| The root-soil system is a critical part in the soil-plant-atmosphere continuum (SPAC) or the earth critical zone (CZ), but it is an extremely complex multiphase system including root system, soil, air, organic matter, microorganism, and so forth. Water transport is the most essential process in the root-soil system, and serves as the base of nutrient transport in plants. Due to the hiding and complexity of root-soil system, to date there remains the lack of quick, non-destructive, effective methods to investigate the water transport process in root-soil system. The objective of this work is to build up an electrical-capacitance-method system consisting of single-frequency capacitance (SFC) method, electrical impedance spectroscopy (EIS) and electrical capacitance tomography (ECT) for investigating water dynamics in the root-soil system. To achieve this objective, we performed various experiments associated with capacitance measurement and the abovementioned electrical capacitance methods. Therefore, this work includes four parts as follows:(1) The factors in relation to capacitance measurementA capacitor is composed of two electrodes and the dielectric between the two electrodes, and capacitance is measured by an equipment. Therefore the measurement of capacitance is relative with three categories of parameters:equipment parameters, electrode parameters, and dielectric parameters. In this work, experiments in relation to these parameters were conducted to provide the backgrounds for designing subsequent experimental schemes and interpreting the experimental results. The experimental results in this part demonstrated that capacitance depends strongly on frequency, temperature, concentration, and soil moisture content, whereas it is slightly affected by voltage, output resistance, measuring velocity, and average times in the measuring range of LCR. Capacitance decreases with the increase of frequency, and especially in the low-frequency domain (lower than 10 kHz) they show a strongly double-logarithmic and negative linear relationship. The frequency of 1 kHz is just located in this frequency, and it is the most representative frequency for LCR. Therefore 1 kHz is generally and firstly selected for the SFC method when LCR is applied. This work also selected 1 kHz as measuring frequency for single-frequency capacitance method.(2) Single-frequency capacitance (SFC) methodThe SFC involves with electrode-material system, mainly including electrodes, growth substrate, and plant roots, and these three parts form an electrode-substrate-root continuum. Respondingly, we conducted electrode experiments, substrate experiments, and changing-root experiments. The obtained experimental results are not consistent with the conceptual model for measuring root capacitance proposed by Dalton (1995), but favor the model by Dietrich (2012). On the basis of many evidences obtained by SCF experiments, di-correlation was presented to explain the reason why capacitance has a linear correlation with root parameters. The bi-correlation means that two correlations exist simultaneously:first, the correlation between the stem capacitance and the stem parameter (e.g. diameter and cross-section area), and second, the correlation between the stem parameter and the root parameter (e.g. area, length, fresh mass, and dry mass) occurring in the allometry, so that a correlation between capacitance and root parameters is established. Besides, hydraulic redistribution of a root system is confirmed by a dividing-root experiment using SFC. A SPAC structure model with two pumps and three pools was proposed in this work to characterize well hydraulic redistribution. Meanwhile, a hypothesis that the endodermis has a breakthrough along with the inverse gradient of water potential was presented to interpret the mechanism of hydraulic redistribution. According to this hypothesis, the hydraulic redistribution occurs when the maximum point of inverse gradient of water potential is located at roots and this inverse gradient can break through the large outward resistance of roots to make water flow into soil.(3) Electrical impedance spectroscopy (EIS)EIS can effectively determine the inner information of material system by interpreting the multi-frequency electrical parameters, whereas SCF determines less information due to one frequency used merely. The multi-frequency data of each compartment of root-soil system, as well as stem, were measured and analyzed. And EIS was applied to the experiments for monitoring the water level dynamics, drought process, waterlogging process of root-soil system. During the data analysis process, SFC was firstly applied to obtain typical points in the water dynamics of root-soil system. Then for these typical points, multi-frequency electrical parameters were analyzed by EIS technique, in which complex nonlinear least squares (CNLS) program was adopted to fit the electrical parameter of selected models. Furthermore, the correlation between the fitted electrical parameters and targeted variables (i.e. water level, moisture content, etc.) was analyzed by a statistical software SPSS to determine some strong correlations. Finally, water dynamics of root-soil system can be predicted according the strong correlations obtained. The experimental results reveal that EIS, accompanying with SFC, can serve as an effective technique to monitor the water dynamics of root-soil system.(4) Electrical capacitance tomography (ECT)Plant root parameters are important in many models of plant growth or water balance; and thus the extent and distribution need to be determined. To meet the demand of determining root parameters, we introduced ECT technique into this work, and built up an ECT system with one LCR and programmable logic controllers (PLC), which was named LCRWin, as measuring unit. For the sensors of this system, electrodes were installed inside the insulating wall, and screens were unused to simplify sensors. Besides, we compiled forward programs of ECT to simulate capacitances, and inversing programs to reconstruct images within the sensing domain. To justify the availability of the proposed system, a number of simulation and physical experiments were conducted. The experimental results demonstrated that this simplified ECT system succeeded in probing anomalous blocks within water in tow and three dimensions; furthermore, it was capable of measuring the rough distribution of root in the nutrient solution and monitoring the infiltration process of water in soil.In this work, three electrical capacitance methods were integrated to experimentally investigate water dynamics of root-soil system. The obtained experimental results reveal that these electrical capacitance methods can serve as in-situ, non-destructive, quick techniques to determine the information of water dynamics of root-soil system. This work will accordingly enhance relative researches on SPAC, CZ, climate change, and so forth; and hence it embraces important scientific implication and practical value. From the disciplinary point of view, the presented work can promote the development of electromagnetic hydrology, which will be a promising sub-discipline of hydrology. |
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