2.2 Cultivation and harvest
To avoid root damage from transplanting and to improve seedling survival, we established monocultures by sowing the seeds and then thinning the seedlings. Before sowing, the surface of the planting pots was evenly gridded using yarn according to the number of seeds to be sown. All the seeds were sterilized with 75% alcohol solution for 10 minutes and then washed with distilled water three times. Then, the seeds were placed into the corresponding grid to a depth of approximately 1 cm from the surface of the substrate. A backup pot was prepared for each species, and seedlings were replanted if they died within two weeks after thinning (Figure S1). The average day/night temperature in the greenhouse was 28.5/18.5 °C, and the relative humidity was between 40% and 60%. The positions of the pots were randomly changed within and between blocks every two weeks. Each pot was watered with a measured amount every 3 to 5 days to ensure plant growth. In the middle and late cultivation stages, we raised the planting pots with nylon netting to prevent ‘competition escape’ of aboveground organs and to make the conditions as similar to field conditions as possible (Figure S2).
The experiment was conducted from August 19, 2019, to January 12, 2020. As the annuals (C. glaucum and S. viridis ) had relatively short life spans, they were harvested in the eighth week. Before harvesting, each plant in the planting pots was numbered according to its grid position and labelled. The reproductive organs were harvested first, followed by the leaves, stems and roots. These separate organs were placed into corresponding envelope bags. The root samples were individually placed in a 2-mm nylon mesh sieve and washed with tap water. Afterwards, all the samples were dried at 65 °C (for 72 h) until reaching a constant mass. The aboveground biomass of an individual plant included the seeds, stems, and leaves, and the belowground biomass included only roots for most species; the exception was L. chinensis , whose belowground biomass included both rhizomes and roots.