Spatial organization of the liquid phase in the rhizosphere

有实验和理论证据表明，根际土壤的物理性质与邻近的块状土壤不同。在干燥过程中，一些物种(如玉米、羽扇豆、小麦)的根际比散装土壤更潮湿，而在重新湿润后，根际暂时变得憎水。粘液被认为是根际这种随时间变化的水分动态的原因。选取粘液进行的试验研究表明，粘液可以很好地解释观测到的根际水分动态。然而，非饱和土壤中粘液和水流之间的相互作用仍然知之甚少。认识有限的原因之一是缺乏粘液与土壤相互作用的全面概念模型。该项目的总体目标是了解决定根际液体形态的物理过程及其对根际宏观水分保持和运移特性的影响。主要的假设是，根际的宏观水力特性产生于粘液和土壤基质之间的相互作用。特别是，我们假设根际的保水性和水力传导性来自表面张力、粘滞力和弹性力之间的相互作用。这个概念的新奇之处在于，通常情况下，土壤中液体的空间构型完全由毛细管力和吸附作用力决定。在这里，我们提出，粘弹性性质也决定了液体的空间构型。我们假设，在低水含量时，当存在粘性粘液(如玉米粘液)时，液体的连接性增加。由于粘液网络的粘度和硬度增加，粘液的细丝在风干条件下也会保持不变。当粘液浓度较高时，粘液网络开始表现为固体，形成额外的基质，可以保持水分并保持液体的连接。我们计划通过测量来自不同植物品种和物种的粘液的物理性质(吸水率、表面张力和粘度)以及不同土壤(砂土和壤土)与粘液混合的新出现的水力性质(保水曲线和水力传导性)来验证这一理论。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，对混合了粘液的土壤和正在生长的植物根际的液体构型进行成像。这一建议的结果可能有助于理解根系对水分和养分的吸收，以及根际微生物的活动。另一方面，为了充分理解粘液在土壤中的作用，需要将这里采用的生物物理方面与本PP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.