Multielectrode Geoelectrical Tomography for the Quantification of Plant Roots

The amount and spatial distribution of plant roots are crucial ecological features, and methods based on soil electrical resistivity (r) tomography (ERT) have been proposed for their non-destructive measurement. ERT allows to map root systems in conditions where the contrast of ρ between soil and roots is high, but the electrical behaviour of resistive or heterogeneous soils may interfere with root-borne effects and requires investigation. We studied the spatial distribution of ρ in different soil-root conditions to test the hypothesis that ERT would allow to detect the spatial distribution of plant roots even when low contrast between roots and background soil variation was expected. High-resolution 2-D and 3-D DC (Direct Current) soil resistivity tomograms were used to compare areas of high and low vegetation density in containers where bare soil (LM), was compared to a Medicago sativa L. (HM) stand, and in resistive soils where a stand of Arundo plinii Turra (HA) was compared with a bare soil (LA) and the area under the canopy of Olea europaea L. (HO) was compared with interrow areas (LO). Destructive measurements of root biomass per unit soil volume (RD), soil electrical conductivity (EC), stone content (S) and water content (q) were made in all treatments. Soil resistivity was significantly affected by vegetation density, with a resistive response in HM, HA and HO. The response was related to RD with significant univariate relationships and the spatial pattern of soil resistivity was dominated by roots and other resistive features like stones in all soils. This allows to conclude that ERT is able to detect plant-root effects even in the presence of a resistive background but resistive features interfere with the mesasurements and need to be taken into account. Abbreviations: ρ = in-situ soil electrical resistivity; EC = electrical conductivity of soil samples; θ = volumetric water content; RD = root biomass per unit soil volume; ERT = electrical resistivity tomography; 2-D = Two-dimensional; 3-D = three-dimensional; DC = Direct Current.