Deciphering the Chemistry of Condensed Aromatic "Black" Carbon and Nitrogen in Amazonian Anthrosols

Amazonian anthrosols are renowned for their high fertility and dark color, properties primarily attributed to the abundance of condensed aromatic carbon (ConAC) in the soil organic matter. ConAC, commonly referred to as black carbon, play a key role in the stability and nutrient retention of these soils. However, the processes governing the formation of ConAC and its transformation into oxygenated derivatives remain poorly understood. In this study, we used multiple analytical platforms to investigate the chemistry of ConAC-rich humic acids (HA) extracted from Terra Mulata de Indio, a type of Amazonian anthrosol. The results reveal that ConAC are predominantly nonprotonated and consist of approximately 4-10 condensed rings. These structures exhibit varying degrees of oxygenation (1-24 oxygen atoms), suggesting that they are produced through oxidative processes. Approximately 20% of ConAC contain nitrogen atoms, referred to as ConAN (condensed aromatic nitrogen), which are part of either heterocyclic ring systems (commonly termed black nitrogen) or present as amine functional groups. As a result, we conclude that HA in Amazonian anthrosols contain polycyclic N-containing aromatic acids (PolyNARA), likely formed through combined charring of plant and animal biomass, abiotic nitrogen incorporation, and/or other soil processes. The mechanisms governing the formation, persistence, and transformation of PolyNARA in Amazonian anthrosols warrant further investigation, particularly given their potential implications for global carbon and nitrogen biogeochemical cycling.