Can plant-soil feedbacks scale up from individual species to communities: tests of the richness and biomass-ratio hypotheses.

Plant-soil feedbacks (PSFs) play an important role in maintaining biodiversity and ecosystem functioning. In nature, multiple plant species collectively shape soil biotic and abiotic properties, thereby influencing subsequent plants (hereafter: community-level PSFs). However, how plant species richness and composition determine community-level PSFs remains poorly explored. We performed a two-phase PSF experiment, in which soil was conditioned by one, two, four, six, or 12 species from a pool of 28 species. PSF effects of these conditioned soils were subsequently tested on an identical six-species assemblage. In our experiment, community-level PSFs were consistently negative, strengthened with increasing conditioning community biomass, but did not vary with conditioning species richness. While both mean and biomass-weighted PSFs of individual component species predicted community-level PSFs, incorporating species identity substantially increased predictive power. Our findings demonstrate that species differ in their contributions to community-level PSFs, underscoring the need to shift PSF research from a species-level focus toward a community-level perspective.