Monitoring Soil-plant Interactions in an Apple Orchard Using 3D Electrical Resistivity Tomography

In this paper we present the application of time-lapse non invasive 3D micro-electrical tomography (ERT) to monitor soil-plant interactions in the root zone. The goal of the study is to better understand the soil-vegetation interactions by the use of non invasive technique, and how this dynamical process can be monitored by time lapse geophysical measurements to explain water balance exchanges in the upper subsoil critical zone. We designed, built and installed a 3D electrical tomography apparatus for the monitoring of the root zone of a single apple tree in an orchard located in the Trentino region, Northern Italy. The micro-ERT apparatus consists of 48 buried electrodes on 4 instrumented micro boreholes plus 24 mini-electrodes on the ground surface. We collected repeated ERT and TDR soil moisture measurements for about one year and performed three different controlled irrigation tests: one during a very dry summer and, during a highly dynamic plant growing spring period and one during a very wet autumn period. We also performed laboratory analysis on soil specimens, in order to evaluate the electrical response at different saturation steps. The results demonstrate that 3D micro-ERT is able to characterize subsoil conditions and monitor root zone activities, especially in terms of root zone suction regions. In particular, we note that in very dry conditions, 3D micro ERT is able to image water plumes on the shall subsoil produced using a drip irrigation system. In the very dynamic growing season, under induced widespread irrigation, micro 3D ERT can detect the main suction zones caused by the tree root activity. Complex interactions between soil moisture and pore water salinity complicate the interpretation. The presented approach is a very promising tool for a better understanding of hydrological interactions in the shallow subsoil, and for the quantification of the soil-plant-atmosphere mass exchanges.