The high content of mineral-free organic matter in soils increases their vulnerability to wildfire in humid-temperate zones

Under a new scenario of global warming, in which wildfire is predicted to affect new forest areas in temperate and boreal climates, information about the properties of soil organic matter (SOM) may be crucial for assessing the impact of wildfire. Unlike Mediterranean soils, the soils in temperate and boreal areas are richer in SOM in the form of unprotected, mineral-free SOM, which is presumably particularly sensitive to heating.To address this topic, three recently burned, SOM-rich Atlantic forest soils, in which different levels of soil burn severity were distinguished, were selected for study. Soil organic matter density fractions (free particulate OM [fPOM], occluded particulate OM [oPOM] and mineral-associated OM [MAF]) and different particle size fractions of MAF (200–63, 63–20 and <20 µm) were obtained and analyzed by thermal analysis (DSC-TG) and solid state nuclear magnetic resonance (¹³C CPMAS NMR). The effect of heating on SOM was also studied in relation to the changes observed in the water repellency and aggregate size of the soil.Heating led to important losses of the three SOM density fractions, particularly the fPOM. The SOM remaining after fire was thus dominated by oPOM and MAF. Although recalcitrance increased in all physical SOM fractions, the effect was greatest in the fPOM, which can be attributed to differences in the combustion conditions and to the composition of these OM pools.Loss and changes in the composition of fPOM explained the reduction in water repellency usually observed in these soils after intense heating. The loss of MAF in severely burned soils seems to be the main driver of the aggregate breakdown. Depletion of this SOM fraction may affect the resilience of the system because of the low rate of replenishment. Although the high degree of recalcitrance of mineral-protected OM may be important for C sequestration in soil, the present findings revealed that the potential effect is relatively small.