¹⁷O‐excess as a detector for co‐extracted organics in vapor analyses of plant isotope signatures

Abstract

Rationale:The stable isotope compositions of hydrogen and oxygen in water (δ 2 H and δ 18 O values) have been widely used to investigate plant water sources, but traditional isotopic measurements of plant waters are expensive and labor intensive.Recent work with direct vapor equilibration (DVE) on laser spectroscopy has shown potential to side step limitations imposed by traditional methods. Here, we evaluate DVE analysis of plants with a focus on spectral contamination introduced by organic compounds. We present 17 O-excess as a way of quantifying organic compound interference in DVE. Methods: We performed isotopic analysis using the δ 2 H, δ 18 O and δ 17 O values of water on an Off-Axis Integrated Cavity Output Spectroscopy (IWA-45EP OA-ICOS) instrument in vapor mode. We used a set of methanol (MeOH) and ethanol (EtOH) solutions to assess errors in isotope measurements. We evaluated how organic compounds affect the 17 O-excess. DVE was used to measure the isotopic signatures in natural plant material from Pinus banksiana, Picea mariana, and Larix laricina, and soil from boreal forest for comparison with solutions. Results: The 17 O-excess was sensitive to the presence of organic compounds in water. 17 O-excess changed proportionally to the concentration of MeOH per volume of water, resulting in positive values, while EtOH solutions resulted in smaller changes in the 17 O-excess. Soil samples did not show any spectral contamination. Plant samples were spectrally contaminated on the narrow-band and were enriched in 1 H and 16 O compared with source water. L. laricina was the only species that did not show any evidence of spectral contamination. Xylem samples that were spectrally contaminated had positive 17 O-excess values. Conclusions: 17 O-excess can be a useful tool to identify spectral contamination and improve DVE plant and soil analysis in the laboratory and in situ. The 17 O-excess flagged the presence of MeOH and EtOH. Adding measurement of δ 17 O values to traditional measurement of δ 2 H and δ 18 O values may shed new light on plant water analysis for source mixing dynamics using DVE.

Tree water deficit and dynamic source water partitioning

“…Barbeta, Jones, et al, 2019;P. Martin-Gomez, Serrano, et al, 2016;Poca et al, 2019], and coextracted organic interference with regards to isotope analysis [P. Martín-Gómez, Barbeta, et al, 2015;M. F. Nehemy, Millar, et al, 2019]).…”

On the use of leaf water to determine plant water source: A proof of concept

“…Such techniques can also be impacted by fractionation associated with water transport within the tree (i.e. passage through membranes) and with co‐extracted volatile substances that plague the laser‐based in‐situ instruments (Nehemy et al, 2019).…”

Triple isotope variations of monthly tap water in China

“…Therefore, combing 17 O-excess and 18 O measurements could separate the temperature (not affecting 17 O-excess) and relative humidity (affecting both 17 O-excess and 18 O) effect on oxygen isotopes. 17 O-excess can also be used to identify spectral contamination and improve direct vapor equilibration in plant and soil analysis 19 . According to the relationship between δ′ 18 O and δ′ 17 O (i.e., the slope of 1000 x ln (δ 18 O + 1) and 1000 x ln (δ 17 O + 1)), synoptic drought related to EI Nino and local drought is distinguishable 20 .…”