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Mucilage produced at the root tips is a soil-born biohydrogel, whose framework is a three-dimensional polysaccharidic polymer network, which can contain over 90% water. The specific biohydrogel properties of mucilage, such as volumetric expansion and shrinkage, affect soil mechanical and hydraulic properties. Still, the physico-chemical mechanisms governing the interactions between mucilage and the porous soil system remain mostly unclear. To our best knowledge, no currently applied method allows the distinction between biohydrogel phases and pore water in the porous soil system. In this work, we used 1 H NMR relaxometry to analyze the presence and properties of biohydrogels in soil. Mucilage in soil leads to a hierarchical pore structure, consisting of the polymeric biohydrogel network surrounded by the surface of soil particles. Water molecules entrapped in mucilage-containing soils revealed an accelerated bulk relaxation and a higher surface relaxivity in comparison with soils not containing mucilage. In model soils, we quantified the gel effect, here defined as the influence of mucilage on proton relaxation. The difference between transversal and longitudinal relaxation rates was plotted as a function of the reciprocal diameter of the model soil particles for soils containing and not containing mucilage. The gel effect was thereby characterized by an accelerated bulk relaxation and an accelerated surface relaxation , traduced respectively by an increased y-intercept and an increased linear coefficient for mucilage-containing soil. [ABSTRACT FROM AUTHOR]

Abstract Soil-born exudates such as mucilage are known to affect soil physicochemical properties. Characterization of the gel properties of mucilage at the pore-scale is necessary to gain mechanistic understanding of the underlying processes leading to changes of soil properties. Yet, mucilage intrinsic properties' complicate its in - situ detection. Longitudinal and transverse magnetic relaxation rates measured with 1H Nuclear Magnetic Resonance (NMR) relaxometry have the potential to study mucilage-water interactions in - situ as they are sensitive to restricted molecular motion of water protons in biohydrogels. However, the relations between water mobility and biohydrogel properties in porous media have remained unknown until now. In this study, the mobility of water molecules in chia seed mucilage in porous systems was systematically investigated by means of 1H NMR relaxometry. Chia seed mucilage was used as it has hydrogel properties shared by a range of biological hydrogels found in soil. Glass beads of several sizes were used to study the influence of the pore size on the NMR signal. A conceptual model based on the equations describing the relaxation of water protons in porous media was developed to integrate these gel effects into the NMR parameters. The increased rigidity of the polymer network and its organization in the pore space, which depended on the particle size and the mucilage concentration, were assessed as the gel effects significantly affecting the bulk relaxation. Our approach, which combines the use of NMR along with other imaging methods, is a promising strategy to detect and characterize the properties of biohydrogel in porous media. Highlights • 1H NMR relaxometry can characterize quantitatively mucilage in model soils. • Effect of mucilage on water mobility in model soils depends on particle size. • Several gel effects affect water mobility in mucilage-treated model soils. • Spider-web effect attests additional rigidity of the polymer network in the pore space. [ABSTRACT FROM AUTHOR]

• How does soil drainage affect biohydrogel-induced soil microstructural stability? • 1H NMR and rheometry to assess biohydrogel-induced particle-particle interactions. • Combined effects of soil drainage and soil composition on biohydrogel swelling. • Soil drainage and polymer-clay interactions promote soil particle gluing. Soil structure formation and its stability against external stress ensure sufficient water, air and nutrient supply for plants under varying environmental conditions. In this context, soil-born biohydrogels glue soil particles together and increase structural stability and water retention via the formation of swollen interparticulate hydrogel structures. However, interparticulate hydrogel behavior in soil under fluctuating water potentials remains still unclear. We addressed this by treating loamy sand and clayey loam with alginate, a hydrogel-forming biopolymer, at various concentrations and adjusted them to matric potentials of ψ = −0.3 kPa, −3.2 kPa and −6.3 kPa. For both soils, we assessed soil-water interactions in terms of mobility, distribution and freezability of water by 1H nuclear magnetic resonance relaxometry and differential scanning calorimetry. The measurements on water entrapment were complemented with the characterization of soil microstructural stability using rheometry (amplitude sweep tests). Alginate hydrogel increased soil microstructural stability and shifted pore-size distributions towards smaller pore sizes with more restricted water mobility. Interestingly, alginate-induced microstructural stability remained constant or further increased with decreasing matric potential, although the effect of alginate on the entrapment and mobility of water decreased with decreasing matric potential for both soils. Moreover, the direction and intensity of alginate-derived effects differed between both soils as water entrapment increased for the loamy sand and decreased for the clayey loam, respectively. This effect was attributed to the mutually restricted swelling of alginate and clay particles as result of various polymer-clay interactions in the clayey loam, which increased the relative amount of less strongly bound water in the soil matrix. The results indicate that the gel effect is composed of several components that strongly depends on various intrinsic and extrinsic factors, including the properties of the hydrogel-forming biopolymer itself and the complex interplay with the available water and mineral surfaces in soil. [ABSTRACT FROM AUTHOR]