Rhizospheric carbon and nitrogen inputs from alfalfa drive microbial degradation of polycyclic aromatic hydrocarbons...

The rhizosphere is increasingly recognized as a hotspot of biogeochemical interactions, where plants and microbes jointly regulate nutrient and pollutant turnover. However, the ecological mechanisms by which plants coordinate microbial consortia to degrade persistent organic pollutants remain unclear, particularly regarding the role of carbon-nitrogen (C-N) interactions in shaping microbial metabolism and community assembly. Here, we established an alfalfa-polycyclic aromatic hydrocarbons (PAHs) system and integrated DNA-stable isotope probing (DNA-SIP) with random forest analysis and structural equation modelling (SEM) to reveal how C-N synergy drives the formation and function of efficient degradation consortia. The alfalfa rhizosphere selectively enriched twelve taxa with PAHs-degrading potential, including the traditionally autotrophic genus Nitrospira. Efficient degradation of low-molecular-weight PAHs was achieved through a feedback loop of carbon input and nitrogen fixation coupling, while the removal of high-molecular-weight PAHs relied on the co-assembly and activation of microbial consortia through C-N synergistic regulation. While readily available root-derived carbon suppressed direct degradation pathways, it indirectly compensated for this limitation by stimulating nitrogen-fixing bacteria that supported degraders via newly fixed nitrogen. SEM analysis indicated that plants appeared to preferentially allocate carbon to diazotrophs, whose fixed nitrogen sustained both primary degraders and auxiliary taxa, thereby enriching the microbes carrying key functional genes (PAHs-RHDα and nifH), and increasing degradation efficiency. Collectively, our study highlights C-N synergy as a central ecological mechanism in rhizosphere pollutant degradation, reframing phytoremediation as a plant-driven assembly of self-sustaining microbial ecosystems and providing a conceptual framework for next-generation, ecosystem-based green remediation strategies.