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

Abstract

In this study we provide data which support the interpretation that tissues of higher plants can constitutea significant source of the short chain n-alkanes with an even/odd predominance (EOP) found in soilorganic 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 (C14C20) 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 nalkane assemblages. These data show that the d13C and dD 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.