The influence of sample matrix on the accuracy of nitrite N and O isotope ratio analyses with the azide method

Physiological Influence of Fe and Cu Availability on Nitrogen Isotope Fractionation during Ammonia Oxidation

Microbially mediated cycling processes play central roles in regulating the speciation and availability of nitrogen, a vital nutrient with wide implications for agriculture, water quality, wastewater treatment, ecosystem health, and climate change. Ammonia oxidation, the first and rate-limiting step of nitrification, is carried out by bacteria (AOB) and archaea (AOA) that require the trace metal micronutrients copper (Cu) and iron (Fe) for growth and metabolic catalysis. While stable isotope analyses for constraining nitrogen cycling are commonly used, it is unclear whether metal availability may modulate expression of stable isotope fractionation during ammonia oxidation, by varying growth or through regulation of metabolic metalloenzymes. We present the first study examining the influence of Fe and Cu availability on the kinetic nitrogen isotope effect in ammonia oxidation (15εAO). We report a general independence of 15εAO from the growth rate in AOB, except at a low temperature (10 °C). With AOA Nitrosopumilus maritimus SCM1, however, 15εAO decreases nonlinearly at lower oxidation rates. We examine assumptions involved in the interpretation of 15εAO values and suggest these dynamics may arise from physiological constraints that push the system toward isotopic equilibrium. These results suggest important links between isotope fractionation and environmental constraints on physiology in these key N cycling microorganisms.

Dual isotopes of nitrite in the Amundsen Sea in summer

Nitrite (NO₂^-) is a key intermediate in the nitrogen (N) cycle, and its transformation is accomplished by microbial communities. However, due to few studies on the nitrite cycle, a clear assessment of the contribution to the marine biogeochemical cycle is missing. Here, we present data on nitrogen and oxygen isotopic composition of NO₂^- in the Amundsen Sea in summer, and explore the biogeochemical processes that influence the NO₂^- cycle. Extremely low δ¹⁵N_NO2 and abnormally high δ¹⁸O_NO2 were found in the upper waters of the Amundsen Sea, with δ¹⁵N_NO2 as low as -58.4 ‰ and δ¹⁸O_NO2 as high as 44.4 ‰. Enzymatic isotopic exchange reactions between nitrate and nitrite have been proposed to be responsible for these isotopic anomalies. The mirror-symmetrical variation between δ¹⁵N_NO2 and δ¹⁸O_NO2 suggests that the isotopic fractionation effects of nitrogen and oxygen are opposite in isotope exchange reactions. Dual isotopes of nitrite indicate that ammonia oxidation is the main source of nitrite, thus nitrification plays an important role in the formation of primary nitrite maximum in the upper Amundsen Sea. The nitrogen and oxygen isotopic compositions of nitrite provide support for clarifying multiple processes of marine nitrogen cycle.

Tracing N2O formation in full-scale wastewater treatment with natural abundance isotopes indicates control by organic substrate and process settings

Nitrous oxide (N₂O) dominates greenhouse gas emissions in wastewater treatment plants (WWTPs). Formation of N₂O occurs during biological nitrogen removal, involves multiple microbial pathways, and is typically very dynamic. Consequently, N₂O mitigation strategies require an improved understanding of nitrogen transformation pathways and their modulating controls. Analyses of the nitrogen (N) and oxygen (O) isotopic composition of N₂O and its substrates at natural abundance have been shown to provide valuable information on formation and reduction pathways in laboratory settings, but have rarely been applied to full-scale WWTPs. Here we show that N-species isotope ratio measurements at natural abundance level, combined with long-term N₂O monitoring, allow identification of the N₂O production pathways in a full-scale plug-flow WWTP (Hofen, Switzerland). Heterotrophic denitrification appears as the main N₂O production pathway under all tested process conditions (0-2 mgO₂/l, high and low loading conditions), while nitrifier denitrification was less important, and more variable. N₂O production by hydroxylamine oxidation was not observed. Fractional N₂O elimination by reduction to dinitrogen (N₂) during anoxic conditions was clearly indicated by a concomitant increase in site preference, δ¹⁸O(N₂O) and δ¹⁵N(N₂O). N₂O reduction increased with decreasing availability of dissolved inorganic N and organic substrates, which represents the link between diurnal N₂O emission dynamics and organic substrate fluctuations. Consequently, dosing ammonium-rich reject water under low-organic-substrate conditions is unfavorable, as it is very likely to cause high net N₂O emissions. Our results demonstrate that monitoring of the N₂O isotopic composition holds a high potential to disentangle N₂O formation mechanisms in engineered systems, such as full-scale WWTP. Our study serves as a starting point for advanced campaigns in the future combining isotopic technologies in WWTP with complementary approaches, such as mathematical modeling of N₂O formation or microbial assays to develop efficient N₂O mitigation strategies.

