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Soils are considered the largest sink of microplastic (MP) in terrestrial ecosystems. However, little is known about the implications of MP on soil physical properties. We hypothesize that low wettability of MP induces soil water repellency, depending on MP content and size of MP and soil particles. We quantified wettability of mixtures of MP and sand. The sessile drop method (SDM) was applied to measure static contact angle (CA) of MP and glass beads at contents ranging from 0 to 100% (w/w). The results are extrapolated to varying combinations of MP and soil particle sizes based on specific surface area. Capillary rise was imaged with neutron radiography quantifying the effect of MP on dynamic CA, water imbibition, and water saturation distribution in sand. At 5% (w/w) MP content, static CA exhibited a steep increase to 80.2° for MP 20–75 μm and 59.7° for MP 75–125 μm. Dynamic CAs were approximately 40% lower than static CAs. Capillary rise experiments showed that MP 20–75 μm reduced water imbibition into sand columns (700–1,200 μm), with average dynamic CA of 40.3° at 0.35% (w/w) MP content and 51.8° at 1.05%. Decreased water saturation and increased tortuosity of flow paths were observed during imbibition peaking at 3.5% (w/w) local MP content. We conclude, in regions with high MP content. water infiltration and thus MP transport are hindered. Local low wettability induced by MP is expected to limit soil wettability and impede capillary rise. Core Ideas: There is high awareness of microplastic contaminating environments raising concern worldwide.Microplastic is found in terrestrial systems, but little is known about soil environments.Microplastics are inherently hydrophobic and represent a hydrophobic surface addition to soil.Consequently, they are potentially increasing soil water repellency.We present an analysis of microplastic within porous media and show effects on water dynamics. [ABSTRACT FROM AUTHOR]

Soil water repellency is traditionally expressed as contact angle (CA) and measured destructively on exposed surfaces or derived from flow behavior or measured forces. These approaches cannot map local heterogeneities in CA, which typically exist in intact soil. Here, we explore the potential and limitations of in situ measurements of three‐dimensional CAs with simplified benchmark tests and in intact sand. The method is based on a protocol for the freely available OpenFOAM software and applied to segmented X‐ray computed tomography (X‐ray CT) data. This study scrutinizes its suitability under typical vadose zone conditions, as it was originally developed for rock studies in petroleum engineering. The assets of the method are that it allows for accurate measurement of locally varying, equilibrium contact angles and that they can be linked to pore scale features that cause them. This is demonstrated with rehydrated mucilage structures in sand and the associated change in local CAs, where they touch sand grains. In general, the image acquisition time of polychromatic X‐ray CT (minutes) precludes the assessments of dynamic CAs that may equilibrate within seconds. Another limitation is that acute CAs are overestimated due to the voxel discretization of interfaces and contact lines in combination with image smoothing. The divergence arises around 60° and is most severe in the limit of vanishing water repellency. In summary, the method enables the mapping of local heterogeneities of equilibrium CAs, though the absolute values should be critically assessed. Core Ideas: An in situ 3D contact angle method based on X‐ray CT is introduced to vadose zone hydrology.Local heterogeneity in equilibrium contact angles is captured and linked to pore‐scale features.The method has limited capability to measure dynamic or acute contact angles. [ABSTRACT FROM AUTHOR]

Here, we present our collective musings on soil research challenges and opportunities and, in particular, the points raised by Philippe Baveye (Baveye, 2020a, 2020b) and Johan Bouma (Bouma, 2020) on I bypass i and I hyperbole i in soil science. Furthermore, developing a healthy and constructive post-publication peer-review system, where bypasses and hyperbolic approaches can be identified and discussed, would ultimately boost publication quality and contribute to a more open discourse in soil science. [Extracted from the article]

Take home messageMucilage secreted by roots and EPS produced by microorganisms alter the physical properties of the soil solution and impact the water dynamics in the rhizosphere. The high viscosity of mucilage and EPS is responsible for the formation of thin filaments and interconnected thin lamellae that span throughout the soil matrix maintaining the continuity of the liquid phase across the pore space even during severe drying. The impact of these mechanisms on plant and microorganisms needs to be explored. [ABSTRACT FROM AUTHOR]

Core Ideas: Our aim was to test whether mucilage promotes diffusion of nutrients in dry soil.Mucilage favors transport of nutrients in drying soil and their uptake by plant.Mucilage increases the soil moisture in the rhizosphere as soil dries.Mucilage maintains the connectivity of liquid phase in the rhizosphere as soil dries. Despite detailed investigations of its distinct biochemical properties and their effects on the availability of nutrients for plants, the biophysical aspects of the rhizosphere, particularly the effect of mucilage on the transport of water and nutrients, are poorly understood. The aim of this study was to investigate the effect of mucilage on the diffusion of nutrients and consequently their transport through the rhizosphere into the plant roots. Phosphor imaging technique determined the temporospatial distribution of 137Cs in a model rhizosphere (a sandy soil mixed with chia seed (Salvia hispanica L) mucilage. The observed profiles of activities were used to estimate the diffusion coefficient of K in soils. A diffusion–convection equation was numerically solved to predict the transport of K and its uptake by a single plant root in drying soil. The results suggest that mucilage (i) keeps the rhizosphere wet and (ii) maintains the connectivity of the liquid phase in drying soil. In these ways, mucilage moderates the drop in diffusive transport. The modeling results showed that the presence of mucilage in the rhizosphere (i) prevents depletion of nutrients in soils with a low nutrient concentration in the soil solution and (ii) delays the risk of nutrient and/or salt accumulation in the vicinity of the root in soils with a high concentration nutrient and/or salt the soil solution. In conclusion, mucilage appears to mitigate the risk of nutrient deficiency and salinity stress as it enhances the diffusive transport in drying soil. In this way, mucilage may favor the transport of nutrients within the rhizosphere and their uptake by plant roots in drying soil. [ABSTRACT FROM AUTHOR]

Core Ideas: Plant mucilage and bacterial extracellular polymeric substances (EPS) prevent the breakup of the soil liquid phase.Formation of continuous structures buffers soil hydraulic properties.The release of viscous polymeric substances represents a universal strategy. Plant roots and bacteria are capable of buffering erratic fluctuations of water content in their local soil environment by releasing a diverse, highly polymeric blend of substances (e.g. extracellular polymeric substances [EPS] and mucilage). Although this concept is well accepted, the physical mechanisms by which EPS and mucilage interact with the soil matrix and determine the soil water dynamics remain unclear. High‐resolution X‐ray computed tomography revealed that upon drying in porous media, mucilage (from maize [Zea mays L.] roots) and EPS (from intact biocrusts) form filaments and two‐dimensional interconnected structures spanning across multiple pores. Unlike water, these mucilage and EPS structures connecting soil particles did not break up upon drying, which is explained by the high viscosity and low surface tension of EPS and mucilage. Measurements of water retention and evaporation with soils mixed with seed mucilage show how these one‐ and two‐dimensional pore‐scale structures affect macroscopic hydraulic properties (i.e., they enhance water retention, preserve the continuity of the liquid phase in drying soils, and decrease vapor diffusivity and local drying rates). In conclusion, we propose that the release of viscous polymeric substances and the consequent creation of a network bridging the soil pore space represent a universal strategy of plants and bacteria to engineer their own soil microhydrological niches where stable conditions for life are preserved. [ABSTRACT FROM AUTHOR]