Comment on: "root orientation can affect detection accuracy of ground-penetrating radar"

In a recent paper, Tanikawa et al. Plant Soil 373:317-327, (2013) reported a considerable impact of root orientation on the accuracy of root detection and root diameter estimation by ground-penetrating radar (GPR). In Tanikawa et al. Plant Soil 373:317-327, (2013), buried root samples in a sand box were scanned from multiple cross angles between root orientation and GPR transecting line under controlled conditions. Changes in radar waveform parameter of roots to different cross angles were investigated. Tanikawa et al. Plant Soil 373:317-327, (2013) clarified that 1) the variation in amplitude area (a signal strength related waveform parameter) to different cross angles fitted a sinusoidal waveform; and 2) the impact of root orientation on root diameter estimation by GPR could be mathematically corrected by applying a grid transect survey. However, we found that the quantitative relationship established in Tanikawa et al. Plant Soil 373:317-327, (2013) between amplitude area and cross angle was incorrect, and the application of a grid transect survey still underestimated root diameter. The change in amplitude area to cross angle between transecting line and root orientation fits a sinusoidal waveform but different to that reported in Tanikawa et al. Plant Soil 373:317-327, (2013). The polarization of GPR wave may explain such sinusoidal variation in amplitude area to cross angle. The effect of root orientation on GPR-based root diameter estimation remains to be calibrated.