Characterizing the Importance of Denitrification for N₂O Production in Soils Using Natural Abundance and Isotopic Labeling Techniques

Abstract

Nitrous oxide (N2O), a potent greenhouse gas that contributes significantly to climate change, is emitted mostly from soils by a suite of microbial metabolic pathways that are nontrivial to identify, and subsequently, to manage. Using either natural abundance or enriched stable isotope methods has aided in identifying microbial sources of N2O, but each approach has limitations. Here, we conducted a novel pairing of natural abundance and enriched assays on two dissimilar soils, hypothesizing this pairing would better constrain microbial sources of N2O. We incubated paired natural abundance and enriched soils from a corn agroecosystem and a subalpine forest in the laboratory at 10%–95% soil saturation for 28 hr. The natural abundance method measured intramolecular site preference (SP) from emitted N2O, whereas the enriched method measured emitted 15N2O from soils amended with 15N‐labeled substrate. The isotopic composition of emitted N2O was measured using a laser‐based N2O isotopic analyzer, yielding two key findings. First, both methods revealed that denitrification was the primary source of N2O in all soils: isotopic enrichment revealed clear normalNnormalO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ reduction to N2O, while SP indicated a likely combination of fungal and bacterial denitrification. Second, we quantified, to our knowledge for the first time, persistent (>55%) β‐position‐specific enrichment in N2O emitted from 15normalNnormalO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$‐amended soils. This counter‐intuitive enrichment pattern could be indicative of codenitrification, an understudied but potentially important contributor to N2O emissions. Our work revealed the ubiquity of denitrification among the soils tested. Future pairings of natural abundance and enriched methods could better characterize diverse denitrification pathways.

Using isotope pool dilution to understand how organic carbon additions affect N2O consumption in diverse soils

“…48 ). At the end of the incubation period, we mixed each jar and gas bag's air by attaching a 60 ml syringe to the jar's gas sampling port and, by opening the stopcock valve connecting the jar headspace to the gas bag, then we pumped the syringe for ~ 60 seconds to homogenize the jar headspace and Tedlar gas bag(Stuchiner and von Fischer 2022b). At the time of sampling, we connected our in-…”

“…Stuchiner and von Fischer (2022b) we provide a detailed description of our incubation apparatus, which includes a 1 L Tedlar gas bag attached to the jar headspace to allow for removal of a large quantity of air for subsequent [N 2 O] and isotopic analysis with our laser-based analyzer (see Section 2.4.3 for details). Prior to incubation, we flushed and filled tion (details in Section 2.4.4).…”

Separating N2O production and consumption in intact agricultural soil cores at different moisture contents and depths

“…Nevertheless, we lack information to be able to source partition the N 2 O generated in this study. Recent advances in N 2 O isotopomer measurements are shedding light on microbial source partitioning of N 2 O, for example, Stuchiner and von Fischer ( 2022b ) demonstrate denitrification was the predominant N 2 O production pathway in soils ranging from 50% to 95% WFPS and Harris et al ( 2021 ) found that he proportion of N 2 O from denitrification did not decrease under even very low WFPS.…”