Soil legacies of a primary invader strongly drive secondary invasions for species that are phylogenetically distant

Invasive plant species can alter soil abiotic and biotic properties, with some changes persisting long after the primary invader's eradication. However, how soil legacies will influence secondary invasions following control of primary invaders remains unclear, hindering development of targeted control and post-removal management strategies. We used Solidago canadensis as the primary invader and established five field soil conditioning treatments: control (bare plots), invasion treatment (unmanaged invaded plots), three management treatments (invaded plots managed by cutting, herbicide application or burning). Subsequently, we assessed responses of nine pairs of secondary invaders and native congeners in these conditioned soils within a greenhouse setting. We found that Solidago invasion decreased soil nutrients including available nitrogen, phosphorus and potassium and increased soil pathogen diversity. While these soil legacies reduced the growth of both secondary invaders and native congeners, they disproportionately enhanced the biomass advantage of secondary invaders, resulting in a predisposition to secondary invasions. Compared to unmanaged invaded plots, cutting did not further modify soil properties, and both herbicide application and burning had no effect on soil pathogen diversity but strongly increased soil available nutrients. Consequently, cutting had no impact on secondary invasions, while increased available nutrients in herbicide application and burning treatments weakened the intensity of secondary invasions. Notably, secondary invaders distantly related to Solidago benefited more from soil legacies, irrespective of management method. These results underscore the role of soil legacy effects in facilitating secondary invasions and highlight phylogenetic distance from the primary invader as a crucial factor in determining secondary invader success.