An understanding of soil moisture content variability is fundamental in hydrological studies of peat soils, whose preservation depend on water-related processes. Dehydration of fens and adapting them for agricultural production have contributed to the degradation of peat soils. The goal of this study was to determine how the critical soil moisture content (CSMC) and soil water repellency (SWR) affect soil moisture patterns in a degraded peat-muck soil profile. SWR was measured under laboratory conditions using the water drop penetration time test, and then the CSMC was assessed. An investigation of moisture patterns was based on soil moisture data collected over short distances in a grasscovered peat-muck soil profile on seven dates. Observed differences in moisture patterns demonstrate that the CSMC can be used for the prediction of preferential flow occurrences in peat-muck soils. Lower values of the CSMC and lower levels of SWR persistence in muck layers than in peat layers indicate that degradation of peat soils improves their wettability. The relatively low values of CSMC and the low shrinkage potential in the muck layer suggest that preferential water flow in the degraded organic soils can occur when heavy rains are preceded by long periods of summer drought.

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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Although soil water repellency (SWR) has been reported under different soils, climates, and vegetation types of the world, especially in forest land and following wildfires, the understanding of this variable continues to be rather limited. This study presented the characterization of SWR from wild fire measurements in a Scots pine Peucedano-Pinetum forest in the Kampinos National Park (central Poland), which is characterized by a temperate continental climate. The main objectives were: [i] To evaluate the potential occurrence, intensity, and persistence of soil water repellency in the surface layers of podzolized rusty soils during a dry summer; [ii] to determine whether a wildfire increased SWR, compared to the unburnt condition of soil; and [iii] to identify changes in hydrophobicity 13 months after a fire. The Water Drop Penetration Time (WDPT) test was used to assess persistence and intensity of soil SWR. Hydrophobicity is a natural phenomenon during periods of drought in temperate continental climates. The extreme class of SWR was observed in surface layers of up to 20 cm. A higher hydrophobicity was noted in the older habitats of the Peucedano-Pinetum forest. Maximum WDPT values (10,800 s) were found for an older ecosystem cover, during a dry summer. SWR in fire-affected soils is dependent on the intensity of the fire, as well as displaying spatial and seasonal variability. Thirteen months after a fire, the highest variability in the occurrence of non-wettability, was recorded in the surface layers of areas affected by a weak fire. A positive relationship between soil pH and WDPT values was determined to a 20 cm depth. Prolonged dry periods resulting from global climate change, may enhance the effects of increasing SWR; it therefore seems reasonable for future research on biosphere–climate interactions, to take the presence of hydrophobicity into account.

Progressing climate change increases the frequency of droughts and the risk of the occurrence of forest fires with an increasing range and a dramatic course. The availability of water and its movement within an ecosystem is a fundamental control of biological activity and physical properties, influencing many climatic processes, whereas soil water repellency (SWR) is a key phenomenon affecting water infiltration into the soil system. Focusing on wide-spectrum effects of fire on the soil system, the research was conducted on a pine stand (Peucedano-Pinetum W. Mat. (1962) 1973) in Kampinos National Park located in central Poland, affected by severe and weak fires, as well as control plots. The main aim of the study was to examine the regeneration of the ecosystem 28 months after the occurrence of a fire. The effect of SWR and soil moisture content, total organic carbon, nitrogen and pH, and gain an understanding of the environmental conditions and processes that shaped the evolution of the species structure of soil microorganism communities (fungal vs. bacterial) have been examined. The Water Drop Penetration Time (WDPT) test was used to assess spatial variability of SWR in 28 plots. Soil bacterial and fungal communities were analysed by Illumina’MISeq using 16S rRNA and Internal Transcribed Spacers 1 (ITS1) regions in six selected plots. After a relatively wet summer, elevated hydrophobicity occurred in areas affected by a weak fire as much as 20 cm into the soil depth. The severe fire and subsequent increase in the richness of the succession of non-forest species contributed to the elimination of hydrophobicity. SWR was more closely linked to the structure and diversity of soil microbial communities than soil physicochemical properties that took place in response to the fire. A statistically significant relationship between the relative occurrence of microorganisms (≥ 1.0% in at least one of the samples) and SWR was established for the following fungi and bacteria species: Archaeorhizomyces sp., Leotiomycetes sp., Byssonectria fusispora, Russula vesca, Geminibasidium sp., family Isosphaeraceae and Cyanobacteria (class 4C0d-2, order MLE1-12). Insight into the functional roles of the individual identified microbial taxa that may be responsible for the occurrence of hydrophobicity was also presented.

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Soil water repellency can significantly degrade its agricultural utility and bring aboutnegative environmental consequences (i.e., reduced infiltration capacity, enhanced overland flow,increased erosion rates, and water infiltration occurred in irregular patterns). The presented studyaimed to establish whether excluding albic Podzols from agricultural production and theirspontaneous inhabitation by a pine tree stand affected their hydrophysical properties. Studies withthe application of the water drop penetration time (WDPT) test showed that a change in the landuse increased the potential water repellency of the surface layer (horizon A) and caused itschangeover from strongly repellent class (Class 2) to extremely repellent (Class 5). The relationshipbetween soil moisture content and wettability made it possible to determine the critical soil moisturecontent (CSMC) for the occurrence of the phenomenon of water repellency. It was confirmed thatthe CSMC value increased along with a change in use. For the site under arable use, it was 9–10vol.%, whereas for the site formerly under arable use and currently covered predominantly by apine tree stand, a value in the range of 14–16 vol.% was reached. A laboratory experiment on surfacerunoff of the soil formerly under arable use showed that over half of the rainfall may be transformedinto surface runoff as a result of occurring water repellency. This means that exceeding the criticalsoil moisture content makes the recharge of soil retention difficult and may significantly influencethe water balance of soil, as well as increasing its susceptibility to drought.

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.