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Abstract

Here we present delta C-13 and delta H-2 data of long-chained, even-numbered (C-27-C-31) n-alkanes from C₃ (trees) and C₄ (grasses) plants and from the corresponding soils from a grassland-woodland vegetation sequence in central Queensland, Australia. Our data show that delta C-13 values of the C₄ grassland species were heavier relative to those of C₃ tree species from the woodland (Acacia leaves) and woody grassland (Atalaya leaves). However, n-alkanes from the C₄ grasses had lighter delta H-2 values relative to the Acacia leaves, but showed no significant difference in delta H-2 values when compared with C₃ Atalaya leaves. These results differ from those of previous studies, showing that C₄ grasses had heavier delta H-2 values relative to C₃ grasses and trees. Those observations have been explained by C₄ plants accessing the more evaporation-influenced and isotopically heavier surface water and tree roots sourcing deeper, isotopically lighter soil water ("Two-layered soil-water system"). By comparison, our data suggest that ecosystem changes (vegetation "thickening") can significantly alter the soil hydrological characteristics. This is shown by the heavier delta H-2 values in the woodland soil compared with lighter delta H-2 values in the grassland soil, implying that the recent vegetation change (increased tree biomass) in the woodland had altered soil hydrological conditions. Estimated delta H-2 values of the source-water for vegetation in the grassland and woodland showed that both trees and grasses in open settings accessed water with lighter delta H-2 values (avg. -46 parts per thousand) compared with water accessed by trees in the woodland vegetation (avg. -7 parts per thousand). These data suggest that in semi-arid environments the "two-layer" soil water concept might not apply. Furthermore, our data indicate that compound-specific delta H-2 and delta C-13 analyses of n-alkanes from soil organic matter can be used to successfully differentiate between water sources of different vegetation types (grasses versus trees) in natural ecosystems. (c) 2006 Elsevier Ltd. All rights reserved. [References: 56]