2.2 Experimental design
Our study was conducted under semi-controlled field conditions over 4
years in the arid valley of the Minjiang River. The experiment used a
randomized block design with eight treatments as combinations of four
plant species and four RFC (0%, 25%, 50%, and 75% volumetric
content, v v-1). Each treatment had three replicates
(plots), making a total of 48 plots (4 RFC
× 4 species × 3 replicates) in the
study. The four xerophytic species evaluated
wereArtemisia vestita , Bauhinia brachycarpa , Sophora
davidii , and Cotinus szechuanensis , which are native to the arid
valley. They were chosen because of their ecological importance and
divergent root performance: A. vestita , a fast-growing subshrub
with shallow root distribution; B. brachycarpa , a
non-nitrogen-fixing legume shrub with a thin and high branching root
system (root branching density = 1.47); S. davidii , a
nitrogen-fixing legume shrub with thick and low branching root system
(root branching density = 1.07); and C. szechuanensis , a tall
shrub with developed root hairs and mycorrhiza. For microhabitat
heterogeneity, differences in plant adaptations are recognized based on
aboveground measurements (Li et al., 2008; Wu et al., 2008; Hu et al.,
2021), whereas the variation in rooting profile and its ecological
indicator is not well known.
Each plot was represented by a pit, with dimensions of 1 m × 1 m × 0.5 m
(length × width × depth) and 50 cm spacing between plots. The soil at a
depth of 0–50 cm was excavated from each pit. Subsequently, the walls
of each pit were lined with polyethylene film to prevent interference
from external conditions, and the bottom of the plot was left unlined to
allow natural drainage. Fine soil
particles (< 2 mm in
diameter) and rock fragments (10–20 mm in diameter) were collected,
mixed uniformly, and filled back into each pit to obtain the desired
RFC.
Soil from all pits was then air dried for a week, and fine soil
particles were collected by passing the dried soil through a 2 mm sieve
and all the soil was then mixed uniformly. Initial soil properties in
the samples (n = 6) were as follows: total carbon, 15.3 ± 0.09 g
kg-1; total nitrogen, 2.31 ± 0.02 g
kg-1; and total phosphorus, 0.61 ± 0.01 g
kg-1. Thin-bedded limestone (dominated by phyllite),
which is commonly found in regional soil, was used for the
rock fragments in this study. A
sufficient amount of rock fragments was collected from the coarse soil
part (≥ 2 mm) that had not passed through the sieve and from nearby
land. The crushed rock materials were first passed through a 10 mm sieve
and then a 20 mm sieve, leaving rock fragments with a particle size of
10–20 mm for use. The density of thin-bedded limestone was 2.56 ± 0.03
g cm-3 (n = 12), as measured by the water displacement
method (Wang et al., 2017). After uniform mixing, fine soil particles
and rock fragments were filled back into each pit at the desired RFC in
April 2018. Each plot was irrigated with 100 L of water and left for
soil to stabilize.
Sowing in each pit was performed at a depth of
0.5–1 cm with an equidistant
interval pattern of nine points (25 cm equidistance between two points)
in April 2018 (Hu et al., 2021). Seeds of the four species were
collected from their natural habitats in the arid valley of the Minjiang
River (31°42′N, 103°53′E, altitude range of 1600–1920 m) in August to
October 2017, air-dried for 4–8 days, and stored at room temperature
(10–25 °C). All the seeds were
disinfected by immersion in 2.5% NaClO for 1 h and then sown in each
plot. Seedlings were watered weekly after sprouting to prevent early
losses. They were thinned 2 months after sprouting (Hu et al., 2021),
leaving four average-sized seedlings per plot and an interval of
approximately 50 cm between seedlings. The plots were weeded twice a
week to ensure normal plant growth.