Substantial nitrogen abatement accompanying decarbonization suppresses terrestrial carbon sinks in China

Domain: earth_science
Published: 2024-09-04
Editorial outcome: major_revision

Abstract
China faces challenges in reaching its carbon neutrality goal by the year 2060 to meet the Paris Agreement and improving air quality simultaneously. Dramatic nitrogen emission reductions will be brought by this ambitious target, yet their impact on the natural ecosystem is not clear. Here, by combining two atmospheric chemistry models and two process-based terrestrial ecosystem models constrained using nationwide measurements, we show that atmospheric nitrogen deposition in China’s terrestrial land will decrease by 44–57% following two emission control scenarios including one aiming at carbon neutrality. They consequently result in a pronounced shrinkage in terrestrial net ecosystem production, by 11–20% depending on models and emission scenarios. Our results indicate that the nitrogen emission reductions accompanying decarbonization would undermine natural carbon sinks and in turn setback progress toward carbon neutrality. This unintended impact calls for great concern about the trade-offs between nitrogen management and carbon neutrality.

Introduction
As the world’s largest emitter of carbon dioxide (CO2) (approximately 10.7 billion tons of CO2 in 2020)1, China is committed to achieving carbon neutrality by 2060, driven by the Paris Agreement and Glasgow Climate Pact goals of limiting the global temperature increase preferably to 1.5°C2. China has also been dedicated to curbing severe air pollution3. To meet these targets, the government aims to synergize the mitigation of air pollution and CO2 emissions with the phase-out of fossil fuel combustion, the dominant source of such emissions4,5, as well as reducing agricultural NH3 emissions6. Meanwhile, boosting natural carbon sinks is of great importance to the transition toward carbon neutrality. Terrestrial ecosystems have served as a net carbon sink in China, effectively absorbing a portion of CO2 emitted to the atmosphere9–11.

However, the evolution of the natural carbon sinks through 2060 may be unintentionally affected by anthropogenic emissions controls. In the pathway toward carbon neutrality in China (i.e., carbon emissions peaking by 2030 and reaching net-zero by 2060), tremendous emissions of reactive nitrogen (Nr), composed of nitrogen oxides (NOx = NO + NO2) and ammonia (NH3), which are released in larger quantities from China than from the United States and European Union12, are projected to be substantially decreased by 2060 (see Methods for details on Nr emissions reduction from the present to 2060). Such reductions in anthropogenic Nr emissions will undoubtedly lead to a decrease in Nr deposition to terrestrial ecosystems. Because N is an essential nutrient element for the primary production of plants, decreased Nr deposition could exacerbate ecosystem N limitation13,14, thereby inhibiting carbon sequestration in the ecosystem. However, the extent to which such policy-driven emission reductions can ultimately alter the evolution of natural carbon sinks is not clear.

This work quantitatively links terrestrial carbon sinks in China to Nr emission changes by climate mitigation and clean air policies. To obtain a robust conclusion, we combined two state-of-the-art atmospheric chemistry models (i.e., Weather Research and Forecasting model coupled with Chemistry, WRF-Chem, and Community Atmosphere Model with Chemistry, CAM-Chem. See Methods and Supplementary Text 2 for model details), the nationwide Nr deposition and CO2 exchanges measurement datasets, and two terrestrial ecosystem process-based models (Community Land Model version 5, CLM5, and Common Land Model, CoLM. See Methods and Supplementary Text 3 for model details). First, we performed parallel atmospheric chemistry simulations separately with historical anthropogenic emissions for the year 2017 (referred to as Base) and projected emissions scenarios for 2060 to derive corresponding present-day and future Nr deposition fluxes (see SI Table S1). Two future emission pathways were examined here, i.e., the end-of-pipe control scenario (EndCtrol) and carbon neutrality scenario (CNeutr), which were previously developed for China based on the Shared Socioeconomic Pathways (see Methods). Next, we integrated these Nr deposition scenarios into the terrestrial ecosystem model projections to ascertain the responses of the terrestrial carbon sinks and the underlying causes.

Results
Reductions in Nr deposition from the present to 2060
The WRF-Chem simulation reproduces the observed spatial pattern and magnitude of Nr deposition reasonably, although there is an underestimation in NOy-N wet deposition (see Supplementary Text 2 and Fig. S2 for details). The typical hotspots (20‒40 kg N ha–1 yr–1) are distributed in eastern China and parts of southwestern China and southern China (Fig. 1b). The annual sum of Nr deposition over Chinese terrestrial land is estimated to be 10.4 Tg N yr–1 for the year 2017 in the Base run (SI Table S2). Our simulation captures the distinct seasonality of Nr deposition fluxes, with the largest amount (3.6 Tg N) in summer (June, July, and August) and the smallest (1.5 Tg N) in winter (December, January, and February) as a joint result of the seasonal cycles of precipitation and nitrogen emissions.

