Experimental set-up
We created experimental plant assemblages composed of four native
herbaceous species that commonly co-occur in grasslands at the Gault
Nature Reserve, located on Mont Saint-Hilaire, Quebec, Canada. The
reserve is comprised of mature temperate forests surrounding a 2-ha
grassland. The herbaceous species Solidago canadensis ,Asclepias syriaca , Achillea millefolium andDesmodium canadense are the dominant species in this grassland
habitat. The experimental plant assemblage consisted of 4 individuals of
different species combinations of these four species, generating a total
of 15 species compositions at 4 plant diversities: 4 monocultures, 6
two-species communities, 4 three-species community and a four-species
community (Table S1). For each species composition, we imposed a soil
treatment which applied either live or sterile soil microbe inoculum,
and a leaf treatment which were high-concentration leaf microbe
inoculum, low-concentration leaf microbe inoculum or sterile leaf
microbe inoculum (Fig S1). Inoculation and sterilization details are
described in the next paragraph. We established 4 replicates for each
composition × treatment combination, resulting in a total of 1440 plant
individuals in 360 plant communities (15 community compositions, 2 soil
treatments, 3 leaf treatments and 4 replicates).
We grew these plant communities in greenhouse conditions beginning in
November 2020. The seeds of the four species were purchased from a
commercial company (Pepiniere Rustique, Quebec, Canada) and surface
sterilized before use by treatment with 75% alcohol for 1 min and then
washed using sterile water. Seeds were placed in propagation trays with
Berger’s BM6 classic peat-and-perlite growing medium (Berger, Quebec,
Canada) to germinate for 10 days, and then transferred to multiple-cell
plug insert trays with the same soil substrate to establish for a week.
After that, four seedlings were transplanted into a plastic pot (27 cm
in diameter and 22 cm tall) containing approximately 4kg BM6 soil
substrate to form a plant community; in total 360 pots were planted.