The hydrogen isotopic composition of plant leaf wax（δDwax） is used as an important tool for paleohydrologic reconstruction. However, the understanding of the relative importance of environmental and biological factors in determining δDwax values still remains incomplete. To identify the effects of soil moisture and plant physiology on δDwax values in an arid ecosystem, and to explore the implication of these values for paleoclimatic reconstruction, we measured δD values of soil water（δDwater） and δDwax values in surface soils along two distance transects extending from the lakeshore to wetland to dryland around Lake Qinghai and Lake Gahai on the northeast Qinghai-Tibetan Plateau. The results showed that the δDwater values were negatively correlated with soil water content（SWC）（R^2=0.9166）, and ranged from –67‰ to –46‰ with changes in SWC from 6.2% to 42.1% in the arid areas of the Gangcha（GCh） and Gahai（GH） transects. This indicated that evaporative D-enrichment in soil water was sensitive to soil moisture in an arid ecosystem. Although the shift from grasses to shrubs with increasing aridity occurred in the arid area of the GH transect, the δDwax values in surface soils from the arid areas of the two transects still showed a negative correlation with SWC（R^2=0.6835）, which may be due to the controls of primary evaporative D-enrichment in the soil water and additional transpirational D-enrichment in the leaf water on the δDwaxvalues. Our preliminary research suggested that δDwax values can potentially be applied as a paleo-humidity indicator on the northeast Qinghai-Tibetan Plateau.
To quantitatively analyze the response of distributions and hydrogen isotopic compositions(δD) of plant leaf wax to moisture, and to better understand their implications for paleoclimatic reconstruction, we measured average chain length(ACL) and δD values of n-alkanes and n-fatty acids(n-FAs) from Orinus kokonorica, a typical and representative plant in Lake Qinghai area, along a distance transect extending from lakeshore to wetland to dryland in the arid ecosystem. The results showed that the ACL values of n-alkanes and n-FAs were negatively correlated with soil water content(SWC) with R2=0.593 and R2=0.924, respectively. This is as a result of plant's response to water loss with more abundance in long-chain n-alkyl lipids under increasing aridity by analyzing relationships between the molecular ratios of long-chain n-alkyl lipids(n-alkanes and n-FAs) from O. kokonorica and SWC. The δD values of C29 n-alkane and C28 n-FA were also negatively correlated with SWC with R2=0.778 and R2=0.760, respectively, which may due to enhanced D-enrichment in leaf water by evapotranspiration(soil water evaporation and leaf water transpiration) with increasing aridity. Our results demonstrated that moisture exerts a significant control on the ACL and δD values from O. kokonorica in an arid ecosystem. This preliminary study on a modern single plant(O. kokonorica) sets a foundation for comprehending these values as quantitative proxies for paleo-humidity reconstruction.
