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

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. 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. 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. 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.