Impact of Hydrogel Content On Water Retention in Soil Micromodels

Many bacteria in soils are found as biofilms attached to the solid surfaces of soil particles by self-made extracellular polymeric substance (EPS).  EPS protects microbes in biofilms from abrupt changes in hydration and promotes aggregation of soil particles.  While both soil structure and bacterial production of EPS are known to individually impact water retention, prior work has not examined these effects simultaneously on a relevant spatial scale.  Here, a microfluidic device was designed to imitate the physical geometry of aggregated soil particles in order to observe, in real-time, the combined effect of EPS content and soil structure on water retention in porous media.  Experiments were performed using EPS produced by the nitrogen-fixing soil bacterium Sinorhizobium meliloti in addition to agar and agarose as model hydrogels.  Flow cells loaded with multiple concentrations of agarose were dried at different controlled humidities to examine the effect of hydrogel content on onset of drying and on drying rate.  As expected, an increase in humidity caused a significant delay in the onset of drying. It was hypothesized that increased agarose concentration should also reduce the drying rate, but experimental results were inconsistent.  Recommendations are provided for future work aimed at examining the combined effect of EPS content and soil structure on water retention in porous media, with the ultimate goal of improving models of moisture retention in soil.