percentage of bare soil (for SWR). Overall, this study has showed that, when a new forest stand is planted, decreases in water infiltration, with subsequent increases in runoff generation capacity) of the soils, can be expected. Conversely, no water repellency is likely to affect new pine plantations.

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The aim of the presented research was to assess the changes in hydro-physical properties of sandy clay under the influence of petroleum hydrocarbon contamination. An understanding of these changes is fundamental in the right remedial actions and for further use of soil. Laboratory tests of inherently wettable sandy clay showed that the petroleum hydrocarbon induced potential soil water repellency (SWR) of extremely repellent class at the contamination of 18 g kg-1. The relationship between soil water potential (pF) and SWR determined by the WDPT test for given hydrocarbon contamination, i.e., 6, 12, 18, 30, 100 g kg-1, showed that the critical soil moisture value (CSMC) corresponds to the pF = 1.0 ÷ 1.5. Soil retention characteristic (pF) showed that an increase in hydrocarbon contamination from 0 to 100 g kg-1 caused a reduction of total available water for plants from about 0.19 to 0.06 cm cm-3. At the same time, in the pF = 1.5 ÷ 2.0 range, intensive soil pore drainage was observed. Statistically, significant effect of hydrocarbon contamination and soil moisture potential on SWR was found. Soil hydrophobicity limits the addition of soil retention, because a significant part of the precipitation can be transformed by surface runoff. The carried out tests showed that at a hydrocarbon contamination of 30 g kg-1, total rainfall amount 14 mm with an intensity of 2 mm h-1 was transformed into a surface drain in approx. 40%. The conducted studies demonstrate the adverse impact of hydrocarbon contamination on the soil's hydro-physical properties. The soil water retention reduction and launching of the surface outflow, as a result of limiting the water penetration process resulting from SWR, change the agrohydrological conditions of the contaminated area. It can result as the imbalance of the flow of energy and matter in the ecosystem. The scenarios of environmental effects, among others, depend on the type of soil, the degree of its pollution, the type of ecosystem, and supporting activities undertaken by man. It should be taken into account that the increasing frequency of drought occurrence associated with climate change is conducive to the phenomenon of SWR regardless of the reasons for its occurrence.

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HYPOTHESIS: While soil water repellency causes a variety of undesirable environmental effects, the underlying mechanism is unknown. We investigate the coupled effects of chemical characteristics and surface topology in a simple model system of two lipids, DSPE (1,2-distearoyl-sn-glycero-3-phosphoethanolamine) and DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine), and a clay substrate. These closely-related lipids allowed the study of how a small change in chemical structure influences the surface hydrophobicity. EXPERIMENTS: Techniques ranging from molecular (simulations) to nanoscopic (atomic force microscopy) to microscopic (fluorescence microscopy) to macroscopic (contact angle measurements) were used to explore interactions at all length scales. The wettability was assessed from initial contact angle and time-dependent changes in droplet shape. FINDINGS: The lipid distribution depended on the lipid's melting temperature: solid lipids did not spread evenly through the film, while liquid ones did. However, the initial contact angle did not change appreciably with the addition of DSPE or DOPE. Only DSPE heated above its melting temperature induced significant changes. In addition to the initial contact angle, quantitative variables extracted from the change in droplet shape over time correlated with the film topography or lipid distribution. These results define a new quantitative approach to investigating partially-wettable soils and provide a potential rationale for why clays can remediate water-repellent soils.

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Biochar addition to soil may change the hydrophobicity of amended soil and influence soil hydraulic properties. Soil hydrophobicity, i.e. soil water repellency (SWR) can interrupt water infiltration and form preferential flow leading to a potential risk of soil erosion or groundwater pollution. Up to date, the effect of different biochars on soil hydrophobicity remains unclear and the association of SWR with soil hydraulic properties is still unknown. To link the biochar hydrophobicity to SWR and soil water holding capacity (WHC), the surface structure and chemical composition of 27 biochars with different feedstocks and pyrolysis temperatures were characterized, and the SWR and soil WHC of biochar-added soil were investigated. Carboxylic groups on the biochar surface, surface area and pore volume were mostly influenced by pyrolysis temperature, which suggested the dominant factor determining the severity of biochar hydrophobicity was pyrolysis temperature. Hydrophilic soil became hydrophobic after biochar amendment. A higher addition rate led to a stronger SWR of hydrophilic soil. Biochar addition increased soil WHC of hydrophilic soil with low total organic carbon (TOC) content. Biochar did not have significant influence on SWR and soil WHC of hydrophobic soil with high TOC content. It implied that the influence of biochar on SWR and soil hydraulic properties mainly depended on soil original hydrophobicity and TOC content. Therefore, the properties of biochar and influence on soil hydrophobicity and hydraulic properties should be considered before processing biochar application.