We find that China’s total Nr deposition decreases by 57% (5.9 Tg yr–1) under the carbon neutrality scenario by 2060 compared with 2017. NOy-N (NOy = NO + its oxidation products) deposition decreases more strongly (by 3.6 Tg N by 2060, equivalent to 78% of the 2017 level) than NHx-N (NHx = NH3 + NH4+) deposition (by 2.3 Tg N, equivalent to 40%) due to the greater reduction of NOx emissions (by 89% compared to the 2017 level by 2060) than that of NH3 emissions (50% reduction). Spatially, the projected decreases of Nr deposition are most pronounced in eastern China and southwestern China, where Nr deposition reductions are larger than 15 kg N ha–1 yr–1, equivalent to more than 60% of that in the 2017 baseline (Fig. 1 and SI Fig. S3). These areas, currently the hotspots of anthropogenic NOx and NH3 emissions due to intensive agricultural and industrial activities, are subject to stringent Nr emissions controls in the target scenario. The EndCtrol scenario, only with end-of-pipe pollutant control technology implemented, exhibits smaller Nr deposition changes (a decrease of 44%, or 4.6 Tg yr–1 by 2060 compared to the 2017 level) from regional to national scales than that of the CNeutr simulation.

Consistently, the CAM-Chem model also shows a reasonable performance of Nr deposition simulation as compared with in-situ observations, with normalized mean biases of –10% and +36% for NOy-N and NHx-N, respectively (SI Fig. S5). The simulations reveal decreases of 40–49% (5.5–6.8 Tg yr–1) in China’s annual Nr deposition under the two examined scenarios (i.e., EndCtrol and CNeutr), comparable to the WRF-Chem results (44–57%) (SI Fig. S3, S4). Despite differences between the two models in atmospheric chemistry mechanisms (see Methods and Supplementary Text 2) and the reactive nitrogen emissions outside China (SI Table S1), their simulation results of domestic Nr deposition changes reach an agreement, manifesting the robustness of our estimations.

It is worth noting that the response of NOy-N deposition to NOx reduction can be modulated by volatile organic compounds (VOCs)6,15. In the CNeutr run, while the decrease of NOy-N deposition results predominantly from the substantial reduction of NOx emissions (by 89%), the decrease of NOy-N deposition (by 85%) is also facilitated by the change in atmospheric oxidation capacity due to the synergistic controls of VOCs and NOx emissions15. Therefore, NOy-N deposition can be efficiently reduced in proportion to the reduction in NOx emissions over most of China.

Terrestrial carbon sinks with reduced Nr deposition
Next, we performed comprehensive terrestrial ecosystem modeling to quantitatively line the terrestrial carbon sinks to future changes in Nr deposition fluxes. A critical metric, net ecosystem production (NEP), defined as the difference between net primary production (NPP) and heterotrophic respiration (CO2 efflux from soil and litter decomposition, RH), can be used to represent the terrestrial ecosystem carbon sinks16. The CLM5 simulation with the Nr deposition field from the Base run yields a terrestrial NEP of 307 Tg C per year in China. The results reasonably reproduce the spatial heterogeneity in gross primary productivity (GPP), ecosystem respiration, and NEP observed by the eddy covariance method (see Supplementary Text 4), showing positive NEP fluxes of 80‒120 g C m–2 yr–1 (i.e., carbon sinks) in southern China and small negative values (i.e., carbon sources) in the west and north part (SI Fig. S8a). Another terrestrial ecosystem model, CoLM, gives similar results concerning the magnitude (260 Tg C per year) and spatial characteristics of land carbon balances (SI Fig. S10a). Both NEP calculations using CLM5 and CoLM fall within the range of previous estimates (SI Table S4, 5).

We then demonstrate the responses of NEP to the reduced Nr deposition using terrestrial ecosystem simulations. Compared to the Base case above, the carbon neutrality target leads to a reduction in NEP of 48.5–52.9 Tg C yr–1 across our two models, or 16–20% in percentage (Fig. 2), while the NEP reductions in the EndCtrol case are only 11–15% (35.5–39.7 Tg C yr–1) due to limited nitrogen control. The model responses are close between CLM5 and CoLM. Given the universality of the former, we adopt the CLM5 simulations hereafter to interpret the process-based ecosystem response to projected Nr deposition under the two emissions scenarios.

For terrestrial natural ecosystems, the nitrogen uptake (NUPTAKE) and NPP are reduced by 2.3–2.9 Tg N yr–1 (3.6–4.6%) and 82.6–105.1 Tg N yr–1 (2.8–3.6%), respectively, since vegetation growth is tightly associated with nitrogen supply17,18. Such decreases for crops are negligible because intensive synthetic fertilizer application in croplands substantially enhances nitrogen availability (SI Fig. S9c). On the other hand, the RH fluxes in terrestrial ecosystems across China are reduced by 46.9–57.2 Tg C yr–1 (1.5–1.8%). This reduction could be explained by the decrease in litterfall (2.1–2.8%), which reduces the carbon supply for soil microorganisms during decomposition19. These cascading effects are linked not only to the direct reduction in Nr deposition but also to the resulting lower N inputs via litterfall, which subsequently reduces organic N mineralization rates. Overall, the larger reduction in NPP than in RH is associated with much higher carbon-to-nitrogen ratios for plants (especially woody tissues) than for soils20,21.

