Soil sampling pits across three hillslope positions - toeslope, backslope, and summit - were dug in 2020 in watershed N4D (burned every 4 years) and N1D (burned annually) to characterize the impacts of woody encroachment on subsurface soil physical, chemical, and biological properties. Pits were hand-dug to 120 cm in the toeslope position and to 60 cm deep at the backslope and summit positions. Soil pits in N4D were dug directly under dogwood shrubs (Cornus drumondii) while pits in N1B were dug under grasses and forbs. Soil pit faces were photographed to determine root fractions with depth, soil monoliths were take to charaterize soil macroporosity with depth while soil cores were taken in each horizon for water retention analysis. Soil sensors were also installed at four soil depths at the toeslope position and 3 soil depths at the backslope and summit positions to record half hourly soil moisture, soil temperature, soil water potential, soil electrical conductivity, and soil carbon dioxide, and soil oxygen. In addition, geophysical measurements were taken in N4D using time-lapse electrical resistivity.
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).
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.