We measured the fraction of soil volume containing fine and coarse roots using images from three soil pits in watershed N4D and three soil pits in watershed N1B. After opening each pit, the face was cleaned and the surface of the pit face was photographed. Each image was overlain with a 1x1 cm grid and analyzed using ImageJ software. The presence or absence of fine roots (diameter < 1 mm) and coarse roots (diameter > 1 mm) was noted for each grid cell. Only presence/absence and not count data were recorded. For cells containing both fine and coarse roots, the presence of only the coarse root(s) was recorded given their greater volume. The number of grid cells with roots were summed and then divided by the total number of cells evaluated to provide a root abundance fraction (fine, coarse, and total) for each 1-cm thick soil layer.

The face of each soil pit was cleaned and soil horizons were described using the Natural Resources Conservation Service Field Book for Describing and Sampling Soils (Schoeneberger et al., 2012).

WES013 (electrical resistivity monitoring): We conducted a rainfall simulation from August 23 to 24, 2023 to test differences in soil water infiltration and drainage beneath grass and shrubs. A grass and shrub plot were established in N4D. A 24.5 m-long transect that passed through each plot was established for monitoring electrical resistivity (ER). The center of the grass plot was at 7 m along the transect and the center of the shrub plot was at 16.5 m along the transect. The mid-point of the transect marked the transition from predominantly grass to predominantly roughleaf dogwood. The transect was also positioned along an elevation isoline to avoid topographic effects of both the rainfall simulation and the ER measurements. We installed 48 electrodes spaced every 0.5 m along the transect and monitored soil electrical resistivity using a Syscal Pro resistivity meter (IRIS Instruments, France). We used a dipole-dipole configuration with 735 quadripoles. Each quadripole was stacked 3 to 5 times for QA/QC. ER acquisition took place during five data acquisition periods from August 22 to 24, during which multiple ER surveys were automatically collected approximately every 30 minutes, resulting in 40 total data sets. We collected three data sets on August 22 to map natural variability prior to the rainfall simulation. We then collected several data sets during each of three rain pulses from August 23 to 24, and the final collection period took place before dismantling the setup on August 24.

Sampling protocol: Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., 2012. Field book for describing and sampling soils. Version 3.0 Natural Resources Conversation Service. National Soil Survey Center, Lincoln, NE.