Warming and redistribution of N inputs drive long-term increase in terrestrial N2O emission factor

Anthropogenic nitrogen inputs cause major negative environmental impacts, including emissions of the important greenhouse gas N2O. Despite their importance, changes in terrestrial N loss pathways driven by global change and spatial redistribution of N inputs are highly uncertain. We present a novel coupled soil-atmosphere isotope model (IsoTONE) to quantify terrestrial N losses and N2O emission factors from 1850-2020, initialised using a global dataset of natural soil δ15N, and optimized with a tropospheric timeseries of N2O isotopic composition using a Bayesian framework. N inputs from atmospheric deposition caused the majority (51%) of anthropogenic N2O emissions from soils in 2020. Long-term growth in emissions was driven by fertilization and deposition, however biological fixation caused subdecadal variability in emissions. N2O emission factors (EF) show large spatial variability due to climate and soil parameters. The mean effective global EF for N2O (weighted by N inputs) was 4.3±0.3% in 2020, much higher than the land surface area-weighted mean (1.1±0.1%). Climate change and redistribution of fertilisation have driven an increase in global EF 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 focussing on fertiliser management and reduced N deposition are introduced.