Interlaboratory comparisons involving nine European stable isotope laboratories have shown that the routine methods of cellulose preparation resulted in data that generally agreed within the precision of the isotope ratio mass spectrometry (IRMS) method used: +/-0.2 per thousand for carbon and +/-0.3 per thousand for oxygen. For carbon, the results suggest that holocellulose is enriched up to 0.39 per thousand in 13C relative to the purified alpha-cellulose. The comparisons of IRMS measurements of carbon on cellulose, sugars, and starches showed low deviations from -0.23 to +0.23 per thousand between laboratories. For oxygen, IRMS measurements varied between means from -0.39 to 0.58 per thousand, -0.89 to 0.42 per thousand, and -1.30 to 1.16 per thousand for celluloses, sugars, and starches, respectively. This can be explained by different effects arising from the use of low- or high-temperature pyrolysis and by the variation between laboratories in the procedures used for drying and storage of samples. The results of analyses of nonexchangeable hydrogen are very similar in means with standard deviations between individual methods from +/-2.7 to +/-4.9 per thousand. The use of a one-point calibration (IAEA-CH7) gave significant positive offsets in delta2H values up to 6 per thousand. Detailed analysis of the results allows us to make the following recommendations in order to increase quality and compatibility of the common data bank: (1) removal of a pretreatment with organic solvents, (2) a purification step with 17% sodium hydroxide solution during cellulose preparation procedure, (3) measurements of oxygen isotopes under an argon hood, (4) use of calibration standard materials, which are of similar nature to that of the measured samples, and (5) using a two-point calibration method for reliable result calculation.

Transit time distributions (TTDs) are crucial descriptors of flow and transport processes in catchments, which can be determined from stable water isotope data. Recently, the young water fraction (F yw ) has been introduced as an additional metric derivable from seasonal isotope cycles. In this study, we calculated F yw and TTDs using monthly isotope data from 24 contrasting subcatchments in a mesoscale catchment (3,300 km 2 ) in Germany. F yw ranged from 0.01 to 0.27 (mean = 0.11) and was smallest in mountainous catchments. Assuming gamma-shaped TTDs, we determined stationary TTDs with the convolution integral method for each subcatchment. The convolution integral was first calibrated against the isotope data only (i.e., traditional calibration) and, second, using a multiobjective calibration with the F yw estimates as an additional constraint. This yielded largely differing TTD parameters even for neighboring catchments, with F yw values below 0.1 generally involving a delayed peak in TTDs (i.e., gamma-distribution shape parameter > 1). While the traditional calibration resulted in large uncertainties in TTD parameters, these uncertainties were reduced with the multiobjective calibration, thereby improving the assessment of mean transit times (2 years on average, ranging between 9.6 months and 5.6 years). This highlights the need for uncertainty assessment when using simple isotope models and shows that the traditional calibration might not yield an optimum solution in that it may give a TTD nonconsistent with F yw . Given the robustness of F yw estimates, isotope models should thus aim at accurately describing both F yw and measured isotope data in order to improve the description of flow and transport in catchments.Plain Language Summary Information on the age of river water is crucial for assessing the vulnerability of rivers to weather extremes and pollution. The age of river water is defined as the time that water has spent underground after rainfall infiltration and before ending up in the river. The probability distribution of river water age can be determined using environmental tracers, which are tracers that naturally occur in the system such as stable water isotopes. In this study, we used isotope models to analyze time series of stable water isotopes in rainfall and streamwater measured in 24 subcatchments of the Bode catchment in central Germany. We found that the mean age of river water ranges between 9.6 months and 5.6 years depending on catchment characteristics such as climate and soil type. Moreover, river water with an age of below 2 to 3 months accounts for between 1% and 27% of the entire age distribution. We demonstrate how to use this information on young river water to constrain other metrics such as the mean water age. We suggest that this method is valuable for future studies using environmental tracers and models to determine water age in catchments.

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In batch experiments, we studied the isotope fractionation of nitrogen and oxygen during denitrification of two bacterial strains (Azoarcus sp. strain DSM 9056 and Pseudomonas pseudoalcaligenes strain F10). Denitrification experiments were conducted with succinate and toluene as electron donor in three waters with a distinct oxygen isotope composition. Nitrate consumption was observed in all batch experiments. Reaction rates for succinate experiments were more than six times higher than those for toluene experiments. Nitrogen and oxygen isotopes became progressively enriched in the remaining nitrate pool in the course of the experiments; the nitrogen and oxygen isotope fractionation varied between 8.6-16.2 and 4.0-7.3%, respectively. Within this range, neither electron donors nor the oxygen isotope composition of the medium affected the isotope fractionation process. The experimental results provide evidence that the oxygen isotope fractionation during nitrate reduction is controlled by a kinetic isotope effect which can be quantified using the Rayleigh model. The isotopic examination of nitrite released upon denitrification revealed that nitrogen isotope fractionation largely follows the fractionation of the nitrate pool. However, the oxygen isotope values of nitrite are clearly influenced by a rapid isotope equilibration with the oxygen of the ambient water. Even though this equilibration may in part be due to storage, it shows that under certain natural conditions (re-oxidation of nitrite) the nitrate pool may also be indirectly affected by an isotope equilibration.

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