Nevertheless, laboratory simulation experiments have unambiguously documented the selective destruction of humus fractions, the chemical alteration in SOM properties, and external inputs of charcoal and charred lingo-cellulose from the vegetation.

Research theme 3 supports activities aimed at studying refractory molecular structures formed during fire events and the contribution of these structures to long-term soil C storage.

Theme 1: Molecular composition and turnover of soil organic matter

Theme 2: Plant molecular structures as drivers of C stabilisation in soils

Theme 4: Molecular markers in soils

Theme 5: Dissolved organic molecules in soils: origin, functionality and transport

Theme 3: Fire transformations of plant and soil molecular structures

Wildfires have a substantial bearing on the long-term sequestration of C and N in forest soils. These processes should be better understood, since small changes in the C storage potential of the soil may alter the global C cycle. Some fire-affected ecosystems may act as sinks for atmospheric C because vegetation fires and fuel-wood combustion transfer C from the relatively fast biological-atmosphere C turnover to the long-term geological one in the form of refractory C.

Most studies probing the formation of refractory C have focused on thermal degradation of lignocellulosic material. Laboratory experiments have proved that forest fires or controlled burnings of crop residues induce a series of inter- and intra-molecular reactions leading to condensed materials with chaotic structure and close resemblance with humic or kerogen-like matter. In addition, other biomolecules such as peptides or lipids may experience major alterations during heat treatments.

The simultaneous occurrence of different processes during forest fires makes it extremely difficult to accurately describe SOM transformations in burned areas. These may range from the total loss of SOM in the top soil or the surface of swamps and peat deposits, to an increase of the passive SOM pool due to the incorporation of necromass from fire affected vegetation, but also partially and heavily charred plant remains (highly refractory organic matter collectively referred to as “black carbon”) on the ground.