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Granular materials with synthetic water repellent coatings have great potential to be used in ground interfaces (ground-atmosphere-vegetation and ground-structure) as infiltration barriers, due to their altered hydrological properties (suppressed infiltration and decreased sorptivity). However, very few studies have evaluated the impact of synthetic soil water repellency on soil erosion. This paper investigates the effect of water repellency on soil erosional behavior, including splash erosion and rill processes. Twenty-four flume tests were carried out on model slopes under artificial rainfall; soils with three wettability levels were tested, including wettable (contact angle, CA < 90°), subcritical water repellent (CA ~ 90°) and water repellent (CA > 90°). Various rainfall intensities (230 mm/h, 170 mm/h, 100 mm/h and 40 mm/h) and grain sizes (Fujian sand and sand/silt mixture) were adopted. Erosional variables, including splash erosion rate, average sediment concentration, peak sediment concentration and time to peak sediment were measured to quantitatively analyze the behavior. This study confirms the impact of water repellency on soil erosion and unveils the possibility to reduce infiltration at ground-atmosphere interface with controlled soil erosion. The results revealed that: (1) synthetic water repellency does not necessarily lead to increased soil erosion yield; its impact is dependent on grain size with the soil erosion loss increasing for Fujian sand, but decreasing for sand/silt mixtures; (2) splash erosion is positively correlated to soil water repellency and high rainfall intensity, regardless of grain size; (3) the erosion processes for sand/silt mixtures are particle size selective and not affected by soil water repellency, whereas this phenomenon is not observed with Fujian sand.

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Prescribed burnings reduce the biomass and the risk of wildfires but can also alter soil water repellency. The objective of this study is to evaluate the impact of several prescribed burnings in soil water repellency (SWR). In spring 2016, prescribed burns were carried out at three forest sites located in: (i) Beteta in a pure forest of Pinus nigra Arnold ssp. salzmannii; (ii) El Pozuelo in mixed forest stands of Pinus pinaster Aiton and Pinus nigra Arnold ssp. salzmannii; (iii) Lezuza in mixed forest stands of Pinus halepensis Miller and Pinus pinaster Aiton. Six plots were established in each study area: three burned and three unburned. SWR was measured before and immediately after prescribed burns following a 1-year periodic evaluation. There were seven sampling dates at Beteta and El Pozuelo and nine at Lezuza with six plots and six measurement transects in each plot (36 measurement transects on each date). Soil water content (SWC), soil temperature (ST) and soil organic matter (SOM) were also measured. Our results showed that SWR increased after burning to quickly return to normal values in Lezuza, after 1 month in El Pozuelo and after 1 year in Beteta. Moreover, a significant positive relationship between SWR and both SOM and ST, but a negative one with SWC, were observed, which led SWR to increase after fire passage, also in the summer months. Continuous monitoring of these study sites is recommended to determine if low-intensity burnings promote mid- to long-term changes in soil characteristics.

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The contamination of soil with petroleum products is a major environmental problem. Petroleum products are common soil contaminants as a result of human activities, and they are causing substantial changes in the biological (particularly microbiological) processes, chemical composition, structure and physical properties of soil. The main objective of this study was to assess the impact of soil moisture on CO2 efflux from diesel-contaminated albic podzol soils. Two contamination treatments (3000 and 9000 mg of diesel oil per kg of soil) were prepared for four horizons from two forest study sites with different initial levels of soil water repellency. CO2 emissions were measured using a portable infrared gas analyser (LCpro+, ADC BioScientific, UK) while the soil samples were drying under laboratory conditions (from saturation to air-dry). The assessment of soil water repellency was performed using the water drop penetration time test. An analysis of variance (ANVOA) was conducted for the CO2 efflux data. The obtained results show that CO2 efflux from diesel-contaminated soils is higher than efflux from uncontaminated soils. The initially water-repellent soils were found to have a bigger CO2 efflux. The non-linear relationship between soil moisture content and CO2 efflux only existed for the upper soil horizons, while for deeper soil horizons, the efflux is practically independent of soil moisture content. The contamination of soil by diesel leads to increased soil water repellency.

