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.