Causes of bulk carbon and nitrogen isotopic fractionations in the products of vegetation burns: laboratory studies

Bulk stable isotope analysis is a means for the characterization of the sources of carbonaceous and nitrogenous material aerosols derived from biogenic sources. In order to use stable isotope techniques for characterizing the products of vegetation burns the isotope effect of combustion must be known. The C3 vegetation Colospherum mopane and Eucalyptus sp. and the C4 vegetation Cenchris cilliarus, Antephora pubesence and Saccharum officinarum, were burned under controlled conditions in the laboratory in order to better understand how the process of combustion affects the isotopic fractionation of the produced material. Carbon isotopes for aerosol particles formed during controlled laboratory burns of C3 vegetation were higher in δ13C by 0.5‰ compared to the source vegetation. Aerosol particles captured above the controlled laboratory burns of C4 vegetation were lower in δ13C by 3.5‰ compared to the source vegetation. The proposed causes for the different isotope effects shown for C3 and C4 sourced products are differences in the oxidation chemistry of these two plant types. Aerosol particulate material and ashes produced during the controlled laboratory burns of the vegetation are higher in δ15N than the source vegetation by 6.6‰ and 2.5‰, respectively. Furthermore, δ15N values for the residual material produced when Eucalyptus sp. samples were heated at discrete temperatures, suggest that different pools of nitrogenous compounds are accessed at different temperatures of heating.