Plant-soil feedback
To test whether plant-soil feedbacks in monoculture can predict feedbacks in a community (Question 4), we performed a monoculture and a community-level plant-soil feedback experiment using a common, field-collected soil inoculum cultured by each of our plant species. To culture the live inoculum, we collected approximately 1 m3 of topsoil from each of eight sites across Canterbury, New Zealand in June 2016. We grew each of our 39 focal plant species (Table S1) in a 10 L pot (12-20 replicates per species) containing the live soil mixed in a 1:2 ratio with a pasteurized background media (50:50 mineral soil:sand that had been steamed twice for 60 minutes at 100 ° C internal temperature). After approximately 9-10 months of growth, we harvested and discarded aboveground material from each pot, but retained and chopped the roots finely and reincorporated them into the soil. After combining replicates of each species, we had soils from each of the 39 species that could be used as “home” (i.e. cultured by a conspecific) or “away” (i.e. cultured by a heterospecific) soil in the monoculture and community-level experiments.
To establish the monoculture plant-soil feedback experiment, we grew twenty individuals of each of the 39 plant species in 10 L pots containing 7 L of freshly pasteurized soil:sand mixture (described above), with ten individuals receiving 2.5 L of “home” soil and the other ten receiving 2.5 L of “away” soil (one of ten different randomly chosen heterospecific species). After approximately 10 months of growth, we harvested all above and belowground plant material and determined the dry weight of all individuals.
To establish the community-level plant-soil feedback experiment, we grew each 8-species community in pots containing a bottom layer of 22 L of gravel, covered with 88 L of pasteurized soil:sand mixture and topped with 12 L of the live inoculum soil. To prepare the “home” soils, we mixed soil from each of the resident species in that community in equal parts. “Away” soils were mixtures of eight species occurring in one of the other 19 plant communities (a different community where none of the residents occurred). After approximately one year of growth, we recorded the realized richness of each mesocosm community, harvested all above and belowground plant material, and determined dry weight of all individuals. Root samples from each individual plant were retained, washed and frozen immediately at -80 oC for subsequent molecular analysis.