Challenges in measuring nitrogen isotope signatures in inorganic nitrogen forms: An interlaboratory comparison of three common measurement approaches

Abstract

Rationale Stable isotope approaches are increasingly applied to better understand the cycling of inorganic nitrogen (Ni) forms, key limiting nutrients in terrestrial and aquatic ecosystems. A systematic comparison of the accuracy and precision of the most commonly used methods to analyze δ15N in NO3− and NH4+ and interlaboratory comparison tests to evaluate the comparability of isotope results between laboratories are, however, still lacking. Methods Here, we conducted an interlaboratory comparison involving 10 European laboratories to compare different methods and laboratory performance to measure δ15N in NO3− and NH4+. The approaches tested were (a) microdiffusion (MD), (b) chemical conversion (CM), which transforms Ni to either N2O (CM‐N2O) or N2 (CM‐N2), and (c) the denitrifier (DN) methods. Results The study showed that standards in their single forms were reasonably replicated by the different methods and laboratories, with laboratories applying CM‐N2O performing superior for both NO3− and NH4+, followed by DN. Laboratories using MD significantly underestimated the “true” values due to incomplete recovery and also those using CM‐N2 showed issues with isotope fractionation. Most methods and laboratories underestimated the at%15N of Ni of labeled standards in their single forms, but relative errors were within maximal 6% deviation from the real value and therefore acceptable. The results showed further that MD is strongly biased by nonspecificity. The results of the environmental samples were generally highly variable, with standard deviations (SD) of up to ± 8.4‰ for NO3− and ± 32.9‰ for NH4+; SDs within laboratories were found to be considerably lower (on average 3.1‰). The variability could not be connected to any single factor but next to errors due to blank contamination, isotope normalization, and fractionation, and also matrix effects and analytical errors have to be considered. Conclusions The inconsistency among all methods and laboratories raises concern about reported δ15N values particularly from environmental samples.

Use of 15N tracers to study nitrogen flows in agro-ecosystems: transformation, losses and plant uptake

“…All studies employing 15 N as a tool are depending on accurate and precise determination of the stable isotope ratio in environmental samples. Many methods exist for analyzing the 15 N abundance of inorganic N species (ammonium and nitrate) in liquid samples (Biasi et al 2022), all with their own limitations and advantages. Jia et al (2022) conducted an extensive literature review on those methodologies as a base for developing a decision support tool as a guideline for research to select appropriate analysis methods for their samples.…”

Simultaneous 15N Online Analysis in NH4+, NO2–, NO3–, and N2O to Trace N2O Production Pathways in Nitrogen-Polluted Aqueous Environments

“…This difference in performance likely has two reasons: first, a higher background in 28 N 2 , 29 N 2 , and 30 N 2 due to the requirement for a more complex setup with additional connectors (e.g., selector valve, Nafion dryer, ascarite trap); second, the 3N-SPIN-MIMS system uses helium as a carrier gas, which dilutes the analytes, while as established by Eschenbach et al, the analytes diffused directly into the high-vacuum of the QMS. The NH 4 + conversion to N 2 O via NO 2 – or NO 3 – offers potential to improve sensitivity, but was not implemented and tested due to the higher complexity and the extended reaction times, which might impede online conversion …”

“…This difference in performance likely has two reasons: first, a higher background in 28 N 2 , 29 N 2 , and 30 N 2 due to the requirement for a more complex setup with additional connectors (e.g., selector valve, Nafion dryer, − offers potential to improve sensitivity, but was not implemented and tested due to the higher complexity and the extended reaction times, which might impede online conversion. 34 2.1.4.3. Accuracy and Precision for 15 .…”

Discussion on the need for correction during isotopic analysis of nitrogen by the denitrifier method

“…17 However, it has been reported that high variability of the nitrogen isotope results have been produced by the denitrifier method by conducting an interlaboratory comparison of denitrifier methods and other methods on standards and environmental samples. 24 And one of the potential reasons which caused the big variability is that the measured results needed to be corrected by the nitrogen isotope calibration. Therefore, the standard calibration method to correct the measurement results of nitrogen isotopes should be used with caution.…”