Applied Research On In-situ Estimation Of Plant Root Biomass Using Electrical Capacitance Method

Root system is a dynamic interface of substance exchange and energy flow between plant and soil, and yet due to the limitation of opaque properties of soil, the directly observation and measurements of root system seemed to be difficult. Electrical capacitance method as a rapid and non-destructive technique of root measurements in-situ, clearly promoting further development of root system. In the study, the maize that cultivated in loess soil was used as material, through the experiments of root electrical parameters measurements and root harvest, relationship between root electrical parameters and root mass were estblished, and influence of internal factors such as measuring frequency and circuit configuration and external conditions such as electrode position and soil water content on root electrical parameters measurements were analyzed, and the measuring criteria of root electrical parameters measurements such as appropriate frequency and circuit configuration, and electrode position and soil water content were confirmed. This study will provide some reference values for the choose of measuring criteria of root electrical capacitance method. Main conclusions were summarized as follows:(1) The measuring frequency of 1KHz could be uesd as an appropriate frequency when measuring root electrical parameters. Under conditions of three measuring frequencies of 0.1, 1 and 10 KHz, there were significant linear correlations between root mass(dry mass and fresh mass) and root capacitance(capacitance and dry mass: 0.5014<R2<0.7152, capacitance and fresh mass: 0.4816<R2<0.6332, P<0.01), and there were significant non-linear correlations between root mass and root impedance(impedance and dry mass: 0.4014<R2<0.6256, impedance and fresh mass: 0.4392<R2<0.6732, P<0.01). Compared with measuring frequencies of 0.1 and 10 KHz, under the frequency of 1KHz, the degrees of fitting between root electrical parameters and root mass were higher, and determination coefficients between capacitance, impedance and dry mass, fresh mass were 0.7152, 0.6332 and 0.6256, 0.6732, respectively. Further more, both volatility of observed electrical parameters and its sensitivity to the variations of root mass varied between low(0.1KHz) and high(10KHz) frequency. Root electrical parameters varied stably following root mass under the frequency of 1KHz.(2) The circuit configuration of two-terminal could be used as an appropriate circuit configuration when measuring root electrical parameters. Compared with four-terminal circuit configuration, the degrees of fitting between root electrical parameters and root dry mass were higher under the configuration of two-terminal, and determination coefficients between capacitance, impedance and root dry mass were 0.6335 and 0.7255, respectively. Under the configuration of two-terminal, both volatility of observed electrical parameters and its sensitivity to the variations of root dry mass were stable.(3) The root electrical parameters were related to the position of stem electrode(height from the ground), and had no relations with the position of soil electrode(the distance between soil electrode and stem, the inserted point of soil electrode and its depth). The stem electrode could be better when positioned in the shoot crown at 1cm. Root capacitance was monotonically decreasing and impedance was monotonically increasing following the height from the ground of stem electrode increasing(0-8cm). Root capacitance decreased by 0.76 times and impedance increased by 3.15 times when the height from the ground of stem electrode increased from 0 to 8cm. Compared with the height from the ground of stem electrode at 5cm, the fitting degrees between root electrical parameters and root dry mass were higher under 1cm, and determination coefficients between capacitance, impedance and root dry mass were 0.7453 and 0.7648, respectively.(4) As soil water content increasing, root capacitance was monotonically increasing and root impedance was decreasing first and then steadying, and root dielectric loss factor and phase were monotonically decreasing. The correlations between root electrical parameters and soil water content were significant(P<0.01). The soil water content should be controled to a higher level such as field capacity of 20% when measuring root electrical parameters.