Root exudates stabilize denitrification yet amplify CO₂ emissions by priming effect in constructed wetlands

Root‑exudated carbon is a key microbial substrate in constructed wetlands (CWs), yet its net impact remains unclear due to its relatively low flux and quantification challenges. Here, we traced its allocation and metabolic pathways using ¹³C-DNA-stable isotope probing coupled with metagenomics, thereby decoupling its role under gradient exogenous carbon inputs. The denitrification potential derived from root‑exudated carbon remained stable regardless of exogenous carbon fluctuations, reducing 1.2-1.44 mg·L^-1 N per mg·L^-1 C. Root‑exudated carbon significantly enhanced the priming effect by 0.9-1.54 times with exogenous carbon inputs (p < 0.05), resulting in an amplification of CO₂ emissions. The denitrification and carbon emissions derived by root exudates was non-linear effects mediated by microbial regulation. DNA-SIP coupled with metagenomics indicated that differences in denitrification and CO₂ emissions response to root‑exudated carbon from substrate quality, microbial community dynamics, and metabolic strategies. The microbial community of the ¹³C-labeled heavy fractions was characterized by active denitrifiers, including Pseudomonas, Aeromonas, and Pseudoxanthomonas, which contributed the highest direct positive effect (20.23 %) to denitrification. Root‑exudated carbon influenced CO₂ emissions by directly altering the DOM composition (contributing 37.01 %) and upregulating C-degrading genes (contributing 13.04 %). These findings revealed root exudated-carbon differentially regulate carbon and nitrogen metabolisms, challenging our understanding of the synergistic enhancement of water purification and climate mitigation functions in CWs.