Crop production with intensive nitrogen (N) application is an important source of atmospheric nitrous oxide (N2O). However, there remains large uncertainty in quantifying cropland N2O emissions and their mitigation potential, especially in regions where cropping systems and farming management practices are highly diverse. Because N2O production in soils is tightly linked to N application rate and type, soil moisture, and oxygen status, improving irrigation is a potential strategy for N2O mitigation. We applied a process‐based biogeochemical model, DeNitrification‐DeComposition, to evaluate the influence of irrigation management on N2O emissions from California cropland, where cropping systems are extremely diverse and irrigation management has changed rapidly. We constructed a database containing data on weather, crop types and areas, soil properties, and farming management practices and predicted N2O emissions from California cropland under four typical irrigation methods (surface gravity, sprinkler, drip, and subsurface drip). We also assessed impact on N2O emissions of large‐scale changes in irrigation management experienced in California from 2001 to 2010 that was driven by the demand of reducing water use in agriculture. Simulations under different irrigation methods indicated that drip and subsurface drip irrigation reduced N2O emissions by 55% and 67%, respectively, compared to surface gravity irrigation. We estimated baseline N2O emissions from California cropland as 7.94 × 103 metric ton (MT) N/year under actual irrigation management in 2002. The large‐scale changes in irrigation management likely reduced N2O emissions by 7.3% from 2001 to 2010. This study showed the potential for reducing N2O emissions by using low‐volume irrigation.

Croplands are important sources of nitrous oxide (N2O) emissions. The lack of both long‐term field measurements and reliable methods for extrapolating these measurements has resulted in a large uncertainty in quantifying and mitigating N2O emissions from croplands. This is especially relevant in regions where cropping systems and farming management practices (FMPs) are diverse. In this study, a process‐based biogeochemical model, DeNitrification‐DeComposition (DNDC), was tested against N2O measurements from five cropping systems (alfalfa, wheat, lettuce, vineyards, and almond orchards) representing diverse environmental conditions and FMPs. The model tests indicated that DNDC was capable of predicting seasonal and annual total N2O emissions from these cropping systems, and the model's performance was better than the Intergovernmental Panel on Climate Change emission factor approach. DNDC also captured the impacts on N2O emissions of nitrogen fertilization for wheat and lettuce, of stand age for alfalfa, as well as the spatial variability of N2O fluxes in vineyards and orchards. DNDC overestimated N2O fluxes following some heavy rainfall events. To reduce the biases of simulating N2O fluxes following heavy rainfall, studies should focus on clarifying mechanisms controlling impacts of environmental factors on denitrification. DNDC was then applied to assess the impacts on N2O emissions of FMPs, including tillage, fertilization, irrigation, and management of cover crops. The practices that can mitigate N2O emissions include reduced or no tillage, reduced N application rates, low‐volume irrigation, and cultivation of nonleguminous cover crops. This study demonstrates the necessity and potential of utilizing process‐based models to quantify N2O emissions from regions with highly diverse cropping systems.

Soils are a source of atmospheric nitrogen oxides (NO x), especially in regions with significant cropland where nitrogen (N) fertilizers are used to enhance crop yields. The magnitude of soil NO x emissions, however, varies substantially by region, depending on the local land use pattern and management activities. We estimated soil NO x emissions in California based on the DeNitrification-DeComposition (DNDC) biogeochemical model, linked to a detailed spatial-temporal differentiated California-specific database. The DNDC-generated surface fluxes were used in the Community Multiscale Air Quality (CMAQ) model to evaluate impacts of soil NO x emissions on formation of ambient particulate (PM 2.5) nitrate in the San Joaquin Valley (SJV) where cropland is the dominant land use. The DNDC-generated soil NO x emissions contribute approximately 1.1% of total anthropogenic NO x emissions in California, at an emission rate of roughly 24 t day −1 (as NO 2) statewide and 9 t day −1 in the SJV. Cropland is the dominant source of soil NO x emissions in California, contributing nearly 60% of statewide soil NO x emissions, driven principally by fertilizer use. The PM 2.5 nitrate concentrations simulated by CMAQ using the DNDC-generated soil NO x emissions are compatible with those observed in the SJV, suggesting that soil NO x emissions have limited impacts on PM 2.5 nitrate formation in the atmosphere. Our emission and air quality modeling results are further supported by long-term ambient NO x-to-carbon monoxide (CO) and satellite NO 2 data analyses in the SJV, which showed diurnal, monthly, and annual trends consistent with characteristics of NO x sources dominated by traffic combustion in both urban and agricultural regions. Plain Language Summary Nitrogen oxides (NO x) are air pollutants that can react with other chemicals in the air to form fine particulate matter and ozone, both of which pose adverse impacts to human health and the environment. Control of NO x emissions is critical to improving air quality in California. Soils are a known NO x source, especially in agricultural areas where large amounts of nitrogen fertilizers are used to increase crop yields. The nitrogen chemicals in soil can be converted into various nitrogen gases, including NO x , by soil microorganisms. The contribution of soil emissions to the total NO x budget varies by region, depending on land uses and management activities. This study modeled soil NO x emissions from different land covers in California and evaluated impacts of soil NO x emissions on the formation of ambient particulate nitrate in the San Joaquin Valley (SJV), where cropland is the dominant land use. Our results indicate that soil NO x is a relatively minor fraction of the total NO x budget in California and has a minor effect on atmospheric concentrations of particulate nitrate in the SJV. Ambient and satellite data analyses show traffic combustions being the dominate source of NO x emissions in both urban and agricultural areas of the SJV.

Abstract:Carbon sequestration occurs when cultivated soils are re-vegetated. In the hilly area of the Loess Plateau, China, black locust (Robinia pseudoacacia) plantation forest and grassland were the two main vegetation types used to mitigate soil and water loss after cultivation abandonment. The purpose of this study was to compare the soil carbon stock and flux of these two types of vegetation which restored for 25 years. The experiment was conducted in Yangjuangou catchment in Yan′an City, Shaanxi Province, China. Two adjacent slopes were chosen for this study. Six sample sites were spaced every 35-45 m from summit to toe slope along the hill slope, and each sample site contained three sampling plots. Soil organic carbon and related physicochemical properties in the surface soil layer (0-10 cm and 10-20 cm) were measured based on soil sampling and laboratory analysis, and the soil carbon dioxide (CO 2 ) emissions and environmental factors were measured in the same sample sites simultaneously. Results indicated that in general, a higher soil carbon stock was found in the black locust plantation forest than that in grassland throughout the hill slope. Meanwhile, significant differences in the soil carbon stock were observed between these two vegetation types in the upper slope at soil depth 0-10 cm and lower slope at soil depth 10-20 cm. The average daily values of the soil CO 2 emissions were 1.27 mol/(m 2 ·s) and 1.39 mol/(m 2 ·s) for forest and grassland, respectively. The soil carbon flux in forest covered areas was higher in spring and less variation was detected between different seasons, while the highest carbon flux was found in grassland in summer, which was about three times higher than that in autumn and spring. From the carbon sequestration point of view, black locust plantation forest on hill slopes might be better than grassland because of a higher soil carbon stock and lower carbon flux.

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