Effect of matric potential and soil-water-hydrogel interactions on biohydrogel-induced soil microstructural stability

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.