the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Uncertainties in fertilizer-induced emissions of soil nitrogen oxide and the associated impacts on ground-level ozone and methane
Abstract. Natural and agricultural soils are important sources of nitrogen oxides (NOx), accounting for about 10 %–20 % of the global NOx emissions. The increased application of nitrogen (N) fertilizer in agriculture has strongly enhanced the N availability of soils in the last several decades, leading to higher soil NOx emissions. However, the magnitude of the N fertilizer-induced soil NOx emissions remains poorly constrained due to limited field observations, resulting in divergent estimates. Here we integrate the results from meta-analyses of field manipulation experiments, emission inventories, atmospheric chemistry modelling and terrestrial biosphere modelling to investigate these uncertainties and the associated impacts on ground-level ozone and methane. The estimated present-day global soil NOx emissions induced by N fertilizer application varies substantially (0.84–2.2 Tg N yr-1) among different approaches with different spatial patterns. Simulations with the 3-D global chemical transport model GEOS-Chem demonstrate that N fertilization enhances global surface ozone concentrations during summertime in agricultural hotspots, such as North America, western Europe and eastern and southern Asia by 0.3 to 3.3 ppbv. Our results show that such spreads in soil NOx emissions also affect atmospheric methane concentrations, reducing the global mean by 7.1 ppbv to 16.6 ppbv as indirect consequence of enhanced N fertilizer application. These results highlight the urgent need to improve the predictive understanding of soil NOx emission responses to fertilizer N inputs and its representation in atmospheric chemistry modelling.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-1416', Anonymous Referee #1, 18 Jun 2025
- AC1: 'Reply on RC1', Cheng Gong, 20 Aug 2025
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RC2: 'Comment on egusphere-2025-1416', Anonymous Referee #3, 08 Jul 2025
Comments:
The manuscript titled “Uncertainties in fertilizer-induced emissions of soil nitrogen oxide and the associated impacts on ground-level ozone and methane,” written by Gong et al., quantifies the uncertainties in soil NOx emissions induced by N fertilizer application (SNOx-Fer) using different estimation approaches and investigates the associated impacts on the simulation of global O3 and CH4 concentrations. Overall, this manuscript is well-structured, and the conclusion is important. However, I would like to raise two major concerns and several minor suggestions for improvement.
Major concerns:
- I can tell by Figures 2, 3, 4, and Section 4.2 that, in general, regions with higher SNOx-Fer have higher O3 enhancement. Is this an approximately linear relationship? Does this relationship vary across different sensitivity experiments and different regions? Providing a more detailed analysis of the response of the O3 simulation to NOx estimations would further highlight the importance of this work. The same concern also applies to the OH/CH4 simulation.
- In Lines 125–129, the growing season is defined using monthly-mean 2-meter temperature and leaf area index instead of using some crop calendar datasets. While this approach is straightforward and climate-driven, it may oversimplify the actual crop phenology in diverse agricultural systems. Given that the rates of N inputs are set to zero during the non-growing season, this definition directly controls the temporal pattern of fertilizer application and thus significantly affects the estimates of fertilizer-induced NOx emissions. If crop-specific growing seasons are not distinguishable in this study, the authors should at least discuss the potential implications of this assumption in the discussion section.
Minor points:
- Are there any top-down methods for estimating NOx emissions? If so, it would be beneficial for the authors also to describe it in the introduction, allowing for a more comprehensive review of the estimation approaches.
- Section 2.2: Consider also adding one or two sentences to describe why this specific inventory is chosen.
- Figure 1: Consider merging (a) and (b) into a single figure using a secondary Y-axis for fertilizer input, which would help the reader better interpret the relationship between nitrogen inputs and SNOx-Fer across approaches.
- Section 4.2 and 4.3: When reporting changes in O3 and CH4 concentrations, consider also providing percentage changes rather than only providing the ppbv changes.
- The HaNi dataset provided N inputs for cropland, pasture, and rangeland. Consider also providing the NOx emissions from cropland, pasture, and rangeland in the Supplementary Information.
- Given the large differences in simulated surface O3 concentrations across the different SNOx-Fer estimation methods (e.g., Fig. 3 and 4), it would be valuable to include a brief comparison with surface O3 observations. While a full validation is beyond the scope of this study, even a qualitative comparison could help indicate which NOx emission estimation method may better reproduce observed O3 levels in key agricultural regions.
Citation: https://doi.org/10.5194/egusphere-2025-1416-RC2 - AC2: 'Reply on RC2', Cheng Gong, 20 Aug 2025
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Summary: This paper assesses the emissions of soil NOx from several different emissions parameterizations, and the global impact of these emissions on atmospheric chemistry. Overall, the large range of differences in the emissions (both in terms of magnitude and spatial pattern) impacts surface ozone and methane concentrations. I agree with the author’s conclusions that this is an important issue to consider and they raise some interesting points. However, this study felt very cursory - results were presented and discussed in very short text. Adding some depth and further discussion to the paper would strengthen the manuscript.
Major points
Minor points