Abstract
Accurate quantification of coarse roots without disturbance represents a gap in our understanding of belowground ecology. Ground penetrating radar (GPR) has shown significant promise for coarse root detection and measurement, however root orientation relative to scanning transect direction, the difficulty identifying dead root mass, and the effects of root shadowing are all key factors affecting biomass estimation that require additional research. Specifically, many aspects of GPR applicability for coarse root measurement have not been tested with a full range of antenna frequencies. We tested the effects of multiple scanning directions, root crossover, and root versus soil moisture content in a sand-hill mixed oak community using a 1500 MHz antenna, which provides higher resolution than the oft used 900 MHz antenna. Combining four scanning directions produced a significant relationship between GPR signal reflectance and coarse root biomass (R2 = 0.75) (p < 0.01) and reduced variability encountered when fewer scanning directions were used. Additionally, significantly fewer roots were correctly identified when their moisture content was allowed to equalize with the surrounding soil (p < 0.01), providing evidence to support assertions that GPR cannot reliably identify dead root mass. The 1500 MHz antenna was able to identify roots in close proximity of each other as well as roots shadowed beneath shallower roots, providing higher precision than a 900 MHz antenna. As expected, using a 1500 MHz antenna eliminates some of the deficiency in precision observed in studies that utilized lower frequency antennas.
Highlights
Traditional methods for measuring root biomass, root architecture, and myriad other root characteristics are difficult, labor-intensive, and destructive [1,2,3,4]
Applying all four scan directions to the 0.25 m2 plots and the grid pattern method of locating core footprints within the plot, the regression developed from the core data estimated 2525 ± 463 g/m2 of coarse root biomass within the 0.25 m2 pRleomtost,e wSenhsi.c2h01w7,a9s, 1n3o37t significantly different from the observed biomass of 2637 g/m2 (p = 0.94). 9 of 16
The results of this study suggest that differences in root moisture content between live and dead roots have a significant effect on Ground penetrating radar (GPR) effectiveness, despite the high level of precision of a 1500 MHz antenna in ideal soil conditions
Summary
Traditional methods for measuring root biomass, root architecture, and myriad other root characteristics are difficult, labor-intensive, and destructive [1,2,3,4]. A major limiting factor in understanding belowground processes and their role in terrestrial carbon sequestration has been the use of destructive measurement techniques (i.e., in-growth cores, soil cores, and pits) that inherently prevent temporal assessments in long-term studies [5,6,9,10,11]. To address these limitations, a number of nondestructive methods have been developed for measuring coarse roots, including radioisotope and stable isotope labeling, sap flow approaches, as well as geophysical imaging techniques [12,13,14,15,16]. Many studies have revealed high correlations between GPR radargrams and root biomass [7,20,21,22,23,24,25,26,27,28], as well as high correlations with root diameter [19,26,29,30]
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