The occurrence of short chain n-alkanes with an even over odd predominance in higher plants and soils

In this study we provide data which support the interpretation that tissues of higher plants can constitute a significant source of the short chain n-alkanes with an even/odd predominance (EOP) found in soil organic matter. Gas chromatographic analyses of vegetation (C3 trees, C4 grasses) and associated soil samples (woodland and grassland) from a study site in central Queensland, Australia, revealed that (1) woody vegetation (leaves) and grasses (leaves, roots) contain short chain n-alkanes (C14−C20) with pronounced EOP and (2) such homologues dominate the n-alkane assemblages in its woodland and grassland soils. The presence of short chain n-alkanes with an EOP in some of the vegetation suggests that these may represent a significant source of such alkanes in the woodland and grassland soils. Previous studies have shown that combustion induced thermal breakdown of long chain n-alkanes may produce short chain homologues with an EOP. A history of repeated bushfires at the study site may have contributed to the presence of these n-alkanes in its soils. The co-occurrence of polyaromatic hydrocarbons derived from fire (e.g. retene) indicates that heat related generation of short chain n-alkanes indeed may have played an additional part in the formation of the observed soil n-alkane patterns. These two potential origins were further investigated by compound specific carbon and hydrogen isotope analyses of the plant and soil n-alkane assemblages. These data show that the δ13C and δD signatures of the short chain n-alkanes in the soil resemble those of the plants. Our study therefore provides strong evidence that EOP among short chain n-alkanes can represent a primary (i.e. non-diagenetic) signature, which originates directly from biological sources. In the case of this Queensland soil, the leaves and roots of higher plants are likely to be the principal sources, together with a smaller secondary contribution from the combustion of associated long chain n-alkanes during bushfires.