How does fire affect the nature and stability of soil organic nitrogen and carbon? A review

After vegetation fires considerable amounts of severely or partly charred necromass (referred to here as char) are incorporated into the soil, with long-term consequences for soil C and N dynamics and thus N availability for primary production and C and N transport within the soil column. Considering results reported in the pyrolysis literature in combination with those obtained from controlled charring of plant material and soil organic matter (SOM), it has become clear that common models claiming char as a graphite-like material composed mainly of highly condensed polyaromatic clusters may be oversimplified. Instead, I suggest a concept in which char is a heterogeneous mixture of heat-altered biopolymers with domains of relatively small polyaromatic clusters, but considerable substitution with N, O and S functional groups. Such a concept allows fast oxidation facilitating both microbial attack and dissolution. Although, char is commonly believed to degrade more slowly than litter, over the long term and under oxic conditions, char may degrade to an extent that it becomes indistinguishable from naturally formed SOM. Oxygen depletion or environments with low microbial activity may be necessary for char to survive without major chemical alteration and in considerable amounts for millennia or longer.