Mechanisms of Pore-scale Moisture Retention: Effects of Physical Structure, Surface Chemistry and Pore Fluid Composition

The rhizosphere is the region of soil directly under the influence of plant roots. In this region, dynamic interactions between plant roots and microorganisms alter the microenvironment, and in turn, are altered by the microenvironment. In rhizosphere soil, key physicochemical properties including physical structure, surface chemistry, and pore fluid composition vary considerably from those in bulk soil. Due to its intrinsic complexity, it is difficult to de-couple the many physical, chemical, and biological factors that promote moisture retention in the rhizosphere. This research project aims to better understand moisture evaporation (rate and extent) and the spatial distribution of water within unsaturated porous media by systematically controlling essential factors acting at the pore-scale. Emulated soil micromodels (ESMs) were designed to mimic the physical structure of both aggregated (rhizosphere) and non-aggregated (bulk) sandy loam soil. First, the effect of physical structure and surface chemistry were examined. Physically identical ESMs were treated to reflect both lower and higher water repellency conditions of typical rhizosphere and bulk soil, respectively. Next, the effect of pore-fluid compositions on moisture regulation was examined. Here we tested the drying behaviors of different solutions of extracellular polymeric substance (EPS) in different chamber geometries. EPS is produced by rhizosphere bacteria and is composed primarily of carbohydrate polymers. EPS swells to promote microbial access to dissolved constituents. EPS also alters soil surface chemistry by increasing water repellency. Some EPS solutions were produced in situ by optogenetically controlled bacteria, and other EPS solutions were collected and purified from bulk bacterial cultures, characterized, and then inserted into chambers with well-defined geometry. Drying and rheological studies pointed to interactions with divalent cations whereby EPS exhibits gel-like properties and modulates properties of the air/water interface, suggesting another mechanism for moisture retention in the rhizosphere. Through the combination of systematic measurements of key factors, we aim to build up a better understanding of pore-scale mechanisms of moisture retention and guide the development of future agriculture biotechnology to achieve greater sustainability and resiliency of agroecosystems.