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Soil water repellency originating from organic coatings plays a crucial role for soil hydraulics and plant water uptake. Focussing on hydrophobicity in the rhizosphere induced by root-mucilage, this study aims to explore the link between macroscopic wettability and nano-microscopic surface properties. The existing knowledge of the nanostructures of organic soil compounds and its effect on wettability is limited by the lack of a method capable to assess the natural spatial heterogeneity of physical and chemical properties. In this contribution, this task is tackled by a geostatistical approach via variogram analysis of topography and adhesion force data acquired by atomic force microscopy and macroscopic sessile drop measurements on dried films of mucilage. The results are discussed following the wetting models given by Wenzel and Cassie-Baxter. Undiluted mucilage formed homogeneous films on the substrate with contact angles >90°. For diluted samples contact angles were smaller and incomplete mucilage surface coverage with hole-like structures frequently exhibited increased adhesion forces. Break-free distances of force curves indicated enhanced capillary forces due to adsorbed water films at atmospheric RH (35 ±â€¯2%) that promote wettability. Variogram analysis enabled a description of complex surface structures exceeding the capability of comparative visual inspection.

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This study delivers new insights into rainfall-induced seal formation through a novel approach in the use of X-ray Computed Tomography (CT). Up to now seal and crust thickness have been directly quantified mainly through visual examination of sealed/crusted surfaces, and there has been no quantitative method to estimate this important property. X-ray CT images were quantitatively analysed to derive formal measures of seal and crust thickness. A factorial experiment was established in the laboratory using open-topped microcosms packed with soil. The factors investigated were soil type (three soils: silty clay loam - ZCL, sandy silt loam - SZL, sandy loam - SL) and rainfall duration (2-14 min). Surface seal formation was induced by applying artificial rainfall events, characterised by variable duration, but constant kinetic energy, intensity, and raindrop size distribution. Soil porosities derived from CT scans were used to quantify the thickness of the rainfall-induced surface seals and reveal temporal seal micro-morphological variations with increasing rainfall duration. In addition, the water repellency and infiltration dynamics of the developing seals were investigated by measuring water drop penetration time (WDPT) and unsaturated hydraulic conductivity (Kun). The range of seal thicknesses detected varied from 0.6 to 5.4 mm. Soil textural characteristics and OM content played a central role in the development of rainfall-induced seals, with coarser soil particles and lower OM content resulting in thicker seals. Two different trends in soil porosity vs. depth were identified: i) for SL soil porosity was lowest at the immediate soil surface, it then increased constantly with depth till the median porosity of undisturbed soil was equalled; ii) for ZCL and SL the highest reduction in porosity, as compared to the median porosity of undisturbed soil, was observed in a well-defined zone of maximum porosity reduction c. 0.24-0.48 mm below the soil surface. This contrasting behaviour was related to different dynamics and processes of seal formation which depended on the soil properties. The impact of rainfall-induced surface sealing on the hydrological behaviour of soil (as represented by WDTP and Kun) was rapid and substantial: an average 60% reduction in Kun occurred for all soils between 2 and 9 min rainfall, and water repellent surfaces were identified for SZL and ZCL. This highlights that the condition of the immediate surface of agricultural soils involving rainfall-induced structural seals has a strong impact in the overall ability of soil to function as water reservoir.

