Spatial organization of the liquid phase in the rhizosphere

有实验和理论证据表明，根际的物理性质不同于邻近的散装土壤。在干燥过程中，某些物种（如玉米，羽扇豆，小麦）的根际比散装土壤更潮湿，而复湿后，根际暂时变成防水。粘液被认为是负责这种时间依赖性的根际水分动态。选择的粘液的实验研究表明，粘液可以很好地解释所观察到的根际水分动态。然而，在非饱和土壤中的粘液和水流之间的相互作用仍然知之甚少。一个原因是有限的理解是缺乏一个全面的概念模型的粘液与土壤的相互作用。本项目的总体目标是了解决定根际液体配置的物理过程及其对根际宏观水分保持和运输特性的影响。其主要假设是，根际的宏观水力特性出现从粘液和土壤基质之间的相互作用。 特别是，我们假设水的保留和水力传导的根际出现的表面张力，粘性和弹性力之间的相互作用。这一概念的新奇在于，通常土壤中液相的空间构型仅通过考虑毛细管力和吸附力来确定。在这里，我们建议，也粘弹性决定的液相的空间配置。我们假设，在低水含量时，当存在粘性粘液（例如玉米粘液）时，液相的连通性增加。由于粘液网络的粘度和硬度增加，粘液丝在空气干燥条件下也能持久存在。 在高粘液浓度下，粘液网络开始表现为固体，形成可以保持水并保持液相连接的额外基质。我们计划通过测量不同植物品种和物种的粘液的物理特性（吸水性，表面张力和粘度）和不同土壤（砂和壤土）与粘液混合的新兴水力特性（保水曲线和水力传导率）来测试这一理论。我们计划使用互补的成像方法，如X射线CT和中子射线照相术，在土壤中的液体配置与粘液混合，并在生长植物的根际成像。这一建议的结果是潜在的有用的了解水分和养分吸收的根，以及在根际微生物的活动。另一方面，为了充分理解土壤中粘液的功能，需要将本文采用的生物物理方面与本PP中计划的补充生物地球化学研究相结合。

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.