Soil organic matter quality and microbial catabolic functions along a gradient of wildfire history in a Mediterranean ecosystem

The principal aim of this research was to determine the influence of an increasing wildfire history on the recovery at short and long term of soil organic matter (SOM) composition and microbial properties. The contemporary wildfire events (since 1950) were recorded for 27 plots located on the siliceous part of the French Mediterranean region (Maures mountain ranges). A wildfire history index was built, tested and calculated in order to display numerical values representative of the different wildfire history parameters (i.e. number of fires, time since fire and mean fire interval). Microbial basal respiration and biomass were analysed as well as intensity of the use of 31 C-substrates, catabolic diversity and C-substrates utilisation profiles. Furthermore, a qualitative characterisation of the SOM was carried out by solid state 13C NMR. Potential drivers of the microbial recovery were identified by studying the relationships between microbial activities and chemical functions of SOM. Our results showed that fire histories resulting in considerable losses or alterations of SOM, such as recent or close fires, decreased the microbial catabolic evenness. This could be attributed to a preferential utilisation of N-containing compounds and complex substrates such as aromatic and polymers reflecting a greater N microbial demand and a selection of specific catabolic functions. Moreover, a large number of fires (4 fires in 57 years compared to 1–2) resulted in lasting degradation of the relative intensity of methyl C function in polymethylene, O-Alkyl C, aromatic C and phenolic C functions inducing a slow-down in recovery of microbial properties. These results also confirm our hypothesis that some chemical functions of SOM can be in equilibrium with wildfire history. Finally, this research demonstrates that FT-NIR analysis can be used as a valuable tool to assess both the wildfire history and the vulnerability of soil quality to shifts in historical fire regimes.

Research highlights
► The developed wildfire history index efficiently combines various wildfire parameters.
► Mediterranean soils are in equilibrium with wildfire history depending on the number and time since fire.
► Lasting degradation of SOM quality by frequent fires decreases the microbial catabolic evenness.
► The recovery in catabolic profiles is related to the recovery of the more decomposable fraction of SOM.
► FT-NIR is a valuable tool to assess the wildfire history and soil quality.