Plant-mediated and drought legacies differentially impact root traits and intra- and interspecific competition strength

Abstract Drought and plant species identity can alter soil conditions, thereby influencing subsequent plant performance and competitive interactions. Although trait differences play a role in determining competition outcomes, most studies have focused on aboveground traits, leaving root-mediated responses under explored. We conducted a two-phase soil conditioning experiment: in Phase 1, species-specific microbial legacies were established through field-derived inoculation of Stipa krylovii, Artemisia frigida, and Agropyron cristatum; Phase 2 examined the plant-mediated legacy of these species along with drought legacy, comprising normal watering and drought treatment. The three focal species were then planted under three competition modes: no competition, intraspecific competition, and interspecific competition. Three key findings emerged: First, plant-mediated legacies showed species-specific effects, primarily influencing A. cristatum 's biomass allocation and root growth. In contrast, drought legacy mainly affected root morphological traits (including specific root length, root tissue density, and average root diameter), with these effects strongly modulated by competition mode. Second, plant legacy (but not drought legacy) significantly impacted both intraspecific and interspecific competition strength, with consistent effects across competition modes. Root growth decreased with increasing intraspecific and interspecific competition strength, while root morphological traits responded to intraspecific competition strength in a species-specific manner. Finally, multidimensional root trait differences and root volume differences explained changes in the interspecific competition strength of A. cristatum. This study emphasizes that plant responses to soil legacy effects and competition modes drive variations in intra- and interspecific competition strength, providing insights into species coexistence and community assembly mechanisms.