Abstract: Invasive plants often generate more positive plant-soil feedbacks (PSFs) than natives. The direction and strength of PSFs have been affected by resource availability and conspecific plant density, but their joint effects on PSFs of invasive plants have not been examined. We conducted a two-phase PSF experiment to examine how soil nutrient availability and planting density affect feedback with soil biota between a community of five invasive Asteraceae plants (two clonal species) and four co-occurring native plants (one clonal species). Soil biota from invaders did not inhibit plant growth, but soil biota from natives did so. The difference in PSFs between geographic origins was most pronounced under high-nutrient and high-density conditions where the biomass of natives in conspecific soils was much lower than that in soils conditioned by heterospecific invaders. Clonality and its interactions with nutrient and density did not affect PSFs. Soils from invaders had a higher diversity of arbuscular mycorrhizal fungi, and soils from invaders and natives differed in the composition of pathogenic fungi. Our results may explain why invaders but not natives often form dense monocultural stands. That is, as plants take advantage of increased nutrient supply and conspecific density increases, PSFs change little for invaders, but they change to be highly negative for natives, preventing them from forming dense monocultural stands. This invasion mechanism may be particularly pertinent to clonal invaders, as they can swiftly proliferate within habitats via clonal reproduction without encountering negative density dependence, thus establishing dense monocultural stands rapidly from just a few individuals.

Key words: clonal invader, density, nutrient addition, plant invasion, plant-soil feedback, soil biota