2.3.2 Vertical distribution
After separating root segments from the soil, root samples were separated into coarse and fine roots (< 2 mm in diameter). Fine root samples were carefully washed with deionized water and scanned using a root image scanner (Epson V800; Seiko Epson Corp., Japan). The average diameter, total length, and total volume of fine roots in each plot were determined using the software Win-RHIZO 2020 (Regent Instruments, Canada). The scanned fine roots were oven-dried at 65 °C for 48 h and then weighed. Finally, the biomass, length density, specific length, and tissue density of fine roots were calculated using the following formulas:
Fine root biomass (g m-2) = Fine root dry mass (g) / Soil core sectional area (m2)
Fine root length density (cm cm-3) = Fine root length (cm) / Soil core volume (cm3)
Specific fine root length (m g-1) = Fine root length (m) / Fine root dry mass (g)
Tissue density (g cm-3) = Fine root dry mass (g) / Fine root volume (cm3)
The rooting depth is indicated by β value. Low β values correspond to shallow root allocation, whereas high β values correspond to an increased proportion of roots with depth.β is a simple numerical index of the rooting distribution based on the asymptotic equation:
Y = 1 – βd
where d is the soil depth (cm), Y is the cumulative root biomass fraction from the surface to a soil depth of d , andβ is the fitted coefficient.