Figure 3 illustrates the spatial changes in NEP fluxes over China for the CNeutr scenario in the year 2060. Of the three regions with pronounced declines in Nr deposition (SI Fig. S3b), the regional decrease in NEP is larger in southern China and southwestern China than in eastern China (Fig. 3d). The region-dependent changes in NUPTAKE, NPP, and RH are all more pronounced in southern than in eastern China, even though the decreases in Nr deposition is largest in eastern China. Specifically, the NPP in natural vegetation decreases more strongly than RH (31.4 g C m–2 yr–1 vs. 21.1 g C m–2 yr–1) in southern China, yielding a larger decrease in NEP fluxes of 10.5 g C m–2 yr–1 compared to that of eastern China (4.4 g C m–2 yr–1). This is because southern China features a wide distribution of natural vegetation, whose growth is more N-limited (SI Fig. S9a). Similarly, the CoLM simulation also shows a much larger decrease in NEP in southern China (9.9 g C m–2 yr–1) than in eastern China (4.8 g C m–2 yr–1) (SI Fig. S10b).

The changes in NEP are seasonally dependent. In our present-day simulation (Base run), the NEP fluxes are generally positive across China during the warm season (spring and summer) (Fig. 4a and SI Fig. S11a), which dominate the annual carbon sinks. Following the carbon neutrality scenario by 2060, the NEP in warm seasons is reduced by 37.6–41.3 Tg C (6.1–8.3%) in the two models’ simulations (Fig. 4b, SI Fig. S12a, b, and Fig. S13a, b). This reduction is associated with the decrease in natural vegetation NPP (Fig. 4b and SI Fig. S14e, f) when Nr deposition is substantially decreased in summer and spring (accounting for 26% and 35% of the total annual Nr deposition decrease, respectively). In contrast, the NEP values are mostly negative during the cold season (autumn and winter), indicating that the land is a net carbon source (Fig. 4a and SI Fig. S11b). Following the carbon neutrality pathway, they become even more negative (7.2–15.3 Tg C) (Fig. 4b, SI Fig. S12-14). Overall, such decreases in annual carbon sinks over Chinese natural terrestrial ecosystems are a net result of the large decrease in natural vegetation NPP in the warm season and a relatively small decrease in RH.

Methods
Future nitrogen emissions scenarios
Anthropogenic air pollutant mitigation pathways that link various climate goals with national clean air actions were designed using an emissions projection model (Dynamic Projection for Emission in China, DPEC)4. For future NOx emissions, we adopted two scenarios to represent potential upper and lower bounds of anthropogenic Nr reduction following different mitigation pathways. First, we employed the Ambitious-pollution-1.5°C-goals reduction pathway as the carbon neutrality emissions scenario (CNeutr) in this study. This scenario follows climate constraints of Representative Concentration Pathway 1.9 (RCP1.9) and Shared Socioeconomic Pathway 1 (SSP 1), which implement low-carbon energy transitions in power and industry sectors. The CNeutr scenario aims to achieve the long-term goal of the 1.5°C temperature limits stated in the Paris Agreement and Glasgow Climate Pact2 which encompassing China’s carbon neutrality target by 2060 aligned with this goal, and supports clean air goal of reducing PM2.5 exposure to the World Health Organization’s annual guideline level of 10 μg/m34. We also employed the Ambitious-pollution-NDC-goals reduction pathway as the end-of-pipe control emissions scenario in our study (EndCtrol), a less ambitious emission reduction trajectory, which follows climate constraints of RCP4.5, and SSP2 socioeconomic developments with only deploying the end-of-pipe control technologies. The EndCtrol scenario aims to achieve China’s near-term climate and clean air goals, including reaching peak CO2 emissions by 2030 and meeting the current PM2.5 annual standard of 35 μg/m34.

China’s coordinated policy aiming to reduce both pollutants and carbon entails substantial reductions of NH3 emissions. As a huge agricultural country, nitrogen fertilization and extensive livestock industry collectively contribute to over 80% of total NH3 emissions in China. According to the ammonia emission model developed in our group34, such improvements in agricultural nitrogen management could decrease NH3 emission by up to 50% in the coming decades, while maintaining agricultural yields6 (see Supplementary Text 1 for estimates of NH3 emission reduction based on various mitigation measures). In this study, we adopted this 50% reduction target for NH3 in 2060, which represents the maximum feasible reduction in China. This scenario is expected to yield a higher estimate of ecosystem response compared to scenarios with weaker NH3 control.

For this study, 2017 was selected as the baseline year that the anthropogenic Nr emissions level could reach 16.4 Tg N yr–1 in China (calculated NOx emissions are from the Multi-resolution Emission Inventory for China (MEIC) database (0.25 degree) and NH3 is from PKU-NH3 (0.1 degree)35,36). Under the CNeutr emissions reduction scenario projected by DPEC, anthropogenic atmospheric NOx emissions will decrease by 89% in 2060. Meanwhile, we assume that NH3 emissions will decrease by 50% in 2060 compared to the 2017 baseline as stated in the preceding. Therefore, total Nr emissions in China are projected to fall by up to 72% from 2017 to 2060 with the CNeutr emissions reduction pathway, resulting in Nr emission of 4.6 Tg N yr–1. Under another emissions reduction scenario, EndCtrol, NOx emissions will decrease by only 62% in 2060 because the benefits of end-of-pipe control reductions are mostly exhausted by 2030. In this scenario, we also assumed the 50% in NH3 emissions, following the maximum potential reduction as in the CNeutr scenario. As a result, total Nr emissions in China will fall by only 57% from 2017 to 2060 with the EndCtrol emissions scenario, resulting in Nr emission of 7.1 Tg N yr–1 (SI Fig. S1).

