Wildfire effects on soil organic matter quantity and quality in two fire-prone Mediterranean pine forests

Pine forests bordering the Mediterranean Sea are often affected by severe wildfires, which cause major changes to soil organic matter (SOM). In this paper, recently burned and adjacent unburned areas of two coastal pine forests of Tuscany, Central Italy, were compared in terms of SOM quantity and quality to assess the nature and intensity of such fire-induced changes. Quantitative modification of the SOM stock was investigated by sampling in a grid pattern. Pre-fire and post-fire bulk SOM were analysed for stable carbon and nitrogen isotopes concentrations (δ13C and δ15N) and spectroscopic properties (solid-state CP-MAS 13C NMR). In addition, we determined the fraction of SOM resistant to acid dichromate oxidation, here assumed to be “pyrogenic” carbon, and the lignin composition after CuO oxidation.The results showed that at the two sites there were similar, significant losses in soil C, in spite of different times passed since the fire. Such C losses are attributable to the almost complete elimination of the litter layer, whereas the underlying mineral soil did not show any significant change in its carbon content. In terms of composition, the bulk SOM did experience a minor decrease in O-alkyl C and a parallel increase in aromatic C, but no significant change in the relative concentrations of 13C and 15N and the amount of pyrogenic C. On the contrary, fire markedly modified soil lignin composition. We recorded changes in the coumaryl to vanillyl and syringyl to vanillyl ratios of lignin-derived phenols, both of which are indicators of the lignin source. Furthermore, at one site, fire significantly increased the acid to aldehyde ratio of vanillyl and syringyl phenols, indicating that fire can affect soil lignin degradation and the overall C cycle. In conclusion, this study suggests that in fire-prone Mediterranean pine forests each fire may influence total SOM stocks and its chemical composition.

► Wildfire had a marked impact on SOM of two fire-prone Mediterranean pine forests.
► Wildfire significantly decreased the soil C stock, mainly removing the litter layer.
► No major change of SOM in terms of C/N ratio and concentrations of 13C and 15N was detected.
► Soil lignin experienced fire-induced alterations in the distribution and composition of phenols.