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Soil water repellency (hydrophobicity) prevents water from wetting or infiltrating soils, triggering changes in the ecosystems. Fire may develop, enhance or destroy hydrophobicity in previously wettable or water-repellent soils. Soil water repellency is mostly influenced by the quality and quantity of soil organic matter, particularly the lipid fraction. Here we report the results of a study on the effect of fire on the distribution of soil lipids and their role in the hydrophobicity grade of six particle size fractions (2-1, 1-0.5, 0.5-0.25, 0.25-0.1, 0.1-0.05 and <0.05mm) of an Arenosol under Quercus suber canopy at the Doñana National Park (SW-Spain). Hydrophobicity was determined using water drop penetration time test. Field emission scanning electron microscopy (FESEM) was used to assess the presence and morphology of the inorganic and organic soil components in the particle size fractions. Soil lipids were Soxhlet extracted with a dichloromethane-methanol mixture. Fatty acids (FAs) and neutral lipids were separated, derivatized, identified and quantified by gas chromatography/mass spectrometry and gas chromatography/flame ionization detection. The hydrophobicity values of soil samples and fractions were statistically different (P < 0.05), for both, the unburnt and burnt soils, and particle size fractions. All samples displayed a similar distribution of FAs, straight-chain saturated acids in the C14-C32 range, and neutral lipids (n-alkan-1-ols, n-alkanes), only differing in their relative abundances. Among possible biogeochemical mechanisms responsible for the changes in soil lipids, the observed depletion of long chain FAs (C≥24) in the coarse fraction is best explained by thermal cracking caused by the heat of the fire. The enrichment of long chain FAs observed in other fractions suggests possible exogenous additions of charred, lipid-rich, material, like cork suberin or other plant-derived macromolecules (cutins). Principal component analysis was used to study the relationships between hydrophobicity with soil organic matter and its different components. Extractable organic matter (EOM) and specifically long chain FAs content were positively correlated to soil hydrophobicity. Therefore, the latter could be used as biomarkers surrogated to hydrophobicity in sandy soils.

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Wildfires can create or intensify water repellency in soil, limiting the soil's capacity to wet and retain water. The objective of this research was to quantify soil water repellency characteristics within burned piñon-juniper woodlands and relate this information to ecological site characteristics. We sampled soil water repellency across forty-one 1,000 m2 study plots within three major wildfires that burned in piñon-juniper woodlands. Water repellency was found to be extensive-present at 37% of the total points sampled-and strongly related to piñon-juniper canopy cover. Models developed for predicting SWR extent and severity had R2adj values of 0.67 and 0.61, respectively; both models included piñon-juniper canopy cover and relative humidity the month before the fire as coefficient terms. These results are important as they suggest that postfire water repellency will increase in the coming years as infilling processes enhance piñon-juniper canopy cover. Furthermore, reductions in relative humidity brought about by a changing climate have the potential to link additively with infilling processes to increase the frequency and intensity of wildfires and produce stronger water repellency over a greater spatial extent. In working through these challenges, land managers can apply the predictive models developed in this study to prioritize fuel control and postfire restoration treatments.

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Soil water repellency (SWR) markers are defined as hydrophobic compounds in soil causing SWR and are mainly derived from plants. Previous studies have shown the types and abundance of SWR-markers in soils. However, how these SWR-markers are exactly related to SWR and their origin is poorly understood. This study aims to understand the relationship between SWR-markers, vegetation type and cover and SWR for a simple sandy soil ecosystem, consisting of oaks with sedge and six grass species. All the soil (at different depth) and vegetation samples were collected in the field along a 6m transect, starting from an oak tree. Further along the transect grasses and sedges became more abundant. Free and ester-bound lipids from soils and plant leaves/roots were obtained using a sequential extraction method and identified by gas chromatography-mass spectrometry. Significant linear correlations were found between the main soil characteristics, such as total organic carbon content, and SWR. Single long-chain (>C20) SWR-markers derived from both plant leaf waxes and roots positively related to SWR. Both ester-bound ω-hydroxy fatty acids and C22 and C24 α,ω-dicarboxylic acids were predominantly present in the grass roots, but to a lesser extent in the roots of oak and sedge. These suberin-derived ω-hydroxy fatty acids and α,ω-dicarboxylic acids characteristic of roots could well predict the SWR. Additionally, the SWR predictors abundantly present in the soils matched well with high concentrations of the corresponding biomarkers in the dominant vegetation species that covered the soils. Our analyses demonstrated that grass roots influenced SWR more due to their more substantial contribution of organic matter to the topsoils than oak roots. This led to a stronger SWR of the soils covered with grass than those covered with oak vegetation.