Atmospheric chemistry model simulations
Two three-dimensional sophisticated atmospheric chemistry models were applied to project the evolution of Nr deposition following the current and future Nr emissions scenarios, i.e., Weather Research and Forecasting model with Chemistry (WRF-Chem version 3.6.1) and Community Atmosphere Model with Chemistry (CAM-Chem version 5.3). Both WRF-Chem and CAM-Chem have been widely used for regional- and global-scale modeling of tropospheric chemistry, owing to their comprehensive representation of atmospheric chemical processes37,38. The parallel simulation settings and the details of Nr emissions scenarios for China are summarized in SI Table S1.

For the WRF-Chem model, our simulation domain encompassed the entire China’s terrestrial area as well as surrounding countries to consider both domestic and transboundary Nr sources. The simulations were conducted for the whole year of 2017 with a horizontal resolution of 50 × 50 km and 24 vertical layers from the surface to 50 hPa; each model run lasted 84 h with a 12 h spin-up time. The initial and boundary meteorological conditions were obtained from 6 h National Centers for Environmental Prediction reanalysis data with 1° × 1° spatial resolution. The anthropogenic emissions outside China follow the MIX inventory for 201039. Biogenic emissions were estimated online by WRF-Chem using the Model of Emission of Gases and Aerosols from Nature40. The dry and wet deposition processes of tracers and aerosols including in-cloud and below-cloud wet removal have been treated in the standard version of WRF-Chem41. The detailed model parameterization schemes used in this study are described in Supplementary Text 2.

To test for robustness, the changes in Nr deposition fluxes were also simulated by the CAM-Chem model at a global scale with a horizontal resolution of 0.9 × 1.25 degrees. This model simulates gas-phase reactions with the Model for Ozone and Related chemical Tracers Version 4 mechanism and aerosol microphysical and chemical processes with the Aerosol Two-dimensional bin module for foRmation and Aging Simulation version 2 (ATRAS) developed in our previous studies42–44. For our CAM-Chem run, the anthropogenic emission inventories in China for the present and future scenarios are the same as those in the WRF-Chem simulations, while the emissions outside China follow the CMIP6 inventory fixed in 201445. Therefore, the differences of Nr deposition between the present and future cases are entirely attributable to the implementation of climate policies in China.

Terrestrial ecosystem model simulations
Two terrestrial ecosystem process-based models, Community Land Model version 5 (CLM5) and Common Land Model version 202X (CoLM202X), were used to quantify the effects of future Nr deposition reductions on carbon neutrality in China (see Supplementary Text 3 for model descriptions)46,47. Both models incorporate a biogeochemical module configured with advanced nitrogen-carbon coupling processes, which make them suitable for ascertaining carbon sink response to nitrogen input (Supplementary Text 3). We conducted parallel simulation experiments for the period of 2005–2014 with three different Nr deposition fluxes from WRF-Chem as inputs due to its higher resolution, i.e., Nr deposition in the Base run, CNeutr run and EndCtrol run. Additional sets of parallel simulations are based on the Base run and CNeutr run, where: (1) all other climate factors are kept constant at the year 2005 level throughout the simulation period, (2) projected elevated CO2 concentrations following the SSP1-1.9 scenario from CMIP6 are used during the simulation period, and (3) a global temperature increase of 1 K is applied during the simulation period. The parallel Nr deposition scenarios, meteorological forcing, and CO2 concentration data are summarized in SI Table S3. The averaged model results of the last three years were used for analysis of the terrestrial carbon sink budget.

In our CLM5 simulation, the model was initialized at a spatial resolution of 2.5° longitude by 1.875° latitude in 1850 through a spin-up process, which recursively uses reanalysis meteorological forcing from the GSWP3v1 set covering 1901 to 1920 to drive ecosystem carbon toward a steady state. Then, historical simulations were conducted from 1850 to 2014 with increasing atmospheric CO2 concentration, warming air temperature, land use changes, and increasing nitrogen deposition. The crop module with annually updated fertilization rates was turned on in conjunction with nitrogen modules. In our CoLM simulation, the model was initialized at a spatial resolution of 0.5° × 0.5°. Similar experimental setups were applied for the CoLM simulations.

Observations and model evaluation
The Nationwide Nitrogen Deposition Monitoring Network (NNDMN), comprised of 28 stations across China48,49, was used to evaluate atmospheric chemistry model performance. Daily measurements of atmospheric nitrate concentrations in particulate matter with a dry diameter of less than 2.5 μm were collected at 35 sites across China for validation of the modeled nitrate and ammonium concentrations. Site information was presented in our previous study6. The Base run outputs from WRF-Chem and CAM-Chem were thoroughly compared to in situ measurements of atmospheric Nr components and their deposition fluxes to land. The evaluation results showed that model outputs were comparable with the observations (Supplementary Text 2).

