Pairing dual-frequency GPR in summer and winter enhances the detection and mapping of coarse roots in the semi-arid shrubland in China

To complement traditional methods of studying coarse roots (>2 mm in diameter) that are often destructive, laborious and point-based, we tested a dual-frequency ground-penetrating radar (GPR) to non-invasively map lateral coarse roots of shrubs (Caragana microphylla Lam.) in the sandy soil of a temperate semi-arid shrubland. The dual-frequency GPR system used in this study simultaneously collects data with two antenna frequencies (400 and 900 MHz). The GPR surveys were repeated in a grid (11 x 4.8 m(2) with a total of 46 survey lines) in summer and winter. Both antenna frequencies and seasonal GPR surveys generated a consistent pattern of the distribution of lateral coarse roots, indicating reliable interpretation of GPR data for coarse roots mapping. The 400 MHz GPR and the winter survey detected more roots in the deeper soil (>0.6 m depth), whereas the 900 MHz GPR and the summer survey detected more in the shallower soil (<0.6 m depth). The higher wave velocity and lower degree of GPR energy attenuation in the frozen soil enhanced the detection of deep coarse roots. Combining root detection results obtained under all conditions revealed a higher distribution density of lateral coarse roots beneath a depression and at the depth of 0.4-0.6 m, but a lower abundance under the intershrub area (i.e. showing the avoidance of intershrub root overlap). These patterns were not evident in GPR images collected using a single frequency antenna in one season. Ground truthing confirmed that pairing GPR data collected at two frequencies improved the detection frequency of the number of lateral coarse roots. More field tests are required to validate the application of repeated dual-frequency GPR surveys to detect and quantify coarse roots in other ecosystems. Highlights The detection of lateral coarse roots by GPR needs to be further improved in field soils. Dual-frequency GPR surveys were repeated in summer and winter to enhance coarse roots mapping. Combining seasonal dual-frequency GPR data improved detection frequency of the number of roots. Topography and intershrub root competition influence the distribution of lateral coarse roots.