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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 evaporation can be delayed by the complex biofilm producer B. subtilis. • An accelerated retreat of the evaporation front initiates decelerated water loss. • Reduced soil hydraulic conductivity and surface tension explain the process. • Modifications are result of a continuous adaptation. When soils dry, water flow and nutrient diffusion cease as the hydraulic microenvironments vital for soil life become fragmented. To delay soil drying locally and related adverse effects, bacteria and plants modify their surroundings by releasing extracellular polymeric substances (EPS). As a result, the physical properties of hotspots like biological soil crusts or the rhizosphere differ from those of the surrounding bulk soil. Specifically, the presence of EPS delays evaporative soil drying. Despite the evidence of reduced evaporation from EPS-amended soils, the mechanisms controlling soil water content dynamics remain elusive. Thus, our study aimed to elucidate the potential of bacteria to modify their local environment when exposed to oscillations in soil water content induced by evaporative drying. We incubated sand microcosms with two contrasting strains of Bacillus subtilis for one week in a flow cabinet. At the end of the incubation period, local water loss was quantified and spatially resolved using time-series neutron radiography. Strain NCIB 3610, a complex biofilm producer steadily modified soil evaporation dynamics during the incubation period resulting in a substantial delay in soil drying due to hydraulic decoupling of the evaporation front from the soil surface. Evaporation dynamics remained largely unaltered in the microcosms inoculated with the domesticated strain 168 trp+ compared to the control treatment. The mechanism of hydraulic decoupling induced by NCIB 3610 was verified by estimates of diffusive fluxes and the position of the evaporation plane in the microcosm. Additionally, the role of polymeric substances in hydraulic decoupling was confirmed by an evaporation experiment using xanthan as an EPS analogue. [ABSTRACT FROM AUTHOR]

Plant roots and bacteria are capable of altering their local soil environment by releasing a diverse, highly polymeric blend of substances (e.g. extracellular polymeric substances EPS and mucilage). Despite the importance of EPS and mucilage for plants and microorganisms 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 CT revealed that upon drying in porous media mucilage and EPS form one-dimensional 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. During drying the polymers are progressively stretched and their viscosity exponentially increases until a critical point when the polymers can no longer follow the receding air-liquid interface. At this critical point the polymers start to behave as a solid matrix that enhances the soil water retention and connectivity of the liquid phase. A model is introduced to predict the formation these one- and two-dimensional polymer structures in drying porous media. Viscosity of the soil solution, drying rate and specific soil surface are the key parameters determining the transition from one- to two-dimensional structures. The model also explains the apparent hysteresis in the relation between water content and water potential in soils embedded with root mucilage. 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 micro-hydrological niches where stable conditions for life are preserved. [ABSTRACT FROM AUTHOR]

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). Despite 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 CT revealed that upon drying in porous media mucilage (from maize 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 impact macroscopic hydraulic properties: i.e. they enhance water retention, preserve the continuity of the liquid phase in drying soils and decreases 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 micro-hydrological niches where stable conditions for life are preserved. [ABSTRACT FROM AUTHOR]

Despite detailed investigations on the distinct biochemical properties of the rhizosphere and their effects on the availability of nutrients for plants, their biophysical aspects, 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 coefficient of nutrients and consequently their transport in the soil and into the plant roots. Repeated phosphor imaging was used to monitor the temporospatial distribution of 137Cs (as an analog of K) within a modeled rhizosphere soil with and without mucilage (a sandy soil amended with mucilage extract from chia seed) under different soil water contents. The monitored profiles of activities were used to estimate the diffusion coefficient of soils with and without mucilage by solving a diffusion equation. Assuming an identical the effect of mucilage on diffusion coefficient of Cs and K, a diffusion-convection equation was numerically solved to predict the transport of K within the soil and its uptake by a single plant root during a soil drying cycle. To this end, the hydraulic and diffusive properties of the soil were parameterized based on the measured data and the K uptake and its concentration in soil were taken from literature data. The results of this study suggest that mucilage in the rhizosphere keeps the rhizosphere wetter and maintains the connectivity of the liquid phase during a soil drying cycle, and thereby could prevent a marked drop in the diffusion coefficient. The results of modeling of nutrient uptake by a single root showed that the presence of mucilage in the rhizosphere could favor nutrient uptake by the plant root. In the case of nutrients with low concentration in the soil solution, it prevents a marked concentration drop in the vicinity of the root as the soil dries and diffusion becomes restricted. This will delay the risk of nutrient deficiency to the plant root. In the case of nutrients with a high concentration in the soil solution, the presence of mucilage in the rhizosphere may delay the risk of salinity stress as the soil dries and the concentration of nutrients increases at the vicinity of the root surface. In conclusion, the results of this contribution show that mucilage may favor the transport of nutrient within the soil and their uptake by plant roots under drying soil condition. [ABSTRACT FROM AUTHOR]