Aromatic Nitrogen-Mediated Enhanced Mineralization of Dissolved Black Carbon through Microbial Dissimilatory Iron Reduction

Microbial dissimilatory iron reduction (DIR) critically influences dissolved organic matter transformation, yet the molecular mechanisms, particularly the nitrogen (N)'s role, remain unclear. Here, we compared dissolved black carbon (DBC, pyrogenic) and leached dissolved organic carbon (LDOC, biogenic) as sole electron donors during DIR. DBC produced 3-fold more ferrous iron than LDOC and achieved higher N-containing compounds mineralization (41% versus 23%), exclusively as ammonium. Fourier-transform ion cyclotron resonance mass spectrometry with N near-edge X-ray absorption fine structure analysis revealed that DBC degradation targets polycyclic aromatic components rich in aromatic N, especially 5-membered rings, accounting for nearly half of dissolved organic nitrogen loss. The estimated electron flux from pyrrolic N degradation alone contributed at least 11% of the total ferrous iron production in DBC. LDOC preferentially removes labile amide N from lignin-like components. N-containing molecular formulas decreased by 51.6% in DBC versus 10.2% in LDOC. Thermodynamic calculations confirm aromatic N dominates degraded DBC. These electron-rich Lewis basic structures form stable iron coordination complexes that facilitate electron transfer and activate adjacent carbon bonds for oxidative ring-cleavage. In contrast, LDOC mineralization followed conventional carbon-centered anaerobic pathways. This study highlights previously overlooked high DBC reactivity in DIR, driven by N-mediated pathways centered on aromatic N.