Climate warming can enhance the growth performance of invasive plants, potentially increasing their nitrogen (N) demand. However, whether this additional demand is supported by free-living N fixation remains unclear. Here, we used a 12-year warming experiment to determine how warming affected diazotrophic abundance, community composition, and nifH transcription in the roots and rhizosphere of the invasive plant Solidago canadensis during the growing and flowering stages. While warming increased plant biomass, it did not increase diazotrophic abundance or nifH transcription in either roots or rhizosphere soil. In the rhizosphere, warming even suppressed nifH transcription during flowering, which coincided with pronounced shifts in the rhizosphere metabolome and diazotrophic community composition, particularly among Bradyrhizobium-affiliated taxa. By contrast, these associations were weak or absent during the growing stage, indicating that plant developmental stage and concurrent seasonal variation modulate the responses of diazotrophs to climate warming. Overall, given the relatively high N availability at our study site, warming-enhanced growth of S. canadensis was unlikely to be supported by increased inputs of newly fixed N. Instead, S. canadensis may have met its increased N demand primarily through morphological and physiological adjustments that favour more efficient uptake and use of existing N pools, as indicated by higher specific root length and increased leaf N-use efficiency under warming. Future studies that directly quantify N fixation rates across gradients of N availability are warranted to validate our findings, especially in N-limited conditions where biological N fixation may contribute more substantially to plant N acquisition.