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

Abstract

AbstractNitrous 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 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 15‐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.