Indirect root distribution characterization using electrical resistivity tomography in different soil conditions

Root cohesion (Cv) represents a fundamental parameter for the stability of a tree as well as for the vegetation contribution to slope stability. Therefore, the finding of information concerning the root presence becomes mandatory to perform realistic predictions. A precise root detection is commonly practiced by a direct analysis of the root system carried out by digging trenches, via air-spade or extracting the entire root apparatus. This methods are very expensive, destructive and not replicable. This paper presents a 2D Electrical Resistivity Tomography (ERT) application in two study sites characterized by different soil textures and soil moisture conditions. The aims of this work were: i) to couple 2D resistivity profiles with high-resolution root density observations carried out along trenches for different soil texture and moisture contents; ii) to determine a regression law for the non-destructive estimation of the root distribution and the Mean Rooting Depth directly from ERT surveys for engineering purposes. The ERT was performed along transects diametrically crossing the root plate. Then, the direct detection of roots was carried out by digging trenches along the transects. The root distribution was coupled with the ERT observations and correlation laws were determined between ERT and root density. Finally, the capability of the obtained regressions in predicting the Mean Rooting Depth (MRD) was tested. Results confirm that soil electrical resistivity is influenced by the presence of the root systems, allowing the non-destructive root detection. Moreover, our findings suggested opposite responses for sandy and clayey soils under different soil moisture conditions. In particular, root detection in clayey soils appeared more efficient in wet conditions, while for sandy soils best results were obtained in dry conditions. The application of ERT for slope stability and tree stability analysis applications appeared promising for further applications both in urban and natural environments.