Spatial organization of the liquid phase in the rhizosphere

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There is experimental and theoretical evidence that the physical properties of the rhizosphere differ from those of the adjacent bulk soil. During drying, the rhizosphere of some species (e.g. maize, lupin, wheat) is wetter than the bulk soil, while after rewetting the rhizosphere turns temporarily water repellent. Mucilage was proposed to be responsible of such time-dependent water dynamics in the rhizosphere. Experimental studies with selected mucilages showed that mucilage could explain well the observed rhizosphere water dynamics. However, the interactions between mucilage and water flow in unsaturated soils remain poorly understood. One reason of the limited understanding is the lack of a comprehensive conceptual model of mucilage interaction with soils. The general objective of this project is to understand the physical processes determining the liquid configuration in the rhizosphere and their impact on macroscopic water retention and transport properties of the rhizosphere. The main hypothesis is that macroscopic hydraulic properties of the rhizosphere emerge from the interactions between mucilage and the soil matrix. In particular, we hypothesize the water retention and hydraulic conductivity of the rhizosphere emerge from the interplay between surface tension, viscous and elastic forces. The novelty of this concept is that typically the spatial configuration of the liquid phase in soils is solely determined by considering capillary and adsorptive forces. Here, we propose that also viscoelastic properties determine the spatial configuration of the liquid phase. We hypothesize that at low water contents the connectivity of the liquid phase increases when a viscous mucilage (e.g. maize mucilage) is present. Filaments of mucilage persist also in air-dry conditions, because of the increasing viscosity and stiffness of the mucilage network. At high mucilage concentrations, mucilage network starts to behave as a solid, forming an additional matrix that can hold water and maintain the liquid phase connected. We plan to test this theory by measuring the physical properties (water adsorption, surface tension and viscosity) of mucilage from different plant varieties and species and the emerging hydraulic properties (water retention curve and hydraulic conductivity) of different soils (sand and loam) mixed with mucilage. We plan to use complementary imaging methods, such as X-ray CT and neutron radiography, to image the liquid configuration in soils mixed with mucilage and in the rhizosphere of growing plants. The results of this proposal are potentially useful for understanding water and nutrient uptake by roots, as well as microbial activity in the rhizosphere. On the other hand, to fully comprehend the function of mucilage in soils it is needed to combine the biophysical aspects adopted here with complementary biogeochemical studies planned in this PP.