Towards an effective characterization of root electrical properties: a spectroscopic approach

The application of geophysical methods to root investigation has been increasing in recent years because of the limitations associated with the use of traditional methods (root excavation, monoliths, minirhizotron etc.). Geophysical methods address these limitations by offering high resolution and non-invasive approaches to root investigation due to their ability to infer properties and structures of the subsurface, at various spatial scales. Recent studies (Weigand and Kemna 2017; Mary et al. 2017) have reported a low frequency polarization of root systems and have shown that SIP/EIT (spectral induced polarization/electrical impedance tomography) holds a promising future for root system characterization. Despite these significant improvements, there is still a knowledge gap regarding the electrical response of fine roots at the segment scale which is essential to enable us to account for the effect of roots in the estimation of soil moisture content of rooted soils. In this study, we use SIP to characterize the electrical properties of single root segments of various plants. A sample holder was designed and tested on ideal resistors and root segments, and was found to be suitable for assessing the electrical properties of root segments of 1-5 cm length and 2 mm diameter in a frequency range of 1Hz – 45 kHz. The system was then used to obtain electrical responses of root segments of Maize and Brachypodium in the laboratory. The resistivity of maize and brachypodium roots at 8 days averaged between 9-14 Ωm, while their polarization averaged 430 mrad and 700 mrad, respectively at high frequency (10 KHz). The resistivity of these roots is much lower than that of saturated sand (110-225 Ωm), dry sand (400-1000 Ωm) and till (17-28 Ωm) as described by Ernston and Kirsch (2006), also they polarized more strongly than geological materials (0.2 - 20 mrad) (Binley et al. 2005; Boerner et al. 1996). These results suggest that fine roots can be differentiated from soils because they show lower resistivity and stronger polarization.