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.