Phase 1: Conditioning phase.
For the conditioning phase, 96 plots of 166 ⋅ 250 cm were each divided
into two 83 ⋅ 250 cm experimental subplots. Plots were allocated into
four blocks and within each block, each plot was randomly allocated to
specific treatment combinations; see below.
To test soil legacy effects in the field, six different conditioning
plant communities were designed, each consisting of six species of
grasses and forbs. The experimental design of the conditioning phase has
been described in full detail in De Long et al. (2019). The plant
species were randomly selected from two separate pools of species, so
that three communities consisted of random combinations from the first
pool and the three remaining communities consisted of random
combinations from the second pool.
To test the effects of plant functional types in the conditioning plant
community, the ratios between grasses and forbs in each seed mixture
were manipulated, and each mixture was prepared in four different
forb:grass ratios (i.e., 0:100%, 25:75%, 75:25% 100:0% forb:grass).
To test whether soil legacy effects would become stronger when the
period of conditioning was longer, the two subplots within each plot
were conditioned for different durations (i.e., a one-year or two-year
legacy). Each treatment combination was replicated across four blocks,
totalling 6 plant communities ⋅ 4 forb:grass ratios x 2 conditioning
durations x 4 replicate blocks (=192) subplots.
In May 2015 (i.e., two-year legacy treatments), for each plot, one of
the two subplots weas randomly assigned to one of the above treatments.
All vegetation of the subplot was removed, while the other subplot was
left untouched in that year. Removal included the top soil layer of
approximately 5 cm, which generally contains the highest density of
roots in this grassland system. This was done to remove the most
dominant roots and prevent re-growth of non-target plant species, as
well as minimize dominant pre-existing soil legacy effects of the
previous plant communities that had grown in the soils. Each stripped
subplot was then sown with one of the prepared target communities
described above.
In May 2016 (i.e., one-year
legacy treatments), all vegetation was removed (as described above) from
the remaining untouched subplot in each of the 96 plots. The subplots
were sown with the same target community as the other subplot in that
plot the year before.