Influence of Physical Microstructure and Surface Properties on the Drying Rate of Water from Simulated Soil Micromodels

Bacteria have been shown to regulate moisture content in soil through the secretion of extracellular polysaccharides (EPS), a polymeric substance whose properties vary with bacterial species and environmental conditions. The presence of small amounts of EPS have been shown to significantly increase water retention in the rhizosphere soil compared with nearby bulk soil that is lacking EPS. The mechanism whereby EPS promotes increased moisture content may include (i) water being held within the swelling polymer matrix during wet conditions and remaining hydrated during dry soil conditions, (ii) promotion of soil particle aggregation which increases the capillary forces on water in small intra-particle soil pores, and (iii) altered soil surface properties, including soil water repellency. Studies conducted using bulk soil may struggle to differentiate among these mechanisms. Here we employed emulated soil micromodels with systematically controlled EPS effects such as soil structure and surface properties to better elucidate the mechanism for EPS-mediated water retention in the rhizosphere. Micromodels were fabricated with aggregated or non-aggregated structures by modifying the spatial distribution of an identical sandy loam-sized particle arrangement between micromodel designs. Micromodels of each design were then surface-treated to exhibit properties of rhizosphere versus bulk soil. Micromodels were initially fully saturated and then allowed to dry in a temperature and humidity controlled environment. Results showed that micromodels with more hydrophobic surfaces took four times longer to completely dry compared to micromodels with less hydrophobic surfaces. Furthermore, microstructure geometry was found to control the spatial distribution of moisture within soil micromodels. These results illustrate the mechanisms whereby soil structure and surface properties influences the drying process in the rhizosphere. Future studies should focus on the effects that different chemical constituents found in bacterial secretions may have on moisture retention using this emulated soil micromodel approach.