Immediate impact of two forest wildfires of different severity on soil organic matter abundance and composition

Soils are subject to direct and indirect effects of fire. As a result of global warming, we have to expect an increase of fire frequency and intensity. Soil organic matter (SOM) is the most affected component, in terms of both content and composition. SOM alteration depends on the degree of soil heating that in turn depends mostly on fire intensity and duration. In this study we investigated the topsoil of two forests involved by wildfires of different severity, moderate one extreme the other one. The aim was to evaluate the effect of a single fire on the quantity and composition of SOM, particularly focusing our attention on non-cellulosic neutral sugars and lignin phenols as well as the stable C isotopic ratio to assess if they can be reliable indicators of fire severity. A study site is Orentano, Tuscany, Central Italy, the other Mt. Gordon, Victoria state, south-east Australia, where the sadly famous ‘Black Saturday’ wildfire, occurring in early February 2009 resulted in the incineration of almost half million hectares of forest. At both study sites a burnt area and an unburned area were compared. Results show that both the fires significantly affected the SOM stock, although only the severe fire in Australia resulted in a tremendous loss in SOM. Isotopic C composition significantly changed at both sites. The moderate severity fire affected the lignin but not the sugar content, whereas the extreme severity fire halved both lignin and sugar contents of soil, although neither SOM component appeared to be selectively oxidized. The NMR analysis clearly showed the footprint of fire on SOM, much stronger in the case of the extreme intensity fire. Overall, however, our findings suggest that none of the parameters we analysed can be used as a reliable index of fire severity in soil.