Effects of Soil Structure and Bacterial EPS on Moisture Retention in Soil Emulating Microfluidic Devices

Soil bacteria are capable of regulating moisture within soils through secretion of extracellular polysaccharides (EPS). In soil, EPS forms a hydrogel and promote an aggregated soil structure. Aggregated soil has a distribution of pore sizes and promotes intermediate moisture conditions. EPS also modulates water by holding moisture in the hydrogel then slowly releasing it during drying conditions.  Here, drying rates of bacterial suspensions were measured in microfluidic devices with pseudo-2D structures emulating different soil structures. Microchannel post dimensions emulating a sandy loam soil were either randomly distributed or collected into aggregated structures. All microfluidic devices were initially saturated with either artificial ground water (AGW) alone or bacteria suspensions in AGW. Two types of bacteria were used: EPS producing bacteria (“EPS+”) and EPS non-producing bacteria (“EPS-”). Moisture retention was measured optically at 80% relative humidity. We found that EPS+ solutions retained moisture twice as long as EPS- solutions in non-aggregated structures. EPS+ suspensions dried at similar rates in aggregated and non-aggregated devices from 100% to until 20% water saturation, then the aggregated structure held moisture better.  Results suggest that the aggregation of soil particles and presence of EPS help retain water. The use of a microfluidic device to observe soil water retention provides a way to replicate intricate soil microstructures; enabling scientists to test effects of different polymers and their concentration on soil water retention.