Tuesday, November 14, 2006

Production of Two Biomass Burning Tracers During A Prescribed Fire : Black Carbon vs Levoglucosan.

Marie A. Alexis1, Cornelia Rumpel1, Patrick Louchouarn2, Nicolas Péchot1, Daniel Rasse1, and André Mariotti1. (1) BIOEMCO, Bldg EGER, Thiverval-Grignon, 78850, France, (2) Texas A&M University, 5007 Ave U, Galveston, TX 77551

Fire is the main disturbance for terrestrial ecosystems, with a strong effect on biogeochemical cycles. Especially, part of the ecosystem organic matter (OM) is chemically modified by temperature elevation. Depending on fire severity, a big variety of chemical structures is produced ranging from slightly altered OM to strongly condensated structures. The fate of these pyrogenic OM when added to soil is unclear. Highly aromatic black carbon (BC) may be the most stable part of the continuum. At molecular level, levoglucosan is the main fire product of cellulose alteration. These two compounds have been separately used as tracers of plant biomass burning in aerosols, soils and sediments. Their combined use may provide closer insight into conditions and OM transformations that occurred during the fire.
We aimed at quantifying BC and levoglucosan in plant residues after fire. Their production rates were compared to improve the understanding of their relative contribution to soil OM. Litter leaves were collected after a prescribed burning. The >2mm fraction was visually separated into charred (black, shiny) and unburned (brown) particles. BC was quantified by chemical oxidation (K2Cr2O7/H2SO4) and elemental analyses. Levoglucosan was identified and quantified by GC/MS analysis of the total lipid extract.
Unburned post-fire leaves contain more levoglucosan than charred leaves, showing that a chemical alteration occurred despite no visual evidence. Moreover BC and levoglucosan concentrations are negatively correlated. This is consistent with their expected production temperatures: levoglucosan may be destroyed at temperature BC is produced. Relative quantity of theses compounds may then provide information about fire severity. However while BC is expected to be stable in soil, levoglucosan may suffer from degradation processes. Consequently, for historical reconstitution their respective fates in soil degrading conditions have to be considered.