CLM5 has been extensively evaluated in International Land Model Benchmarking projects at both the site scale and global scale, with improvements reported in variables such as net ecosystem carbon balance, net ecosystem exchange, and ecosystem respiration (RE)47,50. Observations from the ChinaFLUX, a network measuring the exchanges of CO2, water vapor, and energy by eddy covariance between terrestrial ecosystem and atmosphere, were used to evaluate the model performance in China51. Carbon fluxes from the Base run were thoroughly compared to daily observations at 8 stations in various ecosystems over China. Evaluations indicate our models can reasonably simulate the spatial pattern of GPP, RE, and NEP over China’s terrestrial ecosystem (Supplementary Text 4).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I appreciate authors’ great efforts to improve this manuscript. Particularly, it is really impressive that authors have added many data and model experiments to validate results and provide uncertainty ranges, which significantly enhances the convincement of this study. I am generally satisfied with the new title and the revised results, despite of a scientific concern regarding to the magnitudes of the N deposition effects. However, regarding the manuscript structure and phrasing, I feel the demonstration of the N deposition effect on NEP are too prolix, while other necessary aspects (e.g. the scenario configuration) are not clear in the main text. There are also many questionable expressions throughout the manuscript. As a result, I cannot recommend the manuscript to be published on Nature Communications as the current version, but hope my comments below could be helpful to further improve this study.\nMajor concerns:\n1. I am wondering if the N deposition effect on NEP is overestimated in CLM and CoLM (or at least close to the upper-boundary estimate). Previous studies generally estimated it in ranges of about 45-65 Tg C yr-1 (e.g. Lu et al., 2012; Lu et al., 2016; Zhu et al., 2021; Yu et al., 2022), i.e. all of the present-day N deposition contributes about 5%-15% to the terrestrial ecosystem sinks. However, in this study, with only 44%-57% reduction in N deposition, 48.5-52.9 Tg C yr-1 (15.8%-20.4%) NEP is reduced. Instead of comparing the C responses to N addition with large uncertainty ranges (Lines 231-238), which actually should be compared at PFT level rather than on the national scale (maybe Table 2 in Lu et al. (2012) could be an example to explain what I initially mean), I suggest authors to design a new CLM/CoLM experiment with zero (or pre-industrial) N deposition to isolate the total N deposition contribution, compare N loadings as well as the contributions on C sinks with previous studies and add discussions regarding the model dependence and associated uncertainties.\n2. I think authors should be careful to demonstrate the dependence of Nr reduction on carbon neutrality. Carbon neutrality does not necessarily lead to significant Nr reduction, although they partly share the same sources (e.g. fossil fuel combustion). Amounts of Nr are actually from agricultural activities, and thus N management is even more important to reduce Nr. For example, I don’t understand what pathway that leads to the assumed 50% NH3 reduction under the carbon neutrality target? How robust it is? Will there be any possibility that we have a future scenario with low CO2 emissions but high N emissions, e.g. overuses of fertilizer? As a result, I feel confused with statements like ‘the consequent decrease in Nr deposition’ (Lines 218-221). I suggest authors to revise the phrasing throughout the manuscript, and introduce the future scenarios with more details.\n3. Lines 140-199: The analysis of the main results is unnecessarily too long. It is pretty straightforward to understand reduced N deposition -> reduced N uptake -> weakened NPP and Rh -> lower NEP. The model behaviors are also as expected as most of other terrestrial biosphere models. There is even no figure or table corresponding to the numbers within lines 140-169. Meanwhile, I don’t understand the meaning of NUE analysis. The N limitation claim is quite weak with only modeling results and actually not the key point of this study. As a result, I suggest authors to start their demonstration with Fig.3, and clarify their most significant results within 1-2 paragraphs. Uncertainties or implications can be in discussion.\nSpecific comments:\n1. Lines 51-52: The magnitude of future Nr reduction with carbon neutrality is still quite uncertain. I don’t think it is proper to cite the numbers here, especially where a questionable 50% reduction in NH3 is assumed.\n2. Line 54: N not Nr. I suggest to revise it as ‘N is one of the essential nutrients for plant growth’\n3. Line 59: Remove ‘evolving’?\n4. Line 65: Authors should guide readers to the descriptions of these two models here.\n5. Lines 70-71: It reads not clear why authors use these two scenarios? What are the implications for these two scenarios?\n6. Line 75: Strange sub-title. Nr deposition is not ‘driven’ by carbon neutrality.\n7. Line 77: WRF-Chem significantly underestimates NO3-N wet deposition (Fig. S2e), which should be pointed out in the main text.\n8. Lines 84-98: It would be good if authors can clarify what variables and which year are these percentage numbers relative to.\n9. Lines 107-119: The whole paragraph reads quite confusing to me. I guess authors think particle-phase N is harder to be deposited than gas-phase N? Are there any evidences on that? Why authors say ‘this is contrast to our recent study’? In addition, I don’t understand why authors need to discuss the NH3 impacts on the response of NOy-N deposition. Anyway, it is a bit strange to read uncertainty discussions before the main conclusion. I suggest to move it into discussion and rephrase the statement.\n10. Line 124: NPP is not equal to vegetation biomass accumulation due to varied litterfall responses. Please just use ‘net primary production’.