Rfallow and related environmental, management and sampling factors from published studies.
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Croplands account for ~1/3 of global anthropogenic nitrous oxide (N<sub>2</sub>O) emissions. A number of recent field experiments found substantial fallow-period N<sub>2</sub>O emissions, which have been neglected for decades. However, the global contribution of the fallow-period emissions and the associated drivers remain unclear. Based on 360 observations across global agroecosystems, we simulated the ratio of the fallow to the whole-year N<sub>2</sub>O emissions (R<sub>fallow</sub>) by developing a mixed-effect model and compiling cropping-system-specific input data. Our results revealed that the mean global gridded R<sub>fallow</sub> was 44% (15-75%, 95% confidence interval), with hotspots mainly in the northern high latitudes. For most cropping systems, soil pH was the dominant driver of global variation in R<sub>fallow</sub>. Global cropland emission factors (i.e., the percentage of fertilizer N emitted as N<sub>2</sub>O, EFs) in EF-based models doubled to 1.9% when the fallow-period N<sub>2</sub>O emissions were included in our simulation, similar to EFs estimated by process-based and atmospheric inversion models (1.8-2.3%). Overall, our study highlights the importance of fallow-period N<sub>2</sub>O emissions in annual totals, especially for single cropping systems and croplands in acidic areas. To accurately estimate N<sub>2</sub>O emissions for national greenhouse gas inventories, it is crucial to update current EFs with full consideration of the fallow-period N<sub>2</sub>O emissions in the Intergovernmental Panel on Climate Change (IPCC) Tier 1 method.
