Role of Soil Microstructure in Microbially-mediated Drying Resistance

The retention of soil moisture between rainfall or irrigation events is imperative to the productivity of terrestrial ecosystems. Amplified weather conditions are expected to result in widespread reduction in soil moisture. Extracellular polysaccharides (EPS) produced by soil bacteria have the ability to influence soil moisture by (i) retaining water directly within the hydrogel matrix, and (ii) promoting an aggregated soil structure. We have developed microfluidic devices that emulate realistic soil microstructures and enable direct observation of EPS production and drying resistance. The objective of this study was to compare moisture retention in emulated soil micromodels containing different soil microstructures. "Aggregated" devices contain a greater number of small (<30 μm) and large (>100 μm) pores, while "non-aggregated" devices contained more intermediate-sized (30-100 μm) pores. Particle-size distributions, similar to a sandy loam, were identical in both cases. Dilute suspensions of either of two strains of Sinorhizobium meliloti were introduced into replicate micromodels: one strain produced EPS ("EPS+") and the other did not produce EPS ("EPS-"). Loaded micromodels were equilibrated at saturated conditions, then dried at 83% RH for several days. Direct observation showed micro-scale patterns of air infiltration. The rate and extent of moisture loss was determined as a function of bacterial strain and microstructure aggregation state. Results showed devices loaded with EPS+ bacteria retained moisture longer than devices loaded with EPS- bacteria. Moisture retention by EPS+ bacteria was enhanced in aggregated versus non-aggregated microstructures. This work illustrates how moisture retention in soil is the result of microbial processes acting within pore-scale soil microstructures. Validated microfluidics-based approaches may help quantitatively link pore-scale phenomena to ecosystem function.