Abstract
AbstractPlant roots and bacteria alter the soil physical properties by releasing polymeric blends into the soil pore space (e.g., extracellular polymeric substances and mucilage). The physical mechanisms by which these substances interact with the soil matrix and alter the spatial configuration of the liquid phase and the related hydraulic properties remain unclear. Here, we propose a theory to explain how polymer solutions form one‐dimensional filaments and two‐dimensional interconnected structures spanning across multiple pores. Unlike water, primarily shaped by surface tension, these polymeric structures remain connected during drying due to their high viscosity. The integrity of one‐dimensional structures is explained by the interplay of viscosity and surface tension forces (elegantly characterized by the Ohnesorge number), while the formation of two‐dimensional structures requires consideration of the interaction of the polymer solution with the solid surfaces and external drivers (e.g., drying rate). During drying, the viscosity of the liquid phase increases and at a critical point, when the friction between polymers and solid surfaces overcomes the water absorption of the polymers, the concentration of the polymer solution at the gas‐liquid interface increases asymptotically. At this critical point, polymers are deposited as two‐dimensional surfaces, such as hollow cylinders or interconnected surfaces. A model is introduced to predict the formation of such structures. Viscosity of the soil solution, specific soil surface, and drying rate are the key parameters determining the transition from one‐to two‐dimensional structures. Model results are in good agreement with observed structures formed in porous media during drying.
Highlights
And spatially confined, the rhizosphere is crossed by immense volumes of water (Bengough, 2012) while plant roots and microorganisms enhance biogeochemical fluxes, which turn this thin layer of soil around the roots into a distinct example of a hotspot in soil (Kuzyakov & Blagodatskaya, 2015)
The integrity of one-dimensional structures is explained by the interplay of viscosity and surface tension forces, while the formation of two-dimensional structures requires consideration of the interaction of the polymer solution with the solid surfaces and external drivers
Prominent among these substances are highly polymeric substances, such as EPS and mucilage, which alter the physical properties of the soil solution (Naveed et al, 2019; Read & Gregory, 1997; Read et al, 1999; Stoodley et al, 2002)
Summary
And spatially confined, the rhizosphere is crossed by immense volumes of water (Bengough, 2012) while plant roots and microorganisms enhance biogeochemical fluxes, which turn this thin layer of soil around the roots into a distinct example of a hotspot in soil (Kuzyakov & Blagodatskaya, 2015). The polymeric blends released from the plant and bacterial species are highly diverse (Flemming & Wingender, 2001; Naveed et al, 2017), yet, they appear to share key physical traits which alter the forces holding water in soils Their high polymer content increases the viscosity of the liquid phase (Flemming & Wingender, 2001, 2010; Naveed et al, 2017; Stoodley et al, 2002), polymers can form a network (Flemming & Wingender, 2010; McCully & Boyer, 1997; Roberson et al, 1993) capable of absorbing and holding water (Flemming & Wingender, 2001; McCully & Boyer, 1997; Read et al, 1999; Roberson & Firestone, 1992; Segura-Campos et al, 2014), and surfactants among exuded polymers decrease the surface tension at the gas-liquid interface (Raaijmakers et al, 2010; Read et al, 2003). These three physical properties improve the connectivity of the liquid phase by avoiding the breakup of liquid connections in drying soils, resulting in the formation of long persistent strands between distant particles and wider hollow cylinders
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