Four-terminal configuration enhancing estimation efficiency of root biomass by electrical capacitance method

Abstract: Validity measurement of electrical capacitance across a root system is the precondition for using electrical capacitance method to estimate root biomass. When using two-terminal configuration to measure root electrical capacitance, the parasitic electrode contact effect would affect validity measurement of electrical capacitance. The primary goal of this study was to evaluate the validity of root electrical capacitance by four-terminal configuration compared with two-terminal configuration to measure root biomass. Maize (Zea mays) were grown in pots containing a loessial soil. Each pot contained one individual and its root system was measured under electrode configuration of two-terminal and four-terminal. The measurements were conducted at stage of jointing, tasseling and maturity of maize, respectively. A total of 5 plants were measured for each stage. Electrical capacitance of the root system was determined using a portable alternating current (AC) digital capacitance bridge (MT-4080A) with a pure sine wave excitation at a frequency of 1 kHz. Home-made stainless, low resistance electrode (5 cm in length and 2 cm in diameter), which was sharpened at one end and pushed into the soil vertically, was used to connect root system and capacitance bridge. Before each measurement, the relative soil moisture of pots was controlled to 25%-30%, 55%-60% and 85%-90%, respectively. Soil water content of pots was measured and controlled by TDR-300 combined with the weighing method. In measurements under two-terminal configuration, one electrode was inserted through the center of the stem about 1 cm from crown, and the other one was inserted into the soil about 5 cm from the base. Compared with the two-terminal configuration, the four-terminal configuration used a further 2 electrodes which were placed above the stem electrode and further from the base. After measurement of each pot, the roots were harvested and the biomas was determined. The results showed that soil water content had an evident effect on electrode contact effect. The electrode contact effect was presented and root electrical capacitance of maize measured by the two-terminal configuration was smaller than the four-terminal configuration. The electrode contact effect increased with the decrease in soil water content and it was the most obvious at the relative soil moisture of 25%-30%. In detail, compared with the two-terminal configuration, the root electrical capacitance determined by the four-terminal configuration increased by 1.04, 0.21 and 0.04 times at relative soil moisture of 25%-30%, 55%-60% and 85%-90%, respectively. Additionally, the validity of root electrical capacitance characterizing root biomass was similar under two configurations at the relative soil moisture of 85%-90% (two-terminal configuration: R2=0.63, P<0.05; four-terminal configuration: R2=0.66, P< 0.05). However, compared with the four-terminal configuration, the validity of root electrical capacitance characterizing root biomass under the two-terminal configuration declined sharply when soil water decreased, and it was most pronounced at the relative soil moisture of 25%-30% (R2 decreased from 0.59 to 0.37, P<0.05). In conclusion, compared with the two-terminal configuration, the validity of root electrical capacitance characterizing root biomass under the four-terminal configuration was better when soil water content was lower.