\n11. Lines 126-130: I suggest to make the ‘other carbon fluxes’ more specific, e.g. wildfire and harvest. Besides, the definition of NEP is NPP-Rh, where harvest is already excluded. There is no ‘NEP from agricultural harvest’. Meanwhile, I don’t think harvest will be constant under the carbon neutrality scenario. Although NEP is not the exact terrestrial carbon sink, I think authors only need to acknowledge and clarify that as parts of discussions.\n12. Table S5: Nice table. However, I suggest authors to make the methods more specific, especially for those data-driven results. For example, they can be ‘remote sensing estimates’, ’ground-level biomass investigation’, ’up-scaling of eddy covariance measurement with statics/machine learning method’ and so on.\n13. Lines 144-146: As CLM5 results are used to interpret NEP-Nr response, why authors use numbers from CoLM in Fig. 2? Why not using the average?\n14. Line 150: The decrease of N uptake might not directly link to reduction in N deposition. A more complete pathway should be not only lower N deposition -> lower inorganic N -> lower N uptake, but also lower N deposition -> lower plant biomass and also higher C:N ratio -> lower litterfall N-> lower soil organic N -> lower N mineralization -> lower inorganic N -> lower N uptake. It is related to the whole biosphere interactions and thus it would be good if authors can clarity it a bit.\n15. Line 152: ‘of’ not ‘pf’\n16. Lines 181-183: It reads strange to claim NEP changes are dominated by NPP or Nuptake rather than Rh. Vegetation and soil are strongly connected. Another example of such expression is in lines 214-216. I suggest this manuscript should be revised thoroughly by experts in ecological modeling to avoid such expressions.\n17. Lines 202-204: why excluding crops makes NPP and Rh almost equal?\n18. Line 220: again, why only using CoLM results here?\n19. Line 237: why this uncertainty range is not shown in Fig. 2 as well as other conclusion sentences? How to compare it with the uncertainty ranges obtained by deviations in N deposition and different land surface models?\n20. Lines 246-249: It is quite controversial to state CO2 fertilization effect is mediated by N constraints, and I don’t understand the following statement of N uptakes and N efficient use.\n21. Lines 251-253: P deposition is not important, it makes sense, but it doesn’t mean P limitation is not important. With continuous N inputs, it could be possible that N reaches saturation and thus turns from N limitation to P limitation. Here, please just acknowledge that P limitation is not considered in this study, which may bring uncertainties.\n22. Lines 255-260. It should be REALLY careful with the call for supplementation of N for natural vegetation. The N inputs may not only provide nutrient to plants, but also acidify soil or threaten biodiversity. Therefore, the N fertilization effect may not as strong as we expected now. More importantly, with the continuous N inputs, many ecosystems have already reached N saturation, which not only means terrestrial carbon sequestration will no longer benefit from N inputs, but also indicates higher N loss into atmosphere or water body, further leading air pollution or water eutrophication. Instead of the current implication, calling for improved management practice with higher N use efficiency might be a better way to increase terrestrial C sinks with the Nr reduction.\n23. Figure 2: Besides the numbers of C fluxes as mentioned above, why the N deposition has no uncertainty range?\n24. Figure 3 and Figs. S3-S4: I am curious if the positive NPP and NEP grids are induced by the increased N deposition? Could authors change the color bars in Figs. S3-S4 to show the positive values?\n25. Lines 325: What is the reference of the 50% reduction in NH3? How realistic it is with the carbon neutrality scenario? What kind of actions could reach such an ambitious goal, while at the same time to meet the human food demands? These questions should be clarified in the manuscript.", "reviewer_2": "Comments on NCOMMS-24-18211-T: “Substantial Nitrogen Abatement Accompanying Decarbonization Suppresses Terrestrial Carbon Sinks in China”\nThank you, the authors, for carefully responding to my suggestions made in the previous review. I have noticed that the author has increased the length of the article in this version and provided more detailed results. I still have some additional concerns and minor suggestions listed below.\n1. Perhaps the author should consider adding some subtitles to better organize the article, so that readers can better keep up with the results and discussion.\n2. To me, it is not a surprised result. Decreased Nr emission -> Decreased N deposition -> Decreased NPP and RH -> Decreased NEP, this cascade relationship is already described and restrained by the model structure. Therefore, the originality and value of the article reduced. The scenarios for predicting the impact of reduced nitrogen deposition on carbon sequestration should be more diverse, and corresponding solutions should be provided.\n3. Currently, predictions of natural carbon sink in China in the context of carbon neutrality tend to focus more on the impacts of natural succession and tree age, while lacking comprehensive consideration of the influence of global change factors on future carbon sink. The author may consider enriching these contents to enhance the quality of the article. The additional sets of simulations, such as eCO2 and increasing temperature, are described very little in the article.\n4. Is it possible to distinguish between the effects of NH4+ and NO3– reduction on carbon sink? NH4+ and NO3– have different sources, emission reduction potentials and costs. Meanwhile, they also have different physicochemical property. For example, NH4+ is more energy efficient than NO3– because NH4+ is a ubiquitous intermediate compound during such physiological processes. NO3– is a mobile nutrient in soil solution, which is beneficial for plant uptake and more easily lost through leaching, while NH4+ is more likely to experience absorption and fixation by soil particulates, resulting in soil NH4+ accumulation. Such effects may be influenced by both the deposition flux of total N and by components of N deposition and its ratio of NH4+ and NO3–.