Influence of fire severity indexes and wildfire vs prescribed fires in soil organic matter quality: an isotopic approach

Resúmen de la Comunicación presentada en el XI Congreso Internacional FuegoRED 16-18 de octubre, Salamanca (España)

Fires are widespread occurrences in many ecosystems, comprising a normal, even crucial, component of a forest's ecology, however recently repeated large-scale wildfires of high intensity and severity that can be severely devastating. The use of stable isotopes in their natural abundance has taken great importance to enhance the knowledge of ecosystems functioning under major alterations such as fire occurrence [1]. This work describes the relationships between the field soil burning severity (SBS) index and the intramolecular¿level changes transformation in the soil organic matter (SOM). The chemical fractionation of SOM was examined by the determination of stable isotopes of C, with varying SBS indices (ranging from 1 to 5) in two sample sets in Pinus pinaster forest stands from the coastal area of Galicia (NW Spain): 1) soils that had been burned by a wildfire, and 2) soils taken from monoliths burned experimentally. Unburned control soils (with an SBS index of 0) were also analyzed. Results show in a natural wildfire, ¿13C values tended to be more depleted from level 2 to level 5, ranging values between -26.5 and -27.3 ¿. This depletion found may be attributed to the methoxyl groups during their conversion to volatile compounds [2]. Conversely, ¿13C values from a prescribed, experimental fire became more 13C enriched with increasing fire severity. As previously found in previous studies [3,4], a general 13C enrichment could be explained by the input from the burned layer of litter and left ashes. Also, this enrichment with increasing burning temperatures indicates that the lighter isotope is preferentially volatizing. However, this is only perceived under controlled fire conditions, where differences could be explained by the higher variability in soil temperatures in wildfires. Further discussion and implications on the use of stable isotopes to assess fire severity in different context will be made.

This study has been partially funded by the Ministry of Science Innovation and Universities (MICIU) through the FIRE2C project (CNS2023-143750). N.T.J-M. thanks the Ramón y Cajal research contract (RyC2021-031253-I). L.M.S.E. thanks the Foundation for Science and Technology (FCT), Portugal for the postdoctoral contract EURAXESS offer IR: 145344. This work was partly supported by the EROFIRE project (PCIF/RPG/0079/2018)