(30f) Mechanisms Whereby Microbes Promote Intermediate Soil Moisture Content

The function of microbial communities is inextricably with the soil microstructure. At the pore scale, water content regulates hydraulic connectivity and thereby modulates microbes’ access to aqueous and gaseous substrates. Furthermore, soil bacteria near plant roots produce extracellular polymeric substances (EPS), a polymeric substance that helps modulate soil moisture and promote plant growth. EPS regulates soil moisture (i) by serving as hydrogel, swelling during wet conditions and remaining hydrated during dry conditions, (ii) by promoting soil particle aggregates via capillary forces, (iii) by altering soil surface properties through creating water repellent surfaces. EPS-mediated moisture retention behavior in soil is complicated but can be understood by systematically controlling the microscale environment. Here, we show the effect of physical microstructure on EPS-mediated moisture retention. We employ physically complex emulated soil micromodels as well as micromodels with uniform macropore or micropore geometries. EPS was collected from stationary-phase Sinorhizobium meliloti cultures and suspended at different concentrations in growth media salts or in artificial groundwater. Experimental results showed that the 0.25× EPS solution dried eight times slower than deionized water, and the 1× EPS solution dried 16 times slower than deionized water in identical soil micromodels. Also, the residual saturation of 0.25× and 1× EPS solutions were 6 ± 1% and 35 ± 21%, compared with a residual saturation of 0 for deionized water. We observed no effect on rate or extent of moisture loss for the micromodels with a uniform macropore geometry. Studies with uniform micropore geometry confirmed that higher EPS concentrations were associated with a reduce rate and increased variance of moisture loss at the pore scale. In this study, we provide a systematic method to evaluate the performance of EPS solutions in modulating soil moisture, and show how EPS concentration decreases the rate of water loss and also amplifies the variability of moisture content at the pore scale. Longer term, we anticipate that the impact of this work will be to aid in the development of sustainable agriculture biotechnology for increased food production using less water.