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.