Microbial particle size mediated effects of transient flow on the transport of bacteria and viruses through unsaturated soil

Understanding microbial transport in the vadose zone under transient flow conditions (such as irrigation and rainfall) is crucial for quantifying soil microbial behaviors, regulating the microbe-mediated belowground processes, and reducing the spread of pathogenic bacteria in soil environment. In this study, we investigated the impact of transient flow on the co-transport of a model bacterium (E. coli 652T7) and a model virus (MS2) in unsaturated soils. More E. coli 652T7 and MS2 broke through the soil under the transient flow condition than under the steady-state flow conditions. At low ionic strength (5 mM), the facilitating transient flow effect was 1.5 times larger for the microsized E. coli 652T7 than for the nanosized MS2 in terms of the maximum relative concentrations of the microorganisms in the effluent. However, this particle size dependent effect of transient flow diminished after the solution ionic strength increased from 5 mM to 20 mM with the increased size of the microorganisms (from 0.74 ± 0.09 µm to 1.54 ± 0.19 µm for E. coli 652T7 and from 0.28 ± 0.09 µm to 0.54 ± 0.09 µm for MS2). This result is attributed to the counteracting effects of electrostatic forces (increasing microbial retention as ionic strength increases) and hydrodynamic forces (decreasing microbial retention due to scouring by mobile air-water interfaces). In addition, the transient flow facilitated transport was 1.8 times larger for the aggregated MS2 under 20 mM than the non-aggregated MS2 under 5 mM. The data indicate that the mobility of colloids with size of 560 nm is most sensitive to the change in hydrodynamic forces. Our research results imply that wetting and drying cycles could significantly change the spatial patterns of microbial community in soils.