2.2 Experimental design
The ecological restoration experiment was conducted from July to
November 2020. According to the previous field survey data of highway,
railway and other construction areas in Nyingchi(Soil gravel content
between 10%~50%), three classes of soils with gravel
contents (>2 mm) of 10% (low), 30% (medium) and 50%
(high) were selected as ecological restoration test materials. Their
physical and chemical properties are summarized in Table 1. In general,
as gravel content increased, the contents of sand, silt, clay, total
porosity, total nitrogen and phosphorus, and organic carbon decreased.
Soil with medium gravel content had the largest bulk density. These
studied soils were neutral with a pH of 6.87~6.98. These
soils were packed into 1 m× 3 m test plots with a thickness of 60 cm
according to native soil
compactness.
Table 1 Physical and chemical properties of studied soils
Three plant species of Festuca elata (Fe,indigenous specie), Medicago
sativa (Ms, introduced specie) and Elymus dahuricus(Ed, introduced specie) were selected as experimental plant
materials according to the commonly used plants in ecological
restoration projects and the natural vegetation of Qinghai-Tibet
Plateau, Among them, Festuca elata and Medicago sativa are
ecological restoration plants introduced in Qinghai Province, andElymus dahuricus is a native plant in Tibet. Considering the
ecological restoration effect of plants and the more obvious competition
effect between plants (same or different species), the method of close
planting is adopted(Huang et al., 2021; Leinauer et al., 2021), seven
typical and widely used plant species communities were set on studied
soils, that is, Festuca elata (seeding rate of 200 kg
hm-2)), Elymus dahuricus (seeding rate of 200
kg hm-2), Medicago sativa seed (seeding rate of
100 kg hm-2), Festuca elata (seeding rate of
200 kg hm-2) plus Elymus dahuricus (seeding
rate of 200 kg hm-2), Festuca elata (seeding
rate of 200 kg hm-2) plus Medicago sativa(seeding rate of 100 kg hm-2), Elymus dahuricus(seeding rate of 200 kg hm-2) plus Medicago
sativa (seeding rate of 100 kg hm-2), andFestuca elata (seeding rate of 200 kg hm-2)
plus Elymus dahuricus (seeding rate of 200 kg
hm-2) plus Medicago sativa (seeding rate of 200
kg hm-2). These plant species treatments are
separately denoted as Fe , Ed , Ms ,Fe +Ed (1:1), Fe +Ms (2:1), Ed+Ms (2:1), and Fe +Ed +Ms (2:2:1).
Three replicates were set for
each treatment, and the same planting method of evenly spreading and
covering 0.2-0.5cm soil was adopted.
After sowing, no additional field
management (such as fertilization and watering) were conducted during
the growth periods (94 days), and the experiments ended in November
2020.
2.3. Sample collection and
measurement
Before the ecological restoration experiments, composited and
undisturbed soil samples were taken from 0-10 cm and 10-20 cm soil
layers, and then brought back to the laboratory for soil physical and
chemical analysis. At the end of plant growth, plant traits including
coverage, natural height, above-ground biomass and root biomass were
measured. Rhizosphere soils (about 3 mm soil fractions around the roots)
were collected.
Soil particle size composition was determined by the sieving-pipette
method after ultrasonic dispersion; field water content by weight method
after oven-dried; bulk density and total porosity by weight method on
undisturbed soil cores (100 cm3); organic carbon of
soils and plant samples by potassium dichromate oxidation and titration
with ferrous sulfate; total nitrogen and phosphorus contents of plant
and soil samples by Smartchem200 automatic discontinuous chemical
analyzer after acid digestion (Tang et al.,2022; Institute of Soil
Science.,1978).