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.