Global soil N2O emissions since the pre-industrial era estimated by an ensemble of Terrestrial Biosphere Models: Magnitude, attribution and uncertainty
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Our understanding and quantification of the magnitude and spatiotemporal patterns of soil nitrous oxide (N2O) emissions and the processes underlying them remain far from robust, limiting our ability to predict future global biogeochemical cycles and climate change. Here we assessed the effects of multiple anthropogenic and natural factors, including nitrogen fertilizer application, atmospheric nitrogen deposition, manure nitrogen application, land cover change, climate change and rising atmospheric CO2?concentration, on global soil N2O emission for the period 1861?2016 using a standard simulation protocol with seven process-based terrestrial biosphere models. Results suggest global soil N2O emissions have increased from 6.3 ? 1.1 Tg N2O-N yr-1?in the pre-industrial period (the 1860s) to 10.0 ? 2.0 Tg N2O-N yr-1?in the recent decade (2007-2016). Cropland soil emissions, in particular, increased from 0.3 Tg N2O-N yr-1?to 3.3 Tg N2O-N yr-1?over the same period, contributing 82% of the increased global soil N2O emissions. In the recent decade, N fertilizer application, N deposition, manure N application and climate change contributed 54%, 26%, 15% and 24%, respectively, to the increase in global soil N2O emissions. Regionally, China, South Asia and Southeast Asia underwent the most rapid increases in cropland N2O emissions since the 1970s. Since the 1980s, US cropland N2O emissions have been relatively flat, and EU cropland N2O emissions appear to have decreased by 14%. Soil N2O emissions from predominantly natural systems accounted for 67% of the global soil emissions in the recent decade but showed only a relatively small increase of 0.7 ? 0.5 Tg N2O-N yr-1?(11%) since the pre-industrial period. Conversely, increasing atmospheric CO2?concentration decreased soil N2O emissions by 15%, and land cover change played a minor role in global soil N2O emissions. The next steps for improving the process-based simulation of global and regional soil N2O emissions include: 1) improving the model representation and parameterizations of key biogeochemical processes responsible for N2O fluxes, including nitrification and denitrification processes, and other N-related fluxes and processes; 2) improving simulations of soil N2O response to individual and combined factors; 3) refining the quality of model input datasets, especially cropland and pasture management practices; and 4) enhancing data-model integration for improving model performance at multiple scales.