Nonpyrogenic "Black" Nitrogen Likely Plays a Major Yet Underrecognized Role in the Global Nitrogen Cycle

Condensed aromatic nitrogen (ConAN) is commonly found in soil and aquatic environments. ConAN is broadly viewed by the biogeochemistry community as a combustion-derived product leading to its frequent designation as pyrogenic "black" nitrogen. Here, we provide evidence that ConAN can also form through nonpyrogenic pathways. In an experiment simulating biomass oxidation, pine wood boards assembled with iron nails were exposed to 12 years of natural environmental conditions. Nitrogen concentrations increased from 0.2 to 1.2 wt % with increasing proximity to the nail. Ultrahigh resolution mass spectrometry showed enrichment of nitrogen-containing compounds within the lignin and condensed aromatic domains. X-ray spectroscopic characterization revealed the development of C-N bonds and formation of pyridinic and pyrrolic nitrogen moieties. We propose plausible nonpyrogenic formation mechanisms for ConAN and perform back-of-the-envelope calculations suggesting that biomass oxidation yields a nonpyrogenic ConAN flux to soils of 3-63 Tg-N yr^-1. These findings demonstrate that ConAN can form under fire-free environmental conditions, highlighting nonpyrogenic ConAN as an underrecognized but potentially major component of the global nitrogen cycle. More broadly, this study reveals a previously ignored pathway for the long-term incorporation of nitrogen into refractory "stable" organic matter in soils and other environments.