Nitrous oxide (N₂O) is a long-lived potent greenhouse gas and stratospheric ozone-depleting substance that has been accumulating in the atmosphere since the preindustrial period. The mole fraction of atmospheric N₂O has increased by nearly 25 % from 270 ppb (parts per billion) in 1750 to 336 ppb in 2022, with the fastest annual growth rate since 1980 of more than 1.3 ppb yr⁻¹ in both 2020 and 2021. According to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR6), the relative contribution of N₂O to the total enhanced effective radiative forcing of greenhouse gases was 6.4 % for 1750–2022. As a core component of our global greenhouse gas assessments coordinated by the Global Carbon Project (GCP), our global N₂O budget incorporates both natural and anthropogenic sources and sinks and accounts for the interactions between nitrogen additions and the biogeochemical processes that control N₂O emissions. We use bottom-up (BU: inventory, statistical extrapolation of flux measurements, and process-based land and ocean modeling) and top-down (TD: atmospheric measurement-based inversion) approaches. We provide a comprehensive quantification of global N₂O sources and sinks in 21 natural and anthropogenic categories in 18 regions between 1980 and 2020. We estimate that total annual anthropogenic N₂O emissions have increased 40 % (or 1.9 Tg N yr⁻¹) in the past 4 decades (1980–2020). Direct agricultural emissions in 2020 (3.9 Tg N yr⁻¹, best estimate) represent the large majority of anthropogenic emissions, followed by other direct anthropogenic sources, including fossil fuel and industry, waste and wastewater, and biomass burning (2.1 Tg N yr⁻¹), and indirect anthropogenic sources (1.3 Tg N yr⁻¹) . For the year 2020, our best estimate of total BU emissions for natural and anthropogenic sources was 18.5 (lower–upper bounds: 10.6–27.0) Tg N yr⁻¹, close to our TD estimate of 17.0 (16.6–17.4) Tg N yr⁻¹. For the 2010–2019 period, the annual BU decadal-average emissions for both natural and anthropogenic sources were 18.2 (10.6–25.9) Tg N yr⁻¹ and TD emissions were 17.4 (15.8–19.20) Tg N yr⁻¹. The once top emitter Europe has reduced its emissions by 31 % since the 1980s, while those of emerging economies have grown, making China the top emitter since the 2010s. The observed atmospheric N₂O concentrations in recent years have exceeded projected levels under all scenarios in the Coupled Model Intercomparison Project Phase 6 (CMIP6), underscoring the importance of reducing anthropogenic N₂O emissions. To evaluate mitigation efforts and contribute to the Global Stocktake of the United Nations Framework Convention on Climate Change, we propose the establishment of a global network for monitoring and modeling N₂O from the surface through to the stratosphere. The data presented in this work can be downloaded from https://doi.org/10.18160/RQ8P-2Z4R (Tian et al., 2023).

Intensive agricultural activities have significantly altered watershed hydrological and biogeochemical processes, resulting in water quality issues and loss of ecosystem functions and biodiversity. A major challenge in effectively mitigating nitrogen (N) loss from agricultural watersheds stems from the heterogeneity of N transformation and transport processes that complicates accurate quantification and modeling of N sources and sinks at the watershed scale. This study utilized stable isotopes of water and nitrate (NO₃^-) in conjunction with spatial stream network modeling (SSNMs) to explore watershed hydrology, N transformation, and sources within a mesoscale river network in the U.S. Corn Belt (Cannon River Watershed, Minnesota) under contrasting hydrological conditions. The results show that the wet season had elevated riverine NO₃^- concentration (medium: 8.4 mg N L^-1), driven by high watershed wetness conditions that mobilizes NO₃^- from the near-surface source zone. Furthermore, the strong hydrologic connectivity also reduced the denitrification potential by shortening water travel times. In comparison, the dry season showed lower NO₃^- concentrations (0.9 mg N L^-1) and stronger denitrification NO₃^- isotope signals. During this period, the decrease in hydrologic connectivity shifted the predominant water source to deep groundwater, with longer water travel time promoting denitrification. After accounting for isotopic fractionations during nitrification and denitrification, we identified fertilizer N as the main NO₃^- source during the wet season (98.2±1.3%), whereas the dry season showed contributions from diverse sources (64.4±11.9% fertilizer, 26.0±15.8% soil N, and 9.5±6.0% manure and sewage). During the dry season, karst regions with high hydrologic connectivity display increased shallow groundwater inputs, carrying elevated NO₃^- levels from leaching of applied chemical fertilizers. These findings highlight the importance of integrating drainage water management and N accumulation in groundwater into nutrient management strategies to develop adaptive measures for controlling N pollution in agricultural watersheds.