Non-invasive monitoring of water and solute fluxes in a cropped soil
Although the influence of root water uptake on solute transport is commonly recognized as important, it has barely been studied throughout the literature. However, plants take up a big amount of the infiltrating water and therefore they influence water flow patterns in the soil and concurrently solute transport processes. For this reason, experiments are required to investigate the relationship between plant root water uptake and flow field variability. Within this PhD project, we tried to elucidate the role of root water uptake on soil moisture distribution and solute transport in two undisturbed soil columns. During three consecutive experimental phases, the soil hydraulic and solute transport characteristics were investigated and the influence of growing barley on water content and tracer movement were studied. Soil water concentration and moisture content in the lysimeters were monitored non-invasively using 3-D electrical resistivity tomography (ERT). ERT is a valuable technique to monitor processes in the unsaturated zone. It is suitable to quantify solute concentration or soil moisture content at the decimeter scale in different soils and under varying conditions. In combination with TDR and effluent measurements, different aspects of the solute transport process and manifestations of preferential flow can be investigated. Steady-state step tracer experiments are very suitable for this purpose. Soil moisture measurements with ERT were conducted as well, but an horizon-specific in-situ calibration of the ERT-measurements for water content was a prerequisite for success. We observed that the solute transport in our silty lysimeters was considerably more heterogeneous than in the loamy-sand soil studied by Koestel (2008; 2009a; 2009b). We observed a clear preferential flow path in one of the lysimeters and found that soil layering had a big influence on the leaching process. The measured water depletion rate, being the result of combined effects of root water uptake and soil water redistribution during the barley experiment without irrigation, was compared with the evaporative demand and root length densities. We could observe a gradual downward movement of the maximum water depletion rate together with periods of redistribution when there was less transpiration. However, we were unable to make the distinction between soil water fluxes and root water uptake, since modeling of the soil water flow field using the time series of water content was not satisfying. We observed root growth at rhizotube surfaces and noted an increasing number of roots with depth. Since the minirhizotron measurements were only conducted at four depths and thus represent a small volume of the entire root zone, we estimated a root architecture model for the barley plants using RootTyp. We were able to set up a simple model, but to obtain better results, the effect of soil constraints and the process of re-iteration should be included. Many aspects of water flow and solute transport in the root zone need to be further investigated. The need for high-quality soil moisture data and simultaneous root architecture data remains. ERT is a promising technique to fill part of this gap, however some issues need to be solved before it can be used without difficulties. Next to measurements, the effort to improve our soil water flow models must be continued in order to improve the estimation of soil water fluxes. Only in this way, we will be able to measure root water uptake at the lysimeter and field scale. This is a necessary step towards a better understanding of the interactions in the soil-plant continuum.
Physical, chemical, mathematical & earth Sciences, Earth sciences & physical geography, Physique, chimie, mathématiques & sciences de la terre, Sciences de la terre & géographie physique
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