Using horizontal borehole GPR data to estimate the effect of maize plants on the spatial and temporal distribution of dielectric permittivity

Agro-ecosystems and their yield productivity are influenced by root water and nutrient uptake. This uptake depends on the crop root architecture and the soil water content distribution within the soil-root zone. Investigating this zone and its processes can help to optimize agricultural practices, like irrigation and fertilization and therefore helps to achieve the goal for sustainable crop production. Mini-rhizotrons have shown to be effective to non-invasively investigate the soil-root zone throughout crop growing seasons using horizontal rhizotubes installed at different depths in the subsurface. In this study, in-situ time-lapse crosshole ground penetrating radar measurements and root images were collected over three maize crop growing seasons at two mini-rhizotron facilities in Selhausen, Germany. These facilities allow to measure data at six different depths ranging between 0.1 m – 1.2 m and for three different plots with varying treatments. The dielectric permittivity was derived from the horizontal crosshole GPR measurements by using standard ray-based analysis along a pair of rhizotubes. Such horizontal permittivity slices can be linked to soil water content using petro-physical relationships. The root architecture is expressed as root length density and is derived from the images, using a workflow combining state-of-the-art software tools, deep neural networks and automated feature extraction. The results of the dielectric permittivity indicate horizontal and vertical variations, depending on weather conditions, soil properties, and root architecture. To quantify the impact of the roots on the spatial and temporal distribution of the dielectric permittivity, we used statistical methods to eliminate the effects of soil heterogeneity, tube deviations and daily evapotranspiration changes. Resulting in permittivity variation along the rhizotubes impacted by the presence of roots.