Ultra‐high resolution mass spectrometry (FT‐ICR‐MS): the effect of a wildfire in soil organic matter

Abstract

Vegetation fires are a global phenomenon occurring in tropical, temperate, Mediterranean and boreal regions. Up to 90% of all forest fires in the EU occur in Mediterranean countries [1]. Wildfires generally affect soil organic matter (SOM) that is the most reactive fraction, resulting in changes to several properties and functions [2]. One of the most recent techniques applied to SOM research is Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). This, together with graphic interpretation tools such as van Krevelen diagrams [3] may be also used to assess alterations to SOM caused by fire. The site of study was a sandy soil under Cork oak (Quercus suber) vegetation in Doñana National Park (SW‐Spain). Alkaline extracts of soil sieve fractions (coarse, 1‐2 m “CF” and fine, <0.05 mm “FF”) collected in a burned (B) area and an adjacent unburned (UB) control site with the same physiographic conditions were studied. The instrument used was a Bruker Daltonics 12 Tesla Apex Qe FT‐ICR‐MS equipped with an Apollo II ESI ion source, operating in negative ion mode [4]. Unique molecular formulas were assigned to peaks using a MatLab script and the compounds detected grouped into 7 main chemical families [3]. Unburnt soil: compounds with high intensity in the lignin/tannin and carbohydrate‐like regions were found for UB CF and in the lipid and protein‐like regions for the UB FF. This suggests different degrees of SOM evolution with that in CF less altered with higher contribution of fresh material than that in FF; the latter believed to be subjected to higher microbial activity/humification processes. Burnt soil: high relative intensity in the carbohydrate‐like and lignin‐like regions and new molecular formulas in the aromatic and condensed aromatics regions are found for the B CF, suggesting fresh organic matter inputs as well as contributions from a more recalcitrant carbon pool. In the B FF, two SOM sources of alteration were identified; i) biogenic (microbial) with high intensity of lipid/protein‐like compounds, similar to UB FF and ii) pyrogenic compounds in the condensed aromatic region together. This indicate both, condensation processes yielding black carbon like materials and additions from new chemical biogenic compounds i.e. from shifts in the microbial activity after fire or from fire mediated distillation processes.

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