Weakened growth of cropland-N 2 O emissions in China associated with nationwide policy interventions.

China has experienced rapid agricultural development over recent decades, accompanied by increased fertilizer consumption in croplands; yet, the trend and drivers of the associated nitrous oxide (N ₂ O) emissions remain uncertain. The primary sources of this uncertainty are the coarse spatial variation of activity data and the incomplete model representation of N ₂ O emissions in response to agricultural management. Here, we provide new data-driven estimates of cropland-N ₂ O emissions across China in 1990-2014, compiled using a global cropland-N ₂ O flux observation dataset, nationwide survey-based reconstruction of N-fertilization and irrigation, and an updated nonlinear model. In addition, we have evaluated the drivers behind changing cropland-N ₂ O patterns using an index decomposition analysis approach. We find that China's annual cropland-N ₂ O emissions increased on average by 11.2 Gg N/year ² (p < .001) from 1990 to 2003, after which emissions plateaued until 2014 (2.8 Gg N/year ² , p = .02), consistent with the output from an ensemble of process-based terrestrial biosphere models. The slowdown of the increase in cropland-N ₂ O emissions after 2003 was pervasive across two thirds of China's sowing areas. This change was mainly driven by the nationwide reduction in N-fertilizer applied per area, partially due to the prevalence of nationwide technological adoptions. This reduction has almost offset the N ₂ O emissions induced by policy-driven expansion of sowing areas, particularly in the Northeast Plain and the lower Yangtze River Basin. Our results underline the importance of high-resolution activity data and adoption of nonlinear model of N ₂ O emission for capturing cropland-N ₂ O emission changes. Improving the representation of policy interventions is also recommended for future projections.
(© 2019 John Wiley & Sons Ltd.)