Growing- and nongrowing-season warming affect invasive plant growth via soil legacy effects

Climate warming is seasonally asymmetric, yet its impact on plant invasions is typically studied using uniform year-round warming, which may obscure season-specific mechanisms. In a pristine grassland, we used a four-year 2 × 2 factorial field-warming experiment coupled with a greenhouse bioassay to test how growing- and nongrowing-season warming influence soil nitrogen, fungal communities, and subsequent invasive plant performance. Growing-season warming increased soil NH₄ ⁺-N, NO₃ ^--N, and available nitrogen, and elevated the effective species number of pathogenic fungi. Nongrowing-season warming increased soil NO₃ ^--N and generated soil legacies that enhanced invasive plant biomass and root allocation. Conversely, year-round warming reduced soil NO₃ ^--N and suppressed invasive biomass relative to nongrowing-season warming. Moreover, invasive plant biomass was positively related to soil NO₃ ^--N but showed no correlation with the effective species number of pathogenic fungi. Our findings show that warming produces seasonally contingent soil legacies mediated by specific nitrogen pathways. Predicting invasion dynamics under climate change requires seasonally explicit frameworks, rather than relying on annual mean temperature increases alone.