Data for"Post-fire soil microbial feedbacks shift from resource competition to mutualism to promote invasive plant dominance."

Wildfires create ecological opportunity windows for exotic plant invasions in degraded ecosystems, yet the belowground mechanisms driving this process remain poorly understood. To address this gap, a greenhouse experiment investigated how increasing the relative density (0%, 25%, 50%, 75%, 100%) of the invasive Bidens pilosa against a native competitor, Cirsium arvense var. integrifolium, alters soil properties, microbial communities, and plant performance in post-fire soils, using an unplanted soil as a control. The key questions were addressed: (1) What invader relative density threshold enables exotic dominance post-fire? (2) How do belowground resource availability, interspecific interactions, and soil microbial feedbacks collectively govern invasion success? Our study identified dual invasion thresholds governing dominance: at 50% invader density, B. pilosa suppressed the native species via aboveground-belowground resource competition. Above 75% density, it recruited a beneficial microbial consortium (notably Proteobacteria and Glomeromycota) that enhanced positive plant-soil feedbacks (PSFs) modifying microenvironments and amplifying invader competitiveness via trait plasticity. Resource preemption, microbial mediation, and trait plasticity collectively amplified the invader's competitive advantage. We propose a phase-shift model where the primary invasion driver transitions from resource competition below a 50% invader density to microbially-mediated positive feedbacks above a 75% threshold. This framework informs targeted post-fire restoration strategies: restricting resource availability when invader density is below 50%, and disrupting key microbial networks when it exceeds 75%. This study reveals the mechanistic basis of the 'post-fire invasion window,' demonstrating how density-dependent PSFs create ecological tipping points. By integrating resource competition and microbial mediation into a unified phase-shift model, this work advances invasion theory and provides operational thresholds for ecosystem management.