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 (N2O) emissions. Process‐based models can quantify N2O 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 N2O emissions from California cropland employing a wide range of cropping systems. DNDC simulated direct N2O 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 N2O emissions ranged from 3,830 to 7,875 tonnes N2O‐N yr−1, representing 0.73%–1.21% of the N inputs. N2O emission rates were higher for hay and field crops and lower for orchard and vineyard. State cropland total N2O 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 N2O emissions declined by 47% from 2000 to 2015. Simulations showed N2O 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 N2O mitigation and highlight the importance of coupling process‐based models with high‐resolution data for characterizing the spatial‐temporal variability of N2O emissions in regions with diverse croplands. Plain Language Summary: Nitrous oxide (N2O) is the third most important greenhouse gas and an important ozone‐depleting substance. It is challenging to quantify N2O emissions from croplands in 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 N2O emissions from California cropland, which has a wide range of cropping systems. DNDC simulated direct N2O emissions from nitrogen (N) inputs from synthetic fertilizers and crop residues during 2000–2015. Simulated direct N2O emissions ranged from 3,830 to 7,875 tonnes N yr−1, representing 0.73%–1.21% of the N inputs from synthetic fertilizers and crop residues. Simulated annual direct N2O emissions from California cropland declined by 47% from 2000 to 2015. This decreasing trend could be explained by reductions of cropland area and associated N inputs, the trend toward growing more orchard crops, and changes in irrigation. This study characterized spatial‐temporal dynamics of N2O emissions from California cropland, and provided knowledge toward effective N2O mitigation. It also highlights the importance of coupling process‐based models with high‐resolution data to quantify the spatial‐temporal variability of N2O emissions in regions with complicated agricultural systems. Key Points: Direct nitrous oxide (N2O) emission from N inputs of fertilizers and residues ranged from 3,830 to 7,875 ton N yr−1 from 2000 to 2015 in California croplandSimulated annual direct N2O emission decreased by 47% from 2000 to 2015, primarily driven by changes in land use and farming managementCoupling model with environmental and activity database improved characterization of direct N2O emissions from California cropland [ABSTRACT FROM AUTHOR]