A widespread nitrogen-fixing invader experiences negative soil feedbacks despite enhancing the abundance of beneficial soil microbes

Background and aims Plant-soil feedbacks may determine the long-term success of introduced species. Here we examined plant-soil feedbacks of a globally invasive shrub, Cytisus scoparius (hereafter Cytisus ), which associates with multiple guilds of microbial mutualists and dominates harvested Douglas-fir forests in the Pacific Northwest. Methods We studied Cytisus root nodulation, mycorrhizal colonization, and growth in two greenhouse experiments. First, we compared invaded to uninvaded field soils. Then we did a soil conditioning experiment with Cytisus (in both invaded and uninvaded field soil) compared to soil conditioned by Douglas-fir. Results Cytisus grown in invaded soils had 67% more root nodules and 72% more AMF colonization than uninvaded soil. Conditioning uninvaded soil with Cytisus increased root nodules by 14% and mycorrhizal colonization by 55%, compared to Douglas-fir conditioned soil. Despite the increased abundance of mutualists, Cytisus grown in Cytisus- conditioned soils were 41% smaller than in uninvaded soil. Conclusions Cytisus increased the abundance of its microbial mutualists in soils, but overall plant-soil feedback was still negative, likely driven by soilborne pathogens, nutrient depletion, and/ or reduced benefits of mutualists. Our results do not support the idea that the high densities reached by Cytisus in its invaded range are caused by positive plant-soil feedbacks.