Effectively quantifying hot moments of nitrous oxide (N2O) emissions from agricultural soils is critical for managing this potent greenhouse gas. However, we are challenged by a lack of standard approaches for identifying hot moments, including (1) determining thresholds above which emissions are considered hot moments, and (2) considering seasonal variation in the magnitude and frequency distribution of net N2O fluxes. We used one year of hourly N2O flux measurements from 16 autochambers that varied in flux magnitude and frequency distribution in a conventionally tilled maize field in central Illinois, USA to compare three approaches to identify hot moment thresholds: 4x the standard deviation (SD) above the mean, 1.5x the interquartile range (IQR), and isolation forest (IF) identification of anomalous values. We also compared these approaches on seasonally subdivided data (early, late, non-growing seasons) vs. the whole year. Our analyses of the datasets revealed that 1.5x IQR method best identified N2O hot moments. In contrast, the 4 SD method yielded hot moment threshold values too high, and the IF method yielded threshold values too low, leading to missed N2O hot moments or low net N2O fluxes mischaracterized as hot moments, respectively. Furthermore, seasonally subdividing the dataset facilitated identification of smaller hot moments in the late and non-growing seasons when N2O hot moments were generally smaller, but it also increased hot moment threshold values in the early growing season when N2O hot moments were larger. Consequently, we recommend using the 1.5x IQR method on whole year datasets to identify N2O hot moments.

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-labelled substrate. The isotopic composition of emitted N2O was measured using a laser-based N2O isotopic analyzer, yielding three key findings. First, isotopic signatures from natural abundance and enriched N2O generally agreed in interpretation. Second, our novel pairing of isotopic methodologies refined understanding of microbial N-transformations in drier agricultural soil. In the 50% saturation agricultural soil, nitrification might have been deemed an important process based on SP alone, but enrichment helped reveal that its contribution to N2O emissions was minor. Finally, we quantified, to our knowledge for the first time, persistent (>50%) β-position-specific enrichment in emitted 15N2O, which is far in excess of SP-level fractionation expectations. This counter-intuitive enrichment pattern raises the possibility of previously unrecognized N-transformations in these soils.