Effects of fire severity on soil organic matter: a multi-isotope (C, N, H, O) comparison of wildfires and experimental burns

Fire severity is key in shaping soil organic matter (SOM) composition, affecting biogeochemical cycles and post-fire recovery. However, accurately assessing fire severity is highly challenging, requiring the implementation of soil burn severity (SBS) indexes. This study employs a multi-isotope and elemental approach (C, N, H, O) to assess fire-induced changes in SOM quantity and quality across soil burn severity (SBS) levels in 0–2 cm mineral soil samples from wildfires and controlled experimental burns on intact soil monoliths in pine forests of Galicia (NW Spain). Results indicate that total organic carbon (TOC) and nitrogen (TN) decreased with increasing burn severity, with TOC losses reaching up to 90% in the most severe burns. Stable isotope analyses revealed divergent fractionation patterns: carbon isotope (δ 13 C) depletion in wildfire-affected soils suggested preferential combustion of lignin-derived compounds, whereas experimental burns exhibited slight δ 13 C enrichment due to the removal of thermolabile organic fractions. Similarly, nitrogen isotope (δ 15 N) values increased with burn severity, which may be linked to the alteration soil microbial community and selective nitrogen volatilization. Hydrogen (δ 2 H) and oxygen (δ 18 O) isotopes showed distinct evaporative and oxidative fractionation trends, with experimental fires displaying stronger δ 2 H enrichment due to more controlled combustion conditions. These findings highlight the utility of elemental and stable isotope analysis in distinguishing between fire types and assessing fire impacts on SOM. These results confirm the suitability of this system of visual indicators for assessing changes in the quality of organic matter in a simple way after a fire.