Experimental design
On 15 July 2018, plants of S. breviflora were dug from a temperate grassland area located at Horinger county, Inner Mongolia, China (111°50′E, 40°29′N). Then, they were hand separated into two types of experimental subjects containing ramets with similar morphological size: individual with single ramet (C1) and individual with three ramets connected by rhizomes (C2). Twenty individuals of each experimental subject were transplanted to one square pot (60 cm length × 35 cm width × 18 cm height) filled with soil mix containing 20% field soil (from Horinger) and 80% vermiculite ((Mg, Fe, Al)3[(Si, Al)4 O10 (OH)2]. 4H2O) at the greenhouse of the College of Grassland, Resources, and Environment of the Inner Mongolia Agricultural University. There was a total of 12 square pots for C1 and 12 square pots for C2. The chemical properties of experimental soils were shown in Table 1. In the greenhouse, there was 14 h light/10 h dark photoperiod. The air temperature was 26 ± 2 ℃ and 20 ± 2 ℃ for day/night, respectively.
The experimental design was a standard Taguchi L8(27) arrays including four factors such as nitrogen (N), phosphorus (P) and water (W) and cluster fragmentation (C) and three interactions (N × P, N × W and P × W). Each of four factors had two levels, and there was a total of 8 treatments (Table 2). This experiment used NO3NH4 and NaH2PO4·2H2O as N and P fertilizer with the addition concentrations of 15 mg N / kg and 120 mg N / kg and 2 mg P2O5 / kg and 24 mg P2O5 (low level/high level) respectively. Water additions included 25 % ~ 30 % field moisture capacity of mixture (FMC) and 75 % ~ 80 % FMC, and cluster fragmentation consisted of individual with single ramet and individual with three ramets (Table 2). MINITAB 18.0 (Minitab, Inc., State College, PA, USA) software was employed in this data analysis step. The experiment was repeated 3 times with 5-day interval between them. Before treatments, individuals were allowed to grow for 21 days to establish roots. Each experimental period lasted 17 weeks. The nitrogen and phosphorus additions were repeated every 20 days, while water additions were repeated every 3 days.