\nLine 63-64: delete “the nationwide Nr deposition and carbon dioxide exchanges measurement dataset”\nLine 152: revise “pf” to “of”\nLine 198-199: “regions with widely distributed natural vegetation are generally more N-limited, as found in southern China”. Need to be careful with this statement. Some forests in southern China are N-rich ecosystems.", "reviewer_3": "General Comments::\nThe manuscript under review addresses a critical and timely topic regarding the interplay between nitrogen abatement and carbon sequestration within the context of China's decarbonization efforts. The study employs a multi-model approach to assess future nitrogen deposition and its effects on terrestrial carbon sinks, which is a robust and commendable methodological choice. The author has provided a detailed response and made revisions to the previous peer review comments. However, the manuscript requires significant revisions to enhance clarity, comprehensiveness, and scientific rigor before it can be considered for publication in Nature Communications.\nMajor Comments:\n1. Model Selection and Validation:\nThe authors have employed two atmospheric chemistry models (WRF-Chem and CAM-Chem) and two terrestrial ecosystem models (CLM5 and CoLM). It is crucial to discuss the rationale behind selecting these specific models and the extent to which their results are consistent or divergent. Additionally, the manuscript would benefit from a more detailed comparison of the model outputs, including any discrepancies and the potential reasons behind them.\n2. Scenario Justification:\nThe scenarios chosen for the study, CNeutr and EndCtrol, need to be better justified and aligned with China's current and projected environmental policies and climate commitments. The authors should elaborate on how these scenarios reflect realistic future conditions and the rationale behind their selection.\n3.Impact of Nitrogen Deposition on Carbon Sinks:\nThe manuscript suggests a significant reduction in terrestrial carbon sequestration due to decreased nitrogen deposition. It is important to discuss the potential ecological implications of this finding, including the long-term effects on biodiversity, soil fertility, and ecosystem services.\n4.Interactions with Other Environmental Factors:\nWhile the focus is on nitrogen and carbon cycles, the authors should consider and discuss potential interactions with other environmental factors such as phosphorus availability, land use changes, and climate change (e.g., temperature and precipitation patterns) that could also influence carbon sequestration.\n5. Economic and Policy Analysis:\nThe brief mention of the economic cost of nitrogen fertilizer application is a valuable point, but the manuscript would benefit from a more in-depth analysis of the economic implications and policy considerations. This could include a discussion on the trade-offs between nitrogen management and carbon neutrality.\n6.Figures and Data Presentation:\nThere are critical issues with the representation of China's geography in the figures, specifically Figure 1 and Figure 3, which do not display the complete map of China as required by official cartographic standards.\nGiven the importance of accurate geographical representation in scientific publications, especially those that are likely to be accessed by a global audience, I recommend that the authors make these revisions before the manuscript can proceed to the next stage of review.\n7.Technical Details and Uncertainty Analysis:\nThe methods section is detailed, but additional information on model calibration, validation, and uncertainty analysis is needed. The authors should provide more details on the input data used for the simulations and conduct a thorough uncertainty analysis.\n8.Language and Grammar:\nThe manuscript is generally well-written, but there are minor grammatical errors and awkward phrasings that should be corrected. It is recommended that the authors engage a professional language editing service to refine the manuscript.\nRecommendation:\nGiven the importance of the topic and the potential contributions of the study to the field, I recommend a \"Major Revision\" followed by a resubmission for further review. The authors should address all the points raised in this review to ensure that the manuscript meets the high standards of Nature Communications."}, "author_response": "Reviewer #1 (Remarks to the Author):\n(Comments are marked in blue)\nReferee comments for NCOMMS-24-18211-T: “Substantial Nitrogen Abatement\nAccompanying Decarbonization Suppresses Terrestrial Carbon Sinks in China.”\nI appreciate authors’ great efforts to improve this manuscript. Particularly, it is really\nimpressive that authors have added many data and model experiments to validate results\nand provide uncertainty ranges, which significantly enhances the convincement of this\nstudy. I am generally satisfied with the new title and the revised results, despite of a\nscientific concern regarding to the magnitudes of the N deposition effects. However,\nregarding the manuscript structure and phrasing, I feel the demonstration of the N\ndeposition effect on NEP are too prolix, while other necessary aspects (e.g. the scenario\nconfiguration) are not clear in the main text. There are also many questionable\nexpressions throughout the manuscript. As a result, I cannot recommend the manuscript\nto be published on Nature Communications as the current version, but hope my\ncomments below could be helpful to further improve this study.\nResponse: Accepted. We have revised the manuscript to address the following concerns.