“Black nitrogen” – an important fraction in determining the recalcitrance of charcoal

The chemical and thermal stability of organic nitrogen (N) in pyrogenic organic matter (PyOM) was examined by charring casein at 350 °C and 450 °C. The alteration was compared with that observed for char derived from lignin, cellulose, grass and wood. With respect to heating, casein showed a considerably higher stability than cellulose. Comparable proportions of carbon (C) and N were recovered, supporting the idea that black nitrogen (BN) represents an integral part of the char structure. Although some amides were still present, they lost importance with increasing temperature. Charring of grass revealed an enrichment in N compounds because of the low thermal stability of cellulose. The similarity in the resulting nuclear magnetic resonance (NMR) spectra to those of casein char confirmed that BN can play a major role in the chemical composition of plant char. ting the chars to oxidation with acidic dichromate demonstrated that, in spite of their relatively high resistance to heat, the N-containing compounds of the chars were less recalcitrant than the components of the cellulose char. Thus, in soil, N-rich chars are likely to be underestimated on the basis of this method. On the other hand, for an ancient paddy soil whose N-containing char compounds were calculated to account for ca. 25% of the total organic C in the soil. This clearly underlines the pedogenic stability of BN and confirms that it has the potential to contribute significantly to the refractory soil organic matter pool.