Nitrogen isotope effects can be used to diagnose N transformations in wastewater anammox systems

Anaerobic ammonium oxidation (anammox) plays an important role in aquatic systems as a sink of bioavailable nitrogen (N), and in engineered processes by removing ammonium from wastewater. The isotope effects anammox imparts in the N isotope signatures (¹⁵N/¹⁴N) of ammonium, nitrite, and nitrate can be used to estimate its role in environmental settings, to describe physiological and ecological variations in the anammox process, and possibly to optimize anammox-based wastewater treatment. We measured the stable N-isotope composition of ammonium, nitrite, and nitrate in wastewater cultivations of anammox bacteria. We find that the N isotope enrichment factor ¹⁵ε for the reduction of nitrite to N₂ is consistent across all experimental conditions (13.5‰ ± 3.7‰), suggesting it reflects the composition of the anammox bacteria community. Values of ¹⁵ε for the oxidation of nitrite to nitrate (inverse isotope effect, - 16 to - 43‰) and for the reduction of ammonium to N₂ (normal isotope effect, 19-32‰) are more variable, and likely controlled by experimental conditions. We argue that the variations in the isotope effects can be tied to the metabolism and physiology of anammox bacteria, and that the broad range of isotope effects observed for anammox introduces complications for analyzing N-isotope mass balances in natural systems.

Dynamics of nitrite in acidic soil during extraction with potassium chloride studied using 15 N tracing

RATIONALE

Nitrite is well known to be unstable, including during soil extraction with KCl (especially in acidic soils), but the source and fate of NO₂ ^- in the short duration of the extraction process remain unclear.

METHODS

A series of ¹⁵ N-tracing studies explored NO₂ ^- transformations during KCl extraction in acidic and alkaline soils. Tests considering multiple factors assessed the interactions of such factors as soil sterilization, extraction time, and pH adjustment. After addition of ¹⁵ NO₂ ^- , ¹⁵ NH₄ ⁺ , and ¹⁵ NO₃ ^- tracers, the concentrations and isotopic compositions of N₂ O, N₂ , NH₄ ⁺ , NO₃ ^- , NO₂ ^- , and dissolved organic nitrogen (DON) were measured to investigate the production and consumption of NO₂ ^- .

RESULTS

Nitrite was stable in alkaline soils during KCl extraction. In contrast, changes did occur in acidic soils during KCl extraction: NO₂ ^- declined rapidly in the first 10 min of extraction although the subsequent rate of decrease lessened as the extraction time progressed. Significant dilution of ¹⁵ NO₂ ^- suggested high rates of NO₂ ^- production and even higher rates of consumption. The soil's organic N was the only source of NO₂ ^- and also its main destination. Soil sterilization showed that NO₂ ^- processes during extraction were chemical, not microbial. The pH adjustment of acidic soil stabilized its NO₂ ^- .

CONCLUSIONS

Overall, the pH adjustment of KCl solution appears favorable for investigating NO₂ ^- dynamics. For example, this work recommends an extraction solution comprising a 4:1 mixture of 2.5 M KCl solution and pH 8.4 buffer, which was more convenient to operate than the method reported by Stevens and Laughlin.