Soil C-sequestration mechanisms driven by wildfires

Abstract

The influence of fire on soil C-sequestration mechanisms is reviewed on the light of recent findings from soil organic matter (SOM) laboratory–controlled heating. Concerning fireinduced accumulation of organic C forms, it is useful to differentiate between: i) stabilization of the originally biodegradable SOM for extended periods (i.e., selective preservation leading to the so called ‘inherited organic matter’), and ii) accumulation of humic-like macromolecules, including pyrogenic soil C- and N-forms in soil. In fact, fire effects may result in SOM structural alteration of functional groups, which may lead to soil hydrophobicity e.g. after severe decarboxylations. Fire may also increase the stability of the links between SOM and minerals through the formation of irreversible H-bridges which remain after rewetting the soil. As described elsewhere, intense fires may also determine the formation of new cyclic, condensed organic structures through the polymerization of reactive compounds of low molecular weight. In general, fire causes structural alteration of biomacromolecules (cellulose, lignin, etc.), which became less biodegradable and not easily recognizable by soil enzymes. In fact, very stable complex SOM forms generated after fire include pseudomelanoidins from the thermal dehydration of carbohydrates , or Maillard products (melanoidins) from N compounds and carbohydrates. It has also been shown the formation of stable macromolecular polyalkyl Cforms from the condensation of lipid compounds. Finally, severe heating leads to the accumulation of a continuum of charred lignocellulosic biomass generically referred to as 'black carbon'. From the experimental viewpoint, it is difficult to observe any microbial assimilation of charcoal, either because it did not occur, or because the methods frequently used are not sufficiently sensitive. A schema showing hypothetical mechanisms involved in soil C sequestration driven by heating is provided and discussed