Microhydrological niches in soils : how mucilage and EPS alter soil hydraulic properties and water dynamics

The soil offers numerous challenges to life residing in its porous environment. One of these challenges are fluctuations in soil water content which are accompanied by shifts in soil hydraulic properties. In order to avoid undesirable alterations and optimise growth conditions, plants and bacteria engineer their local environment by release of mucilage and EPS (extracellular polymeric substances). So far, modifications of soil properties were mainly attributed to the intrinsic properties of these highly polymeric blends. In this work, we focused on deriving a mechanistic understanding of how mucilage and EPS interact with the soil pore space and how these interactions impact soil hydraulic properties and water dynamics in the rhizosphere and other biological hotspots in soils. Mucilage and EPS are capable of absorbing large volumes of water, increase the viscosity of the soil solution and decrease its surface tension. Upon drying, mucilage turns water repellent. Here, we proposed a conceptual model linking the intrinsic physical properties of mucilage to their impact on soil hydrology. The increase in viscosity is related to the high content of polymers which can form an interconnected network. As the soil dries, mucilage and EPS become increasingly concentrated, the viscosity of the soil solution locally increases and its surface tension decreases. When a critical viscosity is reached and parts of the polymer network are adsorbed to drying surfaces, the retreat of the liquid front is delayed and its break-up due to capillary forces is prevented. This concept is confirmed by microscopy imaging and high resolution X-ray CT, which revealed that mucilage and EPS form filaments and two-dimensional structures in this process. Upon drying in porous media, mucilage at low concentrations (mass of dry gel per mass of dry soil) resulted in the formation of filaments. With increase in initial mucilage concentration, two-dimensional surfaces formed when the water content was relatively high and the liquid phase connected. Complementary measurements of soil hydraulic properties of mucilage amended soils showed how the formation of these continuous two-dimensional structures impacts soil physical properties, such as soil hydraulic conductivity, soil water retention and vapour diffusion. The maintained liquid connectivity in drying soils, which is caused by the high viscosity, low surface tension and interaction of the polymer network with the soil porous matrix, explains why the hydraulic conductivity of a mucilage amended sandy loam was higher at low soil water content when compared to its control, as shown in evaporation experiments. Additionally, the delayed retreat of the liquid phase at a critical mucilage concentration creates an additional matric (capillary) potential and enhances soil water retention. To separate and quantify this matric (capillary) effect from the intrinsic property of the polymer network to absorb water remains an open task. Furthermore, upon severe soil drying, the network of two-dimensional structures reduces vapour diffusion and thus delays soil drying. This effect was illustrated using time series neutron radiography to visualise the drying of mucilage amended sandy loam and a water saturated control. Besides affecting soil hydraulic properties and evaporation rates during soil drying, mucilage impacts the rewetting kinetics. Mucilage amended soils showed water repellency. Precisely, a sharp decrease in wettability was observed near mucilage contents at which one-dimensional structures were replaced by two-dimensional continuous surfaces. Simulation of water drop infiltration experiments in mucilage amended soils showed that the creation of continuous clusters of non-wettable pores induced a substantial decrease in soil wettability, indicated by a transition of water drop penetration time from milliseconds to minutes. Although most experiments presented here were based on simplified systems, such as mucilage amended porous media, we propose that the release of highly polymeric blends into the soil pore space represents a universal strategy of soil organisms. Plants and bacteria engineer the physical properties of their local environment in very similar and astoundingly effective ways. The mechanisms discovered in this thesis lead to hydraulic decoupling of biological hotspots (e.g. the rhizosphere or biocrust) and buffer the erratic fluctuations experienced by soil organisms in these microhydrological niches.