The influence of memory, sample size effects, and filter paper material on online laser-based plant and soil water isotope measurements

Branch water uptake and redistribution in two conifers at the alpine treeline

During winter, conifers at the alpine treeline suffer dramatic losses of hydraulic conductivity, which are successfully recovered during late winter. Previous studies indicated branch water uptake to support hydraulic recovery. We analyzed water absorption and redistribution in Picea abies and Larix decidua growing at the treeline by in situ exposure of branches to δ2H-labelled water. Both species suffered high winter embolism rates (> 40-60% loss of conductivity) and recovered in late winter (< 20%). Isotopic analysis showed water to be absorbed over branches and redistributed within the crown during late winter. Labelled water was redistributed over 425 ± 5 cm within the axes system and shifted to the trunk, lower and higher branches (tree height 330 ± 40 cm). This demonstrated relevant branch water uptake and re-distribution in treeline conifers. The extent of water absorption and re-distribution was species-specific, with L. decidua showing higher rates. In natura, melting snow might be the prime source for absorbed and redistributed water, enabling embolism repair and restoration of water reservoirs prior to the vegetation period. Pronounced water uptake in the deciduous L. decidua indicated bark to participate in the process of water absorption.

Organic contamination in online laser-based plant stem and leaf water isotope measurements for pre-extracted samples

Water stable isotopes have been widely used as natural tracers to investigate soil-plant-atmosphere interactions. Recent developments in induction module cavity ring-down spectroscopy (IM-CRDS) have made it possible to rapidly complete isotope analyses, and to combust co-extracted organic compounds at the same time. However, the agreement between IM-CRDS and isotope ratio mass spectrometry (IRMS) analyses has generally been poor and was primarily attributable to spectral interference of IM-CRDS. Here we evaluated the impacts of organic contamination on the isotope ratios using IM-CRDS with two different methods. No spectral interference was observed for solid samples measured directly by IM-CRDS, whereas clear organic contamination occurred in isotope analyses for pre-extracted plant stem and leaf samples. Our results demonstrate that IM-CRDS can fully combust co-extracted organic compounds by in-line oxidation in the direct measurement of solid samples, although this may not guarantee that the IM-CRDS can obtain better isotopic data than IRMS. It may be risky to evaluate the performance of IM-CRDS by measuring pre-extracted water samples because cryogenic vacuum distillation is likely to introduce extra organic compounds, which may not be fully removed during subsequent IM-CRDS measurement. In addition, spectral variables are useful for post-processing corrections.

Triple isotope variations of monthly tap water in China

Tap water isotopic compositions could potentially record information on local climate and water management practices. A new water isotope tracer 17O-excess became available in recent years providing additional information of the various hydrological processes. Detailed data records of tap water 17O-excess have not been reported. In this report, monthly tap water samples (n = 652) were collected from December 2014 to November 2015 from 92 collection sites across China. The isotopic composition (δ2H, δ18O, and δ17O) of tap water was analyzed by a Triple Water Vapor Isotope Analyzer (T-WVIA) based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) technique and two second-order isotopic variables (d-excess and 17O-excess) were calculated. The geographic location information of the 92 collection sites including latitude, longitude, and elevation were also provided in this dataset. This report presents national-scale tap water isotope dataset at monthly time scale. Researchers and water resource managers who focus on the tap water issues could use them to probe the water source and water management strategies at large spatial scales.

Leaf water 18 O and 2 H maps show directional enrichment discrepancy in Colocasia esculenta

Spatial patterns of leaf water isotopes are challenging to predict because of the intricate link between vein and lamina water. Many models have attempted to predict these patterns, but to date, most have focused on monocots with parallel veins. These provide a simple system to study, but do not represent the majority of plant species. Here, a new protocol is developed using a Picarro induction module coupled to a cavity ringdown spectrometer to obtain maps of the leaf water isotopes (¹⁸ O and ² H). The technique is applied to Colocasia esculenta leaves. The results are compared with isotope ratio mass spectrometry. In C. esculenta, a large enrichment in the radial direction is observed, but not in the longitudinal direction. The string-of-lakes model fails to predict the observed patterns, while the Farquhar-Gan model is more successful, especially when enrichment is accounted for along the radial direction. Our results show that reticulate-veined leaves experience a larger enrichment along the axis of the secondary veins than along the midrib. We hypothesize that this is due to the lower major/minor vein ratio that leads to longer pathways between major veins and sites of evaporation.