Controls on the Isotopic Composition of Nitrite (δ15N and δ18O) during Denitrification in Freshwater Sediments

Abstract

The microbial reduction of nitrate, via nitrite into gaseous di-nitrogen (denitrification) plays a major role in nitrogen removal from aquatic ecosystems. Natural abundance stable isotope measurements can reveal insights into the dynamics of production and consumption of nitrite during denitrification. In this study, batch experiments with environmental bacterial communities were used to investigate variations of concentrations and isotope compositions of both nitrite and nitrate under anoxic conditions. To this end, denitrification experiments were carried out with nitrite or nitrate as sole electron acceptors at two substrate levels respectively. For experiments with nitrate as substrate, where the intermediate compound nitrite is both substrate and product of denitrification, calculations of the extent of isotope fractionation were conducted using a non-steady state model capable of tracing chemical and isotope kinetics during denitrification. This study showed that nitrogen isotope fractionation was lower during the use of nitrite as substrate (ε = −4.2 and −4.5‰ for both treatments) as compared to experiments where nitrite was produced as an intermediate during nitrate reduction (ε = −10 and −15‰ for both treatments). This discrepancy might be due to isotopic fractionation within the membrane of denitrifiers. Moreover, our results confirmed previously observed rapid biotic oxygen isotope exchange between nitrite and water.

Enhancing Nitrate Removal With Industrial Wine Residue: Insights From Laboratory Batch and Column Experiments Using Chemical, Isotopic and Numerical Modeling Tools

“…When simulating δ 1 ⁵N-NO 2 of the low NO 2 experiment (B-L), the model fitted the experimental δ 1 ⁵N-NO 2 using the experimentally determined ε 15 N NO2 equal to 4‰ but a higher ε 15 N NO2 of 18‰ instead of 8‰ was imposed in B-V to accurately reproduce the experimental results. Similar δ 1 ⁵N-NO 2 results were obtained by Sebilo et al (2019) for a high NO 2 system, where the model could simulate the experimental results by using a higher ε 15 N NO2 of 10‰ than the value obtained in batch experiments ( 4.2‰). The behavior of ε 15 N NO2 in our models within a high-nitrite system may be associated with nitrite's potential reoxidation to nitrate, which was not previously considered.…”

“…Similar δ 1 ⁵N‐NO 2 − results were obtained by Sebilo et al. (2019) for a high NO 2 − system, where the model could simulate the experimental results by using a higher ε 15 N NO2 of −10‰ than the value obtained in batch experiments (−4.2‰). The behavior of ε 15 N NO2 in our models within a high‐nitrite system may be associated with nitrite’s potential reoxidation to nitrate, which was not previously considered.…”

Predicting Nutrient Incontinence in the Anthropocene at Watershed Scales

“…Stable isotopes can indicate nutrient source and degree of biogeochemical processing (Mariotti et al, 1981;Lehmann et al, 2003;Malone et al, 2018). NO − 3 isotopes are particularly useful because NO − 3 is a dominant form of nitrogen in nutrient saturated ecosystems (Aber et al, 1998), organic and industrial fertilizers have distinct initial δ 15 N and δ 18 O (Bedard-Haughn et al, 2003;Lohse et al, 2013;Denk et al, 2017), and denitrification (both heterotrophic and autotrophic) strongly fractionates NO − 3 isotopes, enriching the residual δ 15 N and δ 18 O (Ayraud et al, 2006;Hosono et al, 2014;Malone et al, 2018;Sebilo et al, 2019). Therefore, we predicted that watersheds with isotopically-enriched NO − 3 would have higher N attenuation (Lehmann et al, 2003) or alternatively that they would have primarily organic fertilizer (Bedard-Haughn et al, 2003).…”

Novel Insight into Microbially Mediated Nitrate-Reducing Fe(II) Oxidation by Acidovorax sp. Strain BoFeN1 Using Dual N–O Isotope Fractionation

“…During microbial nitrate/nitrite reduction, although 15 ε and 18 ε may change with various experimental effects (e.g., nitrate/nitrite concentrations and carbon source concentrations), the kinetic isotope effect of N and O was masked to the same extent, and isotope effects changed synchronously. Thus, the ratios of 18 ε: 15 ε remained unchanged under different experimental conditions. ,− ,, The ratios of 18 ε: 15 ε depend on the type of reductase. These results indicate that the slopes in the dual N and O isotope plots were much less affected by mass-transfer limitations than those of single N or O isotopes.…”

“…Thus, the ratios of 18 ε: 15 ε remained unchanged under different experimental conditions. 7,[12][13][14][15]20,34 The ratios of 18 ε: 15 ε depend on the type of reductase. These results indicate that the slopes in the dual N and O isotope plots were much less affected by mass-transfer limitations than those of single N or O isotopes.…”