Opposing Surfactant and Gel Effects of Soil Borne‐Hydrogels on Soil Water Retention

Soil‐borne hydrogels (e.g., bacterial extracellular polymeric substances, root‐derived mucilage) contain polymeric (i.e., gel‐like) substances as well as surface active substances, which modify the viscosity of soil fluid and wetting properties of soil particles, respectively. In this study, the individual and coupled effects of gel and surfactant on soil water retention were investigated experimentally using xanthan and Triton X‐100 as surrogates for gel and surfactant, respectively. Results show opposing effects that xanthan enhanced soil water retention because of increased solution viscosity and its hygroscopic property while surfactant reduced soil water retention due to reduced surface tension. We applied the modified Kosugi model that considers non‐capillary adsorption to describe the soil water retention curves measured in this study as well as additional curves reported in the literature. The model effectively captured the gel and surfactant effects using two model parameters, ω and μm, representing the weighting factor for capillary retention and the matric potential required to drain the effective mean pore diameter, respectively. Compared with the controls, gel‐dominated samples increased non‐capillary adsorption whereas surfactant‐dominant samples had a smaller (i.e., less negative) matric potential at the onset of drainage. Finally, we provided an explanation for enhanced water retention due to increased solution viscosity considering fluid displacement process. Our study improved the mechanistic understanding of soil water retention as affected by viscosity and surface tension individually as well as their coupled effects. Key Points The effects of gel and surfactant as caused by soil‐born hydrogels were investigated using xanthan and Triton X100 as surrogate Xanthan increased soil water retention while Triton X100 reduced soil water retention The modified Kosugi model that considers non‐capillary adsorption was able to capture the gel and surfactant effects