Experimental design
The experiment was conducted in 2007 at the Pasture Ecological Research Station of Northeast Normal University, Changling, Jilin province, China (44°45’ N, 123°45’ E). Seeds of A. therophrasti were collected from local wild populations near the research station in the late August of 2006 and were dry stored at -4oC. We used a split plot design, with soil conditions as the main factor, density and block as a sub-factor. Two large plots were assigned as two (infertile and fertile) soil conditions, each was divided into nine 2 × 3 m sub-plots and randomly arranged with three treatments of densities and blocks. Seeds of A. theophrasti were sown on June 7, 2007, with three inter-planting distances of 30, 20 and 10 cm, to reach target plant densities of 13.4, 36 and 121 plants per m-2, assigned as Low, Medium and High density treatments respectively. Most seeds emerged 4 d after sowing. Seedlings were thinned to the target densities at four-leaf stage. Plots were hand-weeded when necessary and watered regularly.
We established the infertile soil conditions as a plot using the original soil of experimental field at the station that had been used annually for many years (aeolian sandy soil). The fertile soil conditions was set up by covering the other large plot with 5-10 cm virgin soil transported from a nearby meadow with no cultivation history (meadow soil), with contrasting nutrient contents of the two soil conditions (Wang et al., 2017). The meadow soil is not located far away from the experiment field, which used to be meadow as well and has been reclaimed for experimental use since the establishment of research station. Therefore, basically the soil of the experimental field was the same type as the meadow soil, but with different conditions or qualities. Covering the other plot with meadow soil led to a greater amount of soil or nutrients for the fertile soil treatment, which also led to thicker soil layers of the fertile plot than the infertile one. To keep the soil and resource amounts as even as possible, we crushed the blocky soil into very small bits, and mingled them adequately, before spreading them over the entire plot and compaction. Seeds were sown into all plots at the same burial depth and sowing rate.