2.1 Plant materials
All species (K. humilis, P. annua, O. ochrocephala, and S.
pulchra ) were obtained from a natural K. humilis meadow of
Haibei National Field Research Station of Alpine Grassland Ecosystem
(37°29’ N–37’45 ’N, 101°12’ E–101°23’ E; altitude 3,280 m). The
average annual precipitation, air temperature, and sunshine duration of
the area were 560 mm, -1.7 °C, and 2,462.7 h, respectively. The soil in
the alpine meadow is classified as Mat-Gryic Cambisol, and it is rich in
organic matter. The main vegetation types in this region and on the
Tibetan Plateau consist of K. humilis and K. pymaeameadows (Du et al., 2020).
Approximately one month after the plants turned green, on June 4, 2019,
we collected samples for the experiments. We selected plants with good
and consistent growth. The soil volume with an area of plant largest
projected on the ground and a height of 10 cm underground as the
sampling standard. The samples were then packed in envelope bags and
brought back to the laboratory. Thirty plants of each species (a total
of 120 plants from all four species) were sampled. In order to avoid
irreversible man-made damage to the grassland, the distance between each
collected plant sample was not less than 5 m.