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Mulching is an effective post-fire soil erosion mitigation treatment. Experiments with forest residue mulch have demonstrated that it increased ground cover to 70% and reduced runoff and soil loss at small spatial scales and for short post-fire periods. However, no studies have systematically assessed the joint effects of scale, time since burning, and mulching on runoff, soil loss, and organic matter loss. The objective of this study was to evaluate the effects of scale and forest residue mulch using 0.25m2 micro-plots and 100m2 slope-scale plots in a burnt eucalypt plantation in central Portugal. We assessed the underlying processes involved in the post-fire hydrologic and erosive responses, particularly the effects of soil moisture and soil water repellency. Runoff amount in the micro-plots was more than ten-fold the runoff in the larger slope-scale plots in the first year and decreased to eight-fold in the third post-fire year. Soil losses in the micro-plots were initially about twice the values in the slope-scale plots and this ratio increased over time. The mulch greatly reduced the cumulative soil loss measured in the untreated slope-scale plots (616gm-2) by 91% during the five post-fire years. The implications are that applying forest residue mulch immediately after a wildfire can reduce soil losses at spatial scales of interest to land managers throughout the expected post-fire window of disturbance, and that mulching resulted in a substantial relative gain in soil organic matter.

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Fires in agricultural areas are common, modifying the functioning of agro-ecosystems. Such fires have been extensively studied, and reported to considerably affect soil properties. Yet, understanding of the impact of livestock grazing, or more precisely, trampling, in fire-affected lands is limited. The objective of this study was to assess the impact of low- to moderate-fire severity and livestock trampling (hoof action) on the solid soil's wettability and related properties, and on soil detachment, in burnt vs. non-burnt croplands. The study was implemented by allowing livestock to access plots under high, medium, and low stocking rates in (unintentionally) burnt and non-burnt lands. Also, livestock exclusion plots were assigned as a control treatment. Results showed that fire slightly decreased the soil wettability. At the same time, water drop penetration time (WDPT) was negatively related to the stocking rate, and critical surface tension (CST) was ~13% smaller in the control plots than in the livestock-presence treatments. Also, the results showed that following burning, the resistance of soil to shear decreased by ~70%. Mass of detached material was similar in the control plots of the burnt and non-burnt plots. At the same time, it was three-, eight-, and nine-fold greater in the plots of the burnt×low, burnt×medium, and burnt×high stocking rates, respectively, than in the corresponding non-burnt ones. This study shows that livestock trampling in low- to moderate-intensity fire-affected lands increased the shearing of the ground surface layer. On the one hand, this slightly increased soil wettability. On the other hand, this impact considerably increased risks of soil erosion and land degradation.

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Soil water repellency (SWR) impedes the processes of soil water infiltration and redistribution. Although water repellent soils exist extensively in the world, its causes were not very clear. In this research, three measurement methods including the water droplet penetration time (WDPT), the molarity of ethanol drop (MED) and the contact angle (denoted as θ below) methods, were applied to obtain different SWR indices for the tested soils collected in Manas River Basin, Xinjiang Uygur Autonomous Region, China. The height method and the mass method were conducted to measure θ values. WDPT values of different soil samples were compared to investigate whether WDPT values were affected after being oven-dried or by different bulk densities. The relationships among three SWR indices (WDPT, MED and θ) were compared, and the major soil physico-chemical properties which influenced SWR were analyzed to discuss the intrinsic mechanism that caused SWR. The results showed that WDPT values of higher bulk density were larger than those of lower bulk density, and WDPT values of oven-dried soils were larger than those of air-dried soils. There were correlations between the three SWR indices of WDPT, MED and θ, but MED was insignificantly related to θ, which showed the differences among various SWR indices, although they were related to each other. When measuring θ values of soil-water, θ values obtained using the height method were larger than those obtained using the mass method, and the differences of θ for the three replications with the height method were smaller than those with the mass method. When using octane as the reference liquid for measuring θ values of soil-water, the differences of the three replications were small, being lower than those using ethyl alcohol. Among the studied multi-physico-chemical properties, clay content affected WDPT and θ values more significantly than other soil physico-chemical properties. Concentrations of K+ and Na+ were both positively correlated to θ va-lues, while the content of montmorillonite was negatively correlated to WDPT and MED values. In summary, among the three SWR measurement methods, the WDPT method was quite simple in operation but was susceptible, the MED method shortened the infiltration time but was time-consuming in operation, and the contact angle method was complicated in operation but the results were relatively accurate. When using octane as the reference liquid for the height method, it was more reliable than using the other liquids. Because each index had its inherent nature, it was suggested to use multiple indices to characterize SWR comprehensively.

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