(24d) Pore-Scale Effects of Dilute Organic Macromolecules and Nanoparticles to Inhibit Moisture Loss

The retention of soil moisture is a complex and dynamic process, influenced by physical, chemical, and biological processes all interacting at the pore scale. For example, the function of microbial communities is inextricably linked with soil microstructure, such as when microbes secrete extracellular polymeric substances (EPS) to regulate soil moisture and promote plant growth. EPS regulates moisture three ways: it serves as a hydrogel (swelling during wet conditions and remaining hydrated during dry conditions), it alters soil surface properties (creating more water-repellent surfaces), and it promotes aggregation of soil particles. EPS-mediated moisture retention can be understood by systematically controlling the aqueous composition and the microscale environment simultaneously. Here, we show the combined effects of physical microstructure and EPS concentration on retention of soil moisture in different PDMS micromodels. We employed a physically complex emulated soil micromodel to evaluate the spatial distribution of water under drying pressure versus purified EPS concentration. Here, the 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 15 µg/mL of EPS solution dried eight times slower than deionized water, and the 58 µg/mL of EPS solution dried 16 times slower than deionized water in identical soil micromodels while the residual saturation of 15 µg/mL and 58 µg/mL of EPS solutions were 6 ± 1% and 35 ± 21%, compared with a residual saturation of 0 for deionized water. Studies using colloidal agro-additives including graphene oxide and biochar showed the degree of oxidation might effect on moisture retention. In this project, we build a systematic platform to evaluate pore-scale effects of dilute organic macromolecules and organic nanoparticles on inhibiting moisture loss. We show that relatively low EPS concentration dramatically reduce the rate of water loss, increase steady state moisture content, and enhance the variability of moisture distribution. We anticipate this work will impact the future development of sustainable agriculture biotechnology for increasing food production while using less water.