A Decreasing Trend of Nitrous Oxide Emissions From California Cropland From 2000 to 2015

Mitigation of greenhouse gas emissions from agriculture requires an understanding of spatial‐temporal dynamics of nitrous oxide (N(2)O) emissions. Process‐based models can quantify N(2)O emissions from agricultural soils but have rarely been applied to regions with highly diverse agriculture. In this study, a process‐based biogeochemical model, DeNitrification‐DeComposition (DNDC), was applied to quantify spatial‐temporal dynamics of direct N(2)O emissions from California cropland employing a wide range of cropping systems. DNDC simulated direct N(2)O emissions from nitrogen (N) inputs through applications of synthetic fertilizers and crop residues during 2000–2015 by linking the model with a spatial‐temporal differentiated database containing data on weather, crop areas, soil properties, and management. Simulated direct N(2)O emissions ranged from 3,830 to 7,875 tonnes N(2)O‐N yr(−1), representing 0.73%–1.21% of the N inputs. N(2)O emission rates were higher for hay and field crops and lower for orchard and vineyard. State cropland total N(2)O emissions showed a decreasing trend primarily driven by reductions of cropland area and N inputs, the trend toward growing more orchard, and changes in irrigation. Annual direct N(2)O emissions declined by 47% from 2000 to 2015. Simulations showed N(2)O emission variations could be explained not only by cropland area and N fertilizer inputs but also climate, soil properties, and management besides N fertilization. The detailed spatial‐temporal emission dynamics and driving factors provide knowledge toward effective N(2)O mitigation and highlight the importance of coupling process‐based models with high‐resolution data for characterizing the spatial‐temporal variability of N(2)O emissions in regions with diverse croplands.