Increased nitrous oxide emissions in East Asia as estimated by bottom-up and top-down approaches

Nitrous oxide (N2O) is the third most important greenhouse gas. Since the early 1980s, East Asia (including China, Mongolia, Japan, South Korea and North Korea) has experienced rapid agricultural and industrial developments, accompanied by increased fertilizer use, manure application and atmospheric N deposition; however, the trend and drivers of the associated nitrous oxide (N2O) emissions in this region remain uncertain. Here we use bottom-up (BU: Terrestrial Biosphere Models, emission inventories, and statistical models) and top-down (TD: three inversion frameworks) approaches to estimate the magnitude and trend of N2O emissions in East Asia between 1980 and 2016 and quantify the contributions of different drivers. The results suggest that, for East Asia total emissions, the BU ensemble mean (1.7 Tg N-N2O yr-1) is close to the TD ensemble mean (1.6 Tg N-N2O yr-1) over 2007-2016. A similar large uncertainty range was also detected in BU (1.2-2.4 Tg N-N2O yr-1) and TD (1.1-2.1 Tg N-N2O yr-1) estimates. The anthropogenic emission was the major source, accounting for 85% of the total emissions, among which 'Direct effects of N addition to agriculture', 'Other direct anthropogenic sources', and 'Indirect effects of N additions from all sources' were three dominant contributors. In comparison, 'Natural soil emissions' only accounted for 15% of the total amount. During the study period, N2O emissions rapidly increased. All sectors except 'Natural soil emissions' had significant increasing trends. 'Direct effects of N addition to agriculture' made the largest contribution to the increasing emissions. Among all countries, China alone contributed 85% of the increase in N2O emissions. Our findings suggest that reductions in fertilizer use, manure application and fossil fuel burning, particularly in China, are critical for mitigating N2O emissions in East Asia.