RationaleTechnological advances have motivated researchers to transition from traditional gas chromatography/isotope ratio mass spectrometry to rapid, high‐throughput, laser‐based instrumentation for N2O isotopic research. However, calibrating laser‐based instruments to yield accurate and precise isotope ratios has been an ongoing challenge. To streamline the N2O isotope research pipeline, we developed the calibration protocol for laser‐based analyzers described here. While our approach is targeted at laboratory soil incubations, we anticipate that it will be broadly applicable for diverse types of stable isotope research.MethodsWe prepared standards diluted from USGS52 and from a commercial cylinder to develop a calibration curve spanning from 0.3 to 300 ppm N2O. To calibrate over this broad range, we binned each isotopocule (N2O, N15NO, 15NNO, and NN18O) into low, medium, and high concentration ranges and then used mathematically similar polynomial functions to calibrate the isotopocules within each concentration range. We also assessed the temporal stability of the instrument and the capacity for our calibration approach to work with isotopically enriched gas samples.ResultsOur calibration approach yielded generally accurate and precise data when isotopocules were calibrated in concentration ranges, and the measurements appeared to be temporally stable. For all isotopocules at natural abundance, the residual percentage error was smallest in the medium N2O range. There was more noise in the corrected isotopomers and isotopologue at natural abundance in samples with the lowest and highest N2O concentrations. Corrected isotopomer results from isotopically enriched samples were very precise.ConclusionsDeveloping our calibration strategy involved learning several key lessons: (1) calibrate isotopocules in distinct concentration ranges, (2) use mathematically similar models to calibrate the isotopocules in each range, (3) calibrated N2O concentrations and δ values tend to be most accurate and precise in the medium N2O range, and (4) we encourage users to take advantage of isotopic enrichment to capitalize on laser‐based instrument strengths.

Nitrous oxide (N2O) is a formidable greenhouse gas with a warming potential ~300× greater than CO2. However, its emissions to the atmosphere have gone largely unchecked because the microbial and environmental controls governing N2O emissions have proven difficult to manage. The microbial process N2O consumption is the only know biotic pathway to remove N2O from soil pores and therefore reduce N2O emissions. Consequently, manipulating soils to increase N2O consumption by organic carbon (OC) additions has steadily gained interest. However, the response of N2O emissions to different OC additions are inconsistent, and it is unclear if lower N2O emissions are due to increased consumption, decreased production, or both. Simplified and systematic studies are needed to evaluate the efficacy of different OC additions on N2O consumption. We aimed to manipulate N2O consumption by amending soils with OC compounds (succinate, acetate, propionate) more directly available to denitrifiers. We hypothesized that N2O consumption is OC‐limited and predicted these denitrifier‐targeted additions would lead to enhanced N2O consumption and increased nosZ gene abundance. We incubated diverse soils in the laboratory and performed a 15N2O isotope pool dilution assay to disentangle microbial N2O emissions from consumption using laser‐based spectroscopy. We found that amending soils with OC increased gross N2O consumption in six of eight soils tested. Furthermore, three of eight soils showed Increased N2O Consumption and Decreased N2O Emissions (ICDE), a phenomenon we introduce in this study as an N2O management ideal. All three ICDE soils had low soil OC content, suggesting ICDE is a response to relaxed C‐limitation wherein C additions promote soil anoxia, consequently stimulating the reduction of N2O via denitrification. We suggest, generally, OC additions to low OC soils will reduce N2O emissions via ICDE. Future studies should prioritize methodical assessment of different, specific, OC‐additions to determine which additions show ICDE in different soils.

Natural gas leaks in local distribution systems can develop as underground pipeline infrastructure degrades over time. These leaks lead to safety, economic, and climate change burdens on society. We develop an environmental justice analysis of natural gas leaks discovered using advanced leak detection in 13 U.S. metropolitan areas. We use Bayesian spatial regression models to study the relationship between the density of leak indications and sociodemographic indicators in census tracts. Across all metro areas combined, we found that leak densities increase with increasing percent people of color and with decreasing median household income. These patterns of infrastructure injustice also existed within most metro areas, even after accounting for housing age and the spatial structure of the data. Considering the injustices described here, we identify actions available to utilities, regulators, and advocacy groups that can be taken to improve the equity of local natural gas distribution systems.

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.

Precise water and fertilizer application can increase crop water productivity and reduce agricultural contributions to greenhouse gas (GHG) emissions. Regulated deficit irrigation (DI) and drip fertigation control the amount, location, and timing of water and nutrient application. Yet, few studies have measured GHG emissions under these practices, especially for maize (Zea mays L.). The objective was to quantify N 2 O and CO 2 emission from DI and full irrigation (FI) within a drip-fertigated maize system in northeastern Colorado. During two growing seasons of measurement, treatments consisted of mild, moderate, and extreme DI and FI. Deficit irrigation was managed based on growth stage so that full evapotranspiration (ET) was met during the yield-sensitive reproductive stage, but less than full crop ET was applied during the late vegetative and maturation growth stages. In the first year, mild DI (90% ET) reduced N 2 O emissions by 50% compared with FI. In the second year, compared with FI, moderate DI (69-80% ET) reduced N 2 O emissions by 15%, and extreme DI (54-68% ET) reduced N 2 O emissions by 40%. Only extreme DI in the second year significantly reduced CO 2 emissions (by 30%) compared with FI. Mild DI reduced yield-scaled emissions in the first year, but moderate and extreme DI had similar yield-scaled emissions as FI in the second year. The surface drip fertigation resulted in total GHG emissions that were one-tenth of literature-based measurements from sprinkler-irrigated maize systems. This study illustrates the potential of DI and drip fertigation to reduce N 2 O and CO 2 emissions in irrigated cropping systems.