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Compound-specific hydrogen and oxygen isotope analyzes on leaf wax-derived n-alkanes (δ 2 H n-alkane) and the hemicellulose-derived sugar arabinose (δ 18 O ara) are valuable, innovative tools for paleohydrological reconstructions. Previous calibration studies have revealed that δ 2 H n-alkane and δ 18 O ara reflect the isotopic composition of precipitation, but-depending on the region-may be strongly modulated by evapotranspirative enrichment. Since no calibration studies exist for semi-arid and arid Mongolia so far, we have analyzed δ 2 H n-alkane and δ 18 O ara in topsoils collected along a transect through Mongolia, and we compared these values with the isotopic composition of precipitation (δ 2 H p-WM and δ 18 O p-WM , modeled data) and various climate parameters. δ 2 H n-alkane and δ 18 O ara are more positive in the arid southeastern part of our transect, which reflects the fact that also the precipitation is more enriched in 2 H and 18 O along this part of the transect. The apparent fractionation ε app , i.e., the isotopic difference between precipitation and the investigated compounds, shows no strong correlation with climate along the transect (ε 2H n-C29/p = −129 ± 14 , ε 2H n-C31/p = −146 ± 14 , and ε 18O ara/p = +41 ± 2). Our results suggest that δ 2 H n-alkane and δ 18 O ara in topsoils from Mongolia reflect the isotopic composition of precipitation and are not strongly modulated by climate. Correlation with the isotopic composition of precipitation has root-mean-square errors of 13.4 for δ 2 H n-C29 , 12.6 for δ 2 H n-C31 , and 1.2 for δ 18 O ara , so our findings corroborate the great potential of compound-specific δ 2 H n-alkane and δ 18 O ara analyzes for paleohydrological research in Mongolia.

To investigate C and N rhizodeposition, plants can be 13 C-15 N double-labeled with glucose and urea using a stem-feeding method (wick method). However, it is unclear how the 13 C applied as glucose is released into the soil as rhizorespiration in comparison with the 13 C applied as CO 2 using a natural uptake pathway. In the present study, we therefore compared the short-term fate of 14 C and 15 N in white lupine and pea plants applied either by the wick method or the natural pathways of C and N assimilation. Plants were pulse-labeled in 14 CO 2-enriched atmosphere and 15 N urea was applied to the roots (atmosphere-soil) following the natural assimilation pathways, or plants were simultaneously labeled with 14 C and 15 N by applying a 14 C glucose-15 N urea solution into the stem using the wick method. Plant development, soil microbial biomass, total rhizorespiration, and distribution of N in plants were not affected by the labeling method used but by plant species. However, the 15 N : N ratio in plant parts was significantly (p < 0.05) affected by the labeling method, indicating more homogeneous 15 N enrichment of plants labeled via root uptake. After 14 CO 2 atmosphere labeling of plants, the cumulated 14 CO 2 release from roots and soil showed the common saturation dynamics. In contrast, after 14 C-glucose labeling by the wick method, the cumulated 14 CO 2 release increased linearly. These results show that 14 C applied as glucose using the wick method is not rapidly transferred to the roots as compared to a short-term 14 CO 2 pulse. This is partly due to a slower 14 C uptake and partly due to slow distribution within the plant. Consequently, 14 C-glucose application by the wick method is no pulselabeling approach. However, the advantages of the wick method for 13 C-15 N double labeling for estimating rhizodeposition especially under field conditions requires further methodological research.

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