More frequent and longer drought periods are predicted threatening agricultural yield. the capacity of soils to hold water is a highly important factor controlling drought stress intensity for plants. Biogenic amorphous silica (bASi) pools in soils are in the range of 0-6% and are suggested to help plants to resist drought. In agricultural soils, bASi pools declined to values of ~1% or lower) due to yearly crop harvest, decreasing water holding capacity of the soils. Here, we assessed the contribution of bASi to water holding capacity (WHC) of soil. Consequently, ASi was mixed at different rates (0, 1, 5 or 15%) with different soils. Afterwards, the retention curve of the soils was determined via Hyprop method. Here we show that bASi increases the soil water holding capacity substantially, by forming silica gels with a water content at saturation higher than 700%. An increase of bASi by 1% or 5% (weight) increased the water content at any water potential and plant available water increased by up to > 40% or > 60%, respectively. Our results suggest that soil management should be modified to increase bASi content, enhancing available water in soils and potentially decreasing drought stress for plants in terrestrial ecosystems.

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.

<p>Soils are the largest sink of microplastic particles (MPP) in terrestrial ecosystems. However, there is little knowledge on the implication of MPP contaminating soils. In particular, we don’t know how MPP move and, on the other hand, how they affect soil hydraulic properties and soil moisture dynamics.</p><p>Among the expected effects of MPP on soil hydraulic properties is the likelihood that MPP enhances soil water repellency. This emerges from (1) the MPP surface chemical properties as well as (2) their surface physical properties like size and shape. Here, we tested mixtures of MPP and a model porous media. The Sessile Drop Method was applied and apparent contact angles were measured. We are able to show enlarged contact angles with rising concentrations of MPP. Already in relatively low concentrations of MPP the contact angels exhibit a steep increase and are rapidly reaching areas of super-hydrophobicity. Furthermore, we provide the physical explanation of the apparent contact angles resulting from the three-phase contact line between solid composite surfaces, water and air. The considered modes of a droplet lying on a surface are Wenzel, Cassie-Baxter and Young. The goal here was to differentiate between the involved surfaces building up the apparent contact angle and to pin down the impact of MPP in these systems.</p><p>Thinking about the implications of these results, an increased water repellency alters soil hydraulic properties towards less water content resulting in a shift in the water retention curve. Less water in soils especially at sites of high MPP concentrations leads to a limitation of degradation of MPP by hydrolysis. Additionally, microorganisms themselves and their enzymes cannot migrate in the liquid phase towards the MPP even elongating the process of natural purification.</p>

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<p>Soils are considered the largest sink of microplastic particles (MP) in terrestrial ecosystems. However, there is little knowledge on the implications of MP contaminating soils. In particular, we do not know the extent of and conditions under which MP are transported through porous media and, if they are deposited, how they affect soil hydraulic properties and soil moisture dynamics. We hypothesize that: 1) hydrophobic MP enhance soil water repellency; 2) isolated MP are displaced and transported by the air-water interface; 3) clusters of MP impede water flow and are retained in air-filled pores.</p><p>We tested these hypotheses in mixtures of MP (µm range) and sands (mm range) in a series of experiments. The Sessile Drop Method (SDM) was applied to measure the average contact angle (CA) of the mixtures for MP and model porous media in the same size range, ranging from 0 - 100 % MP content. Based on the specific surface and shape factor of MP and soil particles, the results are extrapolated to different MP and soil particle sizes. Capillary rise experiments were performed to measure the impact of MP on water infiltration. The applied MP contents of 0.35 % and 1.05 % reflect an average CA of 60° and 90° from the SDM extrapolation. Capillary rise of water and ethanol were carried out to estimate the apparent CA. Additionally and with the same MP content, we simultaneously imaged in three-dimensions the movement of deuterated water and MP during repeated drying / wetting cycles using X-Ray and Neutron tomography (at the beamline ICON, PSI). The different neutron attenuation coefficients of deuterated water and MP allows for estimating their distribution in the sand packing.</p><p>Already at MP contents of 5 % the CA measured with the SDM exhibited a steep increase and reached 59° to 81°, depending on the grain size of MP. The capillary rise experiments showed that MP reduce capillary rise. The apparent CA (43° and 53°) were smaller compared to the average CA from the SDM (60° and 90°), but the added MP increased air entrapment during capillary rise leading to a reduced saturation of the pore space (18 % and 16.5 %). Accumulation of MP at the advancing air-water interface was visible. Neutron and X-ray imaging showed at high resolution that regions with major MP content are water repellent and, are bypassed by water flow, and remain in air-filled pores.</p><p>Extrapolation of these results to soils suggests that in microregions with high MP contents, water infiltration is hindered. The low water content in these microregions might limit MP degradation due to reductions in: hydrolysis, coating of MP by e.g. dissolved organic substances, and colonization by microorganisms.</p>

