How important is "Black Nitrogen" for C-sequestration in soils?

Abstract

Charcoal, produced during vegetation fires is well recognized as an important C sink. Whereas severe fire can lead to complete destruction of the organic layer, moderate wildfire result in minor alterations of soil organic matter (SOM) and sometimes even result in an increase of organic C and N due to input of partly charred material. Moderate heating of vegetation residues leads to little changes of the C/N ratios. It can be as wide as 440 to 630 for burnt woody material but also as narrow as 6.0 to 6.9 as it was found for char from for young grass material. These C/N ratios clearly show that during charring, N is incorporated into structures which are fairly resistant to heating. As demonstrated by 15N NMR those structures are mostly pyrrole/indole-type N with minor contribution of pyridine N. In non-woody chars, such N-heteroaromatics can consume up to 17% and sometimes even 60% of their organic C, which indicates that char models assuming a graphite-like structure may be oversimplified. However, compared to most proteinaceous compounds in litter, heteroaromatic N is less accessible for microorganisms. Thus, incorporation of char into SOM increases the stable soil organic carbon content, but also the amount of inert refractory organic nitrogen. The low biological availability of this nitrogen has an impact on biomass production but also on SOM turnover. As demonstrated by its presence in a forest soil 24 years after the last fire, this pyrogenic N seems to be relatively stable and thus is expected to have a long term impact on soil biochemistry. The collected data clearly show that “Black Nitrogen” (BN) certainly needs more attention if a better understanding of SOM stabilization is wanted.