HR: 1340h
AN: B13C-0231    [Abstracts]
TI: Compound-Specific d13C And dD Analyses Of Plant And Soil Organic Matter: Implications For Water Sources And C3-C4 Vegetation Change Studies
AU: Krull, E S
EM: Evelyn.Krull@csiro.au
AF: CSIRO Land & Water, PMB 2, Glen Osmond, 5064 Austria
AU: * Gleixner, G
EM: ggleix@bgc-jena.mpg.de
AF: Max-Planck Institute for Biogeochemistry, Postfach 100164, Jena, 07701 Germany
AU: Sachse, D
EM: dirk.sachse@bgc-jena.mpg.de
AF: Max-Planck Institute for Biogeochemistry, Postfach 100164, Jena, 07701 Germany
AB: Here we present d13C and dD data of C27-C31 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 C4 species ({\it Iseilema} and {\it Astrebla}) from the grassland were consistently 13C-enriched relative to C3 tree plant materials ({\it Acacia} leaves and seedpods and {\it Atalaya} leaves) from the woodland and woody grassland. However, n-alkanes from the C4 grasses were \deltaD depleted (-77\permil) relative to the {\itAcacia} leaves and seedpods, but showed no difference in dD values when compared with C3 {\it Atalaya} leaves. This is contradictory to data from previous studies, showing that C4 plants were enriched in \deltaD relative to C3 plants (the same direction as the d13C values). This past observation has been ascribed to C4 plants accessing the more evaporation-influenced (D-enriched) surface water and tree roots sourcing more D-depleted deeper soil water. Our data, on the other hand, indicate that ecosystem characteristics (woody versus grassy) have a greater influence on the dD values of the vegetation than the type of photosynthetic pathway. Specifically, the differences in dD values from "woodland" trees ({\it Acacia}) compared with "woody grassland" trees ({\it Atalaya}) suggest that the dD of soil water in semi-arid climates is ecosystem-dependent. This concept is supported by d13C and dD analysis of the C31 n-alkane, a grass-specific biomarker, from woodland and grassland soils. The similar d13C values in the woodland (-25.2ñ0.34\permil) and grassland soil (-25.1ñ0.03\permil) confirmed that the C31 n-alkane is grass-derived. The dD values of the C31 n-alkane, on the other hand, were by 12\permil enriched in the soil under the woodland compared with the one under grassland, supporting that the dD values of the soil water profile are governed by the hydrological characteristics of the ecosystem, not by photosynthetic pathway. The C27 and C29 n-alkanes (tree-specific biomarkers) from the woodland soil were more enriched in dD than the C31 n-alkane and were very similar to the values of the Acacia leaves. A crossplot of the d13C and dD values of the long-chained n-alkanes from plants and soil organic matter indicates that isotopic values from soil organic matter n-alkanes faithfully record recent changes in vegetation (in this case C4 to C3-dominated) as well as changes in water sources as the ecosystem changed from grassy to forested. Thus, the data from this study provide insight into soil-water-plant dynamics in semi-arid climate soils and caution against the assumption that d13C and dD differences in C3 and C4 plants are independent of climate and water stress.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting