Extended Data Fig. 2: The methodology for data synthesis of the global N₂O budget.

BU and TD represent bottom-up and top-down methods, respectively. The colour codes are the same as that used in Table 1 and Figs. 1–3. We use both approaches, including 22 bottom-up and five top-down estimates of N₂O fluxes from land and oceans. For sources estimated by the bottom-up approach, we include six process-based terrestrial biosphere modelling studies¹⁶; five process-based ocean biogeochemical models⁹⁹; one nutrient budget model^30,60,61; five inland water modelling studies^{35,36,50,51,68}; one statistical model SRNM based on spatial extrapolation of field measurements¹⁷; and four greenhouse-gas inventories: EDGAR v4.3.2¹⁰⁰, FAOSTAT¹⁰¹, GAINS⁴¹, and GFED4s¹⁰². In addition, previous studies regarding estimates of surface sink^58,73, lightning^53,54, atmospheric production^56,57,103, aquaculture^31,62 and model-based tropospheric sink⁸¹ and observed stratospheric sink¹ are included in the current synthesis. ^aRef. ³¹ and ref. ⁶² provide global aquaculture N₂O emissions in 2013 and in 2009, respectively; and the nutrient budget model^30,60,61 provides nitrogen flows in global freshwater and marine aquaculture over the period 1980–2016. ^bModel-based estimates of N₂O emissions from inland and coastal waters include rivers and reservoirs^35,36, lakes⁵¹, estuaries³⁵, coastal zones (that is, seagrasses, mangroves, saltmarsh and intertidal saltmarsh)⁶⁸ and coastal upwelling⁵⁰.