<p>Pathways of Microplastic (MP) into ecosystems are manifold and range from agricultural mulching practices to atmospheric deposition with soil being considered the largest sink of MP in terrestrial ecosystems. Once deposited there, MP is posing a hydrophobic surface addition. Former experiments showed that pristine MP can cause lower water saturation of pore spaces and so change the liquid configuration within a porous network. If water cannot reach MP, biotic degradation might be hindered. However, in natural soil systems MP can be coated over time by soil abundant substances e.g., iron compounds with the potential effect of decreasing their hydrophobicity. We hypothesize that: 1) ferrihydrite pre-coated MP shows reduced hydrophobicity; 2) in-situ wetting and drying cycles with ferrihydrite leads to partial coating of MP.</p><p>We tested these hypotheses by applying hotspots of MP, pre-coated and pristine, to sand in rectangular columns and performed neutron imaging during capillary rise. Neutron imaging allowed for visualizing and quantifying liquid dynamics and configuration. Water was used for the pre-coated MP (n=6) variants and ferrihydrite suspension (100 mg L<sup>-1</sup>) in three wetting and drying cycles for the pristine MP (n=6) variants. The utilized MP are polystyrene (PS, 20-75 µm) and polyethylene terephthalate (PET, 20-75 µm). The grain size of sand was 0.7-1.2 mm. Pre-coating was achieved by shaking the raw material for 3 h in a 100 mg L<sup>-1</sup> ferrihydrite suspension and subsequent drying in a sieve supported by a vacuum pump.</p><p>Capillary rise of water into pristine MP variants exhibited zero water saturation at the hotspot and water movement around the MP aggregation was observed. Capillary rise of water into pre-coated MP variants differ in result by polymer type. While pre-coated PS is still hydrophobic, the pore space of pre-coated PET was completely water saturated. The rising water accelerated towards the hotspot due to its lower matric potential compared to sand.</p><p>Capillary rise of ferrihydrite suspension in wetting and drying cycles also showed varying results according to polymer type. While there is no effect on water saturation on PS in the hotspot after three wetting cycles, PET exhibits a slightly higher water saturation during the second wetting but stagnating in the third.</p><p>Our results suggest that ferrihydrite coating, being only one of numerous potential coating agents, can bond to MP and change its surface polarity. Differences in completeness of coating can be explained by inherent chemical and physical properties of different polymer types. But once hydrophilic, completely, or only part of the surface, water flow induced colonization and migration of microorganisms and their enzymes can proceed and biotic degradation can take place. The open question lies within the time frame necessary to overcome MP’s inherent hydrophobicity.</p>

Addition of microplastics (MP) to soil has the potential to increase soil water repellency. Absorption of soil abundant substances on MP surfaces has the potential to overcome MP inherent hydrophobicity.

<p>Soil is considered the largest sink of microplastics (MP) in terrestrial ecosystems. Among the expected effects of MP as hydrophobic surface addition is the likelihood that MP enhances soil water repellency. So, crucial for MP fate in soils is the interaction between MP and water. If MP is translocated by water flow and, vice versa, MP impacts water flow, to what extent? Water flow on the pore scale will be impacted with feedbacks on transport and retention of MP. However, we don’t know the extent of and conditions under which MP are transported through porous media and, if deposited, how they interplay with soil water dynamics. We hypothesize that: (i) isolated MP are displaced and translocated by air-water interfaces and (ii) local accumulation of MP is facilitated by bypassing water flow. To approach this question, neutron and x-ray imaging of MP and water in soils was utilized.</p> <p>Dual neutron and x-ray imaging at the beamlines ICON (Paul-Scherrer-Institute) during repeated wetting-drying cycles was applied to trace MP-water interactions in aluminum cylinders filled with sand (0.7-1.2 mm) and MP (PET, 20-75 µm) in gravimetric contents of 0.35, 1.05 and 2.10%. The contents refer to static contact angle estimations of the mixtures resembling < 90°, 90° and > 90°. First, simultaneous neutron and x-ray tomography captured the initial dry MP configuration in samples. Subsequently, neutron radiographies of deuterated water flow through the sample of 1 ml min<sup>-1</sup> were recorded for 200s. After drying, repeated tomography gave insights into MP translocation.</p> <p>Neutron and x-ray imaging results showed that regions of major MP content are water repellent. Water flow bypasses and MP is mainly retained. Resultant air entrapments lead to reduced water contents. In regions of minor MP content water can infiltrate. Here, the air-water interface collects isolated MP and shifts their distribution towards an enhanced accumulation.</p> <p>Extrapolation of these results to natural soil systems suggests that vertical transport of MP can be limited especially at hotspots of high MP contents. Water bypasses here. This might limit the water dependent degradation processes of MP due to reductions in hydrolysis, coating and colonization by microorganisms even elongating the process of natural attenuation.</p>