Anthropogenic nitrogen inputs cause major negative environmental impacts, including emissions of the important greenhouse gas N₂O. Despite their importance, shifts in terrestrial N loss pathways driven by global change are highly uncertain. Here we present a coupled soil-atmosphere isotope model (IsoTONE) to quantify terrestrial N losses and N₂O emission factors from 1850-2020. We find that N inputs from atmospheric deposition caused 51% of anthropogenic N₂O emissions from soils in 2020. The mean effective global emission factor for N₂O was 4.3 ± 0.3% in 2020 (weighted by N inputs), much higher than the surface area-weighted mean (1.1 ± 0.1%). Climate change and spatial redistribution of fertilisation N inputs have driven an increase in global emission factor over the past century, which accounts for 18% of the anthropogenic soil flux in 2020. Predicted increases in fertilisation in emerging economies will accelerate N₂O-driven climate warming in coming decades, unless targeted mitigation measures are introduced.

Soil moisture strongly affects the balance between nitrification, denitrification and N₂O reduction and therefore the nitrogen (N) efficiency and N losses in agricultural systems. In rice systems, there is a need to improve alternative water management practices, which are designed to save water and reduce methane emissions but may increase N₂O and decrease nitrogen use efficiency. In a field experiment with three water management treatments, we measured N₂O isotope ratios of emitted and pore air N₂O (δ¹⁵N, δ¹⁸O and site preference, SP) over the course of 6 weeks in the early rice growing season. Isotope ratio measurements were coupled with simultaneous measurements of pore water ${\mathrm{NO}}_{\mathrm{3}}^{-}$ , ${\mathrm{NH}}_{\mathrm{4}}^{+}$ , dissolved organic carbon (DOC), water-filled pore space (WFPS) and soil redox potential (Eh) at three soil depths. We then used the relationship between SP × δ¹⁸O-N₂O and SP × δ¹⁵N-N₂O in simple two end-member mixing models to evaluate the contribution of nitrification, denitrification and fungal denitrification to total N₂O emissions and to estimate N₂O reduction rates. N₂O emissions were higher in a dry-seeded + alternate wetting and drying (DS-AWD) treatment relative to water-seeded + alternate wetting and drying (WS-AWD) and water-seeded + conventional flooding (WS-FLD) treatments. In the DS-AWD treatment the highest emissions were associated with a high contribution from denitrification and a decrease in N₂O reduction, while in the WS treatments, the highest emissions occurred when contributions from denitrification/nitrifier denitrification and nitrification/fungal denitrification were more equal. Modeled denitrification rates appeared to be tightly linked to nitrification and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ availability in all treatments; thus, water management affected the rate of denitrification and N₂O reduction by controlling the substrate availability for each process (${\mathrm{NO}}_{\mathrm{3}}^{-}$ and N₂O), likely through changes in mineralization and nitrification rates. Our model estimates of mean N₂O reduction rates match well those observed in ¹⁵N fertilizer labeling studies in rice systems and show promise for the use of dual isotope ratio mixing models to estimate N₂ losses.

For the past two decades, the measurement of nitrous oxide (N₂O) isotopocules – isotopically substituted molecules ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁴N¹⁸O of the main isotopic species ¹⁴N¹⁴N¹⁶O – has been a promising technique for understanding N₂O production and consumption pathways. The coupling of non-cryogenic and tuneable light sources with different detection schemes, such as direct absorption quantum cascade laser absorption spectroscopy (QCLAS), cavity ring-down spectroscopy (CRDS) and off-axis integrated cavity output spectroscopy (OA-ICOS), has enabled the production of commercially available and field-deployable N₂O isotopic analyzers. In contrast to traditional isotope-ratio mass spectrometry (IRMS), these instruments are inherently selective for position-specific ¹⁵N substitution and provide real-time data, with minimal or no sample pretreatment, which is highly attractive for process studies. Here, we compared the performance of N₂O isotope laser spectrometers with the three most common detection schemes: OA-ICOS (N₂OIA-30e-EP, ABB – Los Gatos Research Inc.), CRDS (G5131-i, Picarro Inc.) and QCLAS (dual QCLAS and preconcentration, trace gas extractor (TREX)-mini QCLAS, Aerodyne Research Inc.). For each instrument, the precision, drift and repeatability of N₂O mole fraction [N₂O] and isotope data were tested. The analyzers were then characterized for their dependence on [N₂O], gas matrix composition (O₂, Ar) and spectral interferences caused by H₂O, CO₂, CH₄ and CO to develop analyzer-specific correction functions. Subsequently, a simulated two-end-member mixing experiment was used to compare the accuracy and repeatability of corrected and calibrated isotope measurements that could be acquired using the different laser spectrometers. Our results show that N₂O isotope laser spectrometer performance is governed by an interplay between instrumental precision, drift, matrix effects and spectral interferences. To retrieve compatible and accurate results, it is necessary to include appropriate reference materials following the identical treatment (IT) principle during every measurement. Remaining differences between sample and reference gas compositions have to be corrected by applying analyzer-specific correction algorithms. These matrix and trace gas correction equations vary considerably according to N₂O mole fraction, complicating the procedure further. Thus, researchers should strive to minimize differences in composition between sample and reference gases. In closing, we provide a calibration workflow to guide researchers in the operation of N₂O isotope laser spectrometers in order to acquire accurate N₂O isotope analyses. We anticipate that this workflow will assist in applications where matrix and trace gas compositions vary considerably (e.g., laboratory incubations, N₂O liberated from wastewater or groundwater), as well as extend to future analyzer models and instruments focusing on isotopic species of other molecules.

