Biofilm effect on soil hydraulic properties: Experimental investigation using soil‐grown real biofilm

Abstract

Understanding the influence of attached microbial biomass on water flow in variably saturated soils is crucial for many engineered flow systems. So far, the investigation of the effects of microbial biomass has been mainly limited to water‐saturated systems. We have assessed the influence of biofilms on the soil hydraulic properties under variably saturated conditions. A sandy soil was incubated with Pseudomonas Putida and the hydraulic properties of the incubated soil were determined by a combination of methods. Our results show a stronger soil water retention in the inoculated soil as compared to the control. The increase in volumetric water content reaches approximately 0.015 cm3 cm−3 but is only moderately correlated with the carbon deficit, a proxy for biofilm quantity, and less with the cell viable counts. The presence of biofilm reduced the saturated hydraulic conductivity of the soil by up to one order of magnitude. Under unsaturated conditions, the hydraulic conductivity was only reduced by a factor of four. This means that relative water conductance in biofilm‐affected soils is higher compared to the clean soil at low water contents, and that the unsaturated hydraulic conductivity curve of biofilm‐affected soil cannot be predicted by simply scaling the saturated hydraulic conductivity. A flexible parameterization of the soil hydraulic functions accounting for capillary and noncapillary flow was needed to adequately describe the observed properties over the entire wetness range. More research is needed to address the exact flow mechanisms in biofilm‐affected, unsaturated soil and how they are related to effective system properties.

Physics and hydraulics of the rhizosphere network

“…Kroener et al (2018) showed that chia seed mucilage decreased the soil saturated hydraulic conductivity of soils of varying texture, but less is known about its impact on the unsaturated hydraulic conductivity. We expect that the relative unsaturated hydraulic conductivity (ratio between conductivity and saturated conductivity) drops more gradually when mucilage is present, as shown for soils embedded with EPS (Volk et al, 2016), because of the maintained continuity of the liquid phase. But the question, whether mucilage does increase the hydraulic conductivity and diffusion coefficient in dry soils, remains open.…”

“…In a provoking paper entitled “The Physics of Soil Water Physics”, Sposito () challenged soil physicists claiming that the fundamental physics of water flow in soils had already been explained by Buckingham () and Richards (), and since then the achievements in soil physics mainly consisted in technical improvements, either experimental or numerical. Sposito's paper is a plea for researching the fundamental physics of water in soils.…”

Microhydrological Niches in Soils: How Mucilage and EPS Alter the Biophysical Properties of the Rhizosphere and Other Biological Hotspots

“…Due to their high viscosity, mucilage and EPS decrease the saturated soil hydraulic conductivity (Kroener et al, 2014; Volk et al, 2016). The effects on unsaturated conditions are less clear.…”

“…The effects on unsaturated conditions are less clear. The decline in hydraulic conductivity with soil water potential was less steep in soils inoculated with biofilm forming bacteria ( Pseudomonas putida ) (Volk et al, 2016). In fine‐textured soils, EPS was even shown to increase the unsaturated conductivity (Volk et al, 2016).…”

“…The decline in hydraulic conductivity with soil water potential was less steep in soils inoculated with biofilm forming bacteria ( Pseudomonas putida ) (Volk et al, 2016). In fine‐textured soils, EPS was even shown to increase the unsaturated conductivity (Volk et al, 2016). Along with these modifications, EPS‐treated soils (Chenu, 1993; Zheng et al, 2018), soils inoculated with a strain of Bacillus subtilis (Zheng et al, 2018), and soil micromodels inoculated with a mucoid strain of Sinorhizobium meliloti (Deng et al, 2015) dried slower than unamended control soil.…”