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 (SUM). In this paper, recently burned and adjacent unburned areas of two coastal pine forests of Tuscany, Central Italy, were compared in terms of SUM quantity and quality to assess the nature and intensity of such fire-induced changes. Quantitative modification of the SUM stock was investigated by sampling in a grid pattern. Pre-fire and post-fire bulk SUM were analysed for stable carbon and nitrogen isotopes concentrations (delta(13)C and delta(15)N) and spectroscopic properties (solid-state CP-MAS (13)C NMR). In addition, we determined the fraction of SUM 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 SUM 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 (13)C and (15)N 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 SUM stocks and its chemical composition.