Nitrous oxide (N₂O) is the primary atmospheric constituent involved in stratospheric ozone depletion and contributes strongly to changes in the climate system through a positive radiative forcing mechanism. The atmospheric abundance of N₂O has increased from 270 ppb (parts per billion, 10⁻⁹ mole mole⁻¹) during the pre-industrial era to approx. 330 ppb in 2018. Even though it is well known that microbial processes in agricultural and natural soils are the major N₂O source, the contribution of specific soil processes is still uncertain. The relative abundance of N₂O isotopocules (¹⁴N¹⁴N¹⁶N, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, and ¹⁴N¹⁴N¹⁸O) carries process-specific information and thus can be used to trace production and consumption pathways. While isotope ratio mass spectroscopy (IRMS) was traditionally used for high-precision measurement of the isotopic composition of N₂O, quantum cascade laser absorption spectroscopy (QCLAS) has been put forward as a complementary technique with the potential for on-site analysis. In recent years, pre-concentration combined with QCLAS has been presented as a technique to resolve subtle changes in ambient N₂O isotopic composition. From the end of May until the beginning of August 2016, we investigated N₂O emissions from an intensively managed grassland at the study site Fendt in southern Germany. In total, 612 measurements of ambient N₂O were taken by combining pre-concentration with QCLAS analyses, yielding δ¹⁵N^α, δ¹⁵N^β, δ¹⁸O, and N₂O concentration with a temporal resolution of approximately 1 h and precisions of 0.46 ‰, 0.36 ‰, 0.59 ‰, and 1.24 ppb, respectively. Soil δ¹⁵N-${\mathrm{NO}}_{\mathrm{3}}^{-}$ values and concentrations of ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and ${\mathrm{NH}}_{\mathrm{4}}^{+}$ were measured to further constrain possible N₂O-emitting source processes. Furthermore, the concentration footprint area of measured N₂O was determined with a Lagrangian particle dispersion model (FLEXPART-COSMO) using local wind and turbulence observations. These simulations indicated that night-time concentration observations were largely sensitive to local fluxes. While bacterial denitrification and nitrifier denitrification were identified as the primary N₂O-emitting processes, N₂O reduction to N₂ largely dictated the isotopic composition of measured N₂O. Fungal denitrification and nitrification-derived N₂O accounted for 34 %–42 % of total N₂O emissions and had a clear effect on the measured isotopic source signatures. This study presents the suitability of on-site N₂O isotopocule analysis for disentangling source and sink processes in situ and found that at the Fendt site bacterial denitrification or nitrifier denitrification is the major source for N₂O, while N₂O reduction acted as a major sink for soil-produced N₂O.

Chronically elevated nitrogen (N) deposition has led to severe nutrient imbalance in forest soils. Particularly in tropical and subtropical forest ecosystems, increasing N loading has aggravated phosphorus (P) limitation of biomass production, and has resulted in elevated emissions of nitrous oxide (N₂O) and reduced uptake of methane (CH₄), both of which are important greenhouse gases. Yet, the interactions of N and P and their effects on greenhouse gas emissions remain elusive. Here, we report N₂O and CH₄ emissions together with soil N and P data for a period of 18 months following a single P addition (79 kg P ha⁻¹, as NaH₂PO₄ powder) to an N-saturated, Masson pine-dominated forest soil at TieShanPing (TSP), Chongqing, south-western (SW) China. We observed a significant decline in both nitrate (NO₃⁻) concentrations in soil water (5 and 20 cm depths) and in soil N₂O emissions, following P application. We hypothesise that enhanced N uptake by plants in response to P addition, resulted in less available NO₃⁻ for denitrification. By contrast to most other forest ecosystems, TSP is a net source of CH₄. P addition significantly decreased CH₄ emissions and turned the soil from a net source into a net sink. Based on our observation and previous studies in South America and China, we believe that P addition relieves N inhibition of CH₄ oxidation. Within the 1.5 years after P addition, no significant increase of forest growth was observed and P stimulation of forest N uptake by understorey vegetation remains to be confirmed. Our study indicates that P fertilisation of N-saturated, subtropical forest soils may mitigate N₂O and CH₄ emissions, in addition to alleviating nutrient imbalances and reducing losses of N through NO₃⁻ leaching.