Measurement for fire damage level in foliar biomass using pyrolysis compound specific hydrogen isotopic analysis (¿D Py-CSIA)

Fire is one of the most important drivers of soil organic matter (SOM) sequestration. In fact, accurate knowledge of wildfires’ effects on the SOM may provide information on its resilience and physico-chemical properties, as well as on the subsequent soil recovery without human intervention. While the quantitative assessment of fire damage levels on SOM is an issue of great interest to researchers, there is not general consensus on the reliability of the physical and chemical methods frequently used to provide surrogates on the real impact of the passage of fire on the soil. In fact, fire effect on soils depends on a complex interaction of factors (temperature, time, moisture, soil depth…) and in general, characterization of SOM samples from fire-affected ecosystems implies the use of several instrumental techniques. This work aims to provide reliable quantitative proxies for pyrogenic SOM in whole environmental samples, taking advantage from changes in hydrogen isotope ratios (δD) of specific compounds released by analytical pyrolysis (Py-CSIA). With this aim in mind, we carried out a series of controlled burnings of progressive intensity, under laboratory conditions, applied to dry samples of foliar biomass from a typical Mediterranean forest species (Juniperus thurifera) heated at 367 °C for different time periods (5', 5'30>, 6' and 7'). The results showed that isotopic analysis of up to 9 compounds (eicosanol, eugenol, stigmastan-3,5-diene, phenol, phenol, 2-methoxy-4-methyl, pimaric acid, limonene, hexadecenoic acid and methylabietate), common in all samples, allowed a valid assessment of the changes in the δD with the different times of burning. Pimaric acid was the only major compound that had a linear correlation y = 3.7x + 153.6 with a R2 = 0.9209, where y corresponds to δD and x is the burning time. The rest of compounds showed different behaviour with the progressive heating, with no defined trends: after the first heating stage, some compounds suffered depletion of the heavy isotope, others were not isotopically altered and others had slight deuterium enrichment (Fig. 1). Nevertheless, all compounds detected in samples subjected to the most intense heating suffered conspicuous deuterium enrichment. The change of isotope signature between control and sample that was burned for 5 min was high in several pyrolysis compounds; therefore, we need to continue the experiment within a broader range of burning times. Our results illustrate that the combination of analytical pyrolysis and isotopic analysis of specific compounds (PyCSIA) opens a new application for the quantification of fire damage levels, and also in the understanding of fire-induced structural changes in SOM, avoiding sample fractionation or wet chemical extractions. Fig. 1: Juniperus thurifera δD variations of 9 specific pyrolysis compounds through different heating periods at a constant temperature of 367 ºC.Peer Reviewe