TY - JOUR

T1 - Challenges in measuring nitrogen isotope signatures in inorganic nitrogen forms

T2 - An interlaboratory comparison of three common measurement approaches

AU - Biasi, Christina

AU - Jokinen, Simo

AU - Prommer, Judith

AU - Ambus, Per

AU - Dörsch, Peter

AU - Yu, Longfei

AU - Granger, Steve

AU - Boeckx, Pascal

AU - Van Nieuland, Katja

AU - Brueggemann, Nicolas

AU - Wissel, Holger

AU - Voropaev, Andrey

AU - Zilberman, Tami

AU - Jaentti, Helena

AU - Trubnikova, Tatiana

AU - Welti, Nina

AU - Voigt, Carolina

AU - Gebus-Czupyt, Beata

AU - Czupyt, Zbigniew

AU - Wanek, Wolfgang

PY - 2022

Y1 - 2022

N2 - Rationale Stable isotope approaches are increasingly applied to better understand the cycling of inorganic nitrogen (N-i) 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 delta N-15 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 delta N-15 in NO3- and NH4+. The approaches tested were (a) microdiffusion (MD), (b) chemical conversion (CM), which transforms N-i to either N2O (CM-N2O) or N-2 (CM-N-2), 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-N-2 showed issues with isotope fractionation. Most methods and laboratories underestimated the at%N-15 of N-i 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 parts per thousand for NO3- and +/- 32.9 parts per thousand for NH4+; SDs within laboratories were found to be considerably lower (on average 3.1 parts per thousand). 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 delta N-15 values particularly from environmental samples.

AB - Rationale Stable isotope approaches are increasingly applied to better understand the cycling of inorganic nitrogen (N-i) 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 delta N-15 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 delta N-15 in NO3- and NH4+. The approaches tested were (a) microdiffusion (MD), (b) chemical conversion (CM), which transforms N-i to either N2O (CM-N2O) or N-2 (CM-N-2), 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-N-2 showed issues with isotope fractionation. Most methods and laboratories underestimated the at%N-15 of N-i 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 parts per thousand for NO3- and +/- 32.9 parts per thousand for NH4+; SDs within laboratories were found to be considerably lower (on average 3.1 parts per thousand). 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 delta N-15 values particularly from environmental samples.

KW - NATURAL-ABUNDANCE

KW - FRESH-WATER

KW - TERRESTRIAL ECOSYSTEMS

KW - DIFFUSION METHOD

KW - SOIL EXTRACTS

KW - AMMONIA DIFFUSION

KW - ORGANIC-MATTER

KW - NITRATE N

KW - N-15

KW - DENITRIFIER

U2 - 10.1002/rcm.9370

DO - 10.1002/rcm.9370

M3 - Journal article

C2 - 35906712

VL - 36

JO - Rapid Communications in Mass Spectrometry

JF - Rapid Communications in Mass Spectrometry

SN - 0951-4198

IS - 22

M1 - 9370

ER -