Impact of bacterial biomass on the surface wettability of soil particles under varying moisture conditions

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Particle wettability is an important controlling factor for a multitude of processes in soil, including fluid movement and distribution, transport and adsorption of colloids and organic molecules, as well as biological activity. Low wettability of soil particles (soil water repellency) generally is considered to be attributable to the presence of adsorbed organic compounds derived from fungal hyphae or plant material. Recent research revealed first evidence for a potential role of bacterial biomass in the development of water repellency in soil. However, until now it is unclear to which extent bacterial cells and their fragments contribute to the occurrence and persistence of soil water repellency and whether bacterial adaptation to water and salt stress can possibly explain the frequently observed variation of water repellency in response to wetting and drying events. Our project aims at unraveling these questions in a series of closely interlinked joint experiments by (1) investigating the factors and conditions that contribute to the occurrence of bacterial surface hydrophobicity, (2) analyzing how bacterial surface properties are reflected by the surface properties of cell-mineral associations and soil material, (3) evaluating the biological and physical stability of cell/fragment (necromass)-mineral associations, and (4) testing the potential feedback of soil particle wettability on bacteria and their surface properties. Starting from experiments in artificial systems, pure bacterial cultures will be exposed to different kinds of stress (osmotic or matric stress) and the isolated cells and cell fragments subsequently intermixed with mineral particles. Surface properties of bacterial cells, minerals and their associations will be characterized by combining contact angle measurements and surface free energy calculations with information on surface chemical structure and nanomechanical properties obtained by X-ray photoelectron spectroscopy and atomic force microscopy. Using the combination of these techniques will help to explore the underlying mechanisms of changes in bacterial surface properties and will allow identifying important interaction mechanisms between minerals and bacterial cells as a prerequisite for evaluating the persistence of the observed effects. Analysis of phospholipid fatty acids in soils exposed to different drought levels will provide first information on drought-induced shifts in the soil microbial community and on bacteria which are particularly effective in changing the wetting properties of soil particles. Identification of biomarkers with strong relationship to cell surface properties will potentially allow to estimate cell wettability even in complex systems such as natural soil. Ultimately, the synthesis of data obtained in this project will greatly enhance the mechanistic understanding of processes and controlling factors involved in the microbially-mediated development and dynamics of soil water repellency.