\nThanks for your helpful comments.\nMajor concerns:\n1. I am wondering if the N deposition effect on NEP is overestimated in CLM and\nCoLM (or at least close to the upper-boundary estimate). Previous studies generally\nestimated it in ranges of about 45–65 Tg C yr–1 (e.g., Lu et al., 2012; Lu et al., 2016;\nZhu et al., 2021; Yu et al., 2022), i.e., all of the present-day N deposition contributes\nabout 5%–15% to the terrestrial ecosystem sinks. However, in this study, with only\n44%-57% reduction in N deposition, 48.5–52.9 Tg C yr–1 (15.8%–20.4%) NEP is\nreduced. Instead of comparing the C responses to N addition with large uncertainty\nranges (Lines 231-238), which actually should be compared at PFT level rather than\non the national scale (maybe Table 2 in Lu et al. (2012) could be an example to\nexplain what I initially mean), I suggest authors to design a new CLM/CoLM\nexperiment with zero (or pre-industrial) N deposition to isolate the total N\ndeposition contribution, compare N loadings as well as the contributions on C sinks\nwith previous studies and add discussions regarding the model dependence and\nassociated uncertainties.\nResponse: Accepted. We improve the manuscript with more analysis.\n(1) As suggested by reviewer, we conducted additional experiments using CLM and\nCoLM, by setting anthropogenic Nr emissions in China to zero to represent the pre-\nindustrial N deposition levels. We put the results in the context of previous studies\non historical Nr deposition effects on carbon sink (Tian et al. 2011; Lu et al. 2012;\nGu et al. 2015; Lu et al. 2016; Zhu et al. 2021; Yu et al. 2022). As shown in the new\nTable S7, the NEP response to Nr deposition is estimated of 7.6–8.8 g C g-1 N, which\nwas in the range of reported values (6–13 g C g–1 N), despite the disparities in N\ndeposition patterns and the simulation years with changing environmental\nconditions such as climate and CO concentration between our study and others.\nSupplementary Information (SI).\n(2) We add a new table, Table S8, to assess the C responses to N changes in detail, using\nPFT-level data rather than a national mean. The results indicate that our model\nestimates are comparable to those from meta-analysis and stoichiometric scaling\nstudies. Please see Lines 218–223 in the revised manuscript and Table S8 in the\nrevised SI.\n(3) We provided discussions on the model dependence with different model structure\nand parameterizations and the associated uncertainties. Please see Lines 231–233\nin the revised manuscript, and Lines 172–186 in the revised Supplementary Text 3.\n2. I think authors should be careful to demonstrate the dependence of Nr reduction on\ncarbon neutrality. Carbon neutrality does not necessarily lead to significant Nr\nreduction, although they partly share the same sources (e.g., fossil fuel combustion).\nAmounts of Nr are actually from agricultural activities, and thus N management is\neven more important to reduce Nr. For example, I don’t understand what pathway\nthat leads to the assumed 50% NH reduction under the carbon neutrality target?\nlow CO emissions but high N emissions, e.g., overuses of fertilizer? As a result, I\n2\nfeel confused with statements like ‘the consequent decrease in Nr deposition’ (Lines\n218-221). I suggest authors to revise the phrasing throughout the manuscript, and\nintroduce the future scenarios with more details.\nResponse: Accepted. We revise our manuscript following this comment.\n(1) We explain more about the pathways leading to N reductions, particularly NH . As\n3\ncommented, the carbon neutrality leads to NO reduction, as NO is generally co-\nx x\nemitted with CO from fossil fuel combustion. NH reduction in this study, though\n2 3\nnot directly linked to carbon neutrality goal, is promoted by China’s “reduction of both pollutant and carbon policy. NH from agricultural activities has been\n3\nconsidered for regulation to mitigate PM pollution and nitrogen-related\n2.5\neutrophication (Liu et al. 2019). According to the ammonia emission model\ndeveloped in our group (Kang et al. 2016; Liu et al. 2019), such improvements of\nagricultural nitrogen management could decrease NH emission by up to 50% in\n3\nthe coming decades, while maintaining agricultural yields. In this study, we adopted\nthis 50% reduction for the year 2060, which is the largest potential that can be\nachieved in China. We provided the detailed explanation of the pathway and its\nreliability in achieving such reduction. Please see Lines 311–319 in the revised\nmanuscript and the revised Supplementary Text 1 for details.\n(2) While the carbon neutrality goal itself does not necessarily yield significant NH\n3\ncontrol, coordinated polices of air pollution and carbon mitigation is promising. We\nrevised the sentences associated with ‘the consequent decrease in Nr deposition’\nthroughout the manuscript and clarified the relationship between carbon neutrality\ngoals and Nr reduction strategies in China. Please see Lines 42–44 and 204–206 in\nthe revised manuscript.\n3. Lines 140-199: The analysis of the main results is unnecessarily too long. It is\nstraightforward to understand reduced N deposition -> reduced N uptake ->\nweakened NPP and Rh -> lower NEP. The model behaviors are also as expected as\nmost of other terrestrial biosphere models. There is even no figure or table\ncorresponding to the numbers within lines 140-169. Meanwhile, I don’t understand\nthe meaning of NUE analysis. The N limitation claim is quite weak with only\nmodeling results and actually not the key point of this study. As a result, I suggest\nauthors to start their demonstration with Fig.3, and clarify their most significant\nresults within 1-2 paragraphs. Uncertainties or implications can be in discussion.\nResponse: Accepted. As suggested, we largely shorten this part to focus on key findings\nand moved the NUE related analysis into the discussion part. The refined texts are given\nat Lines 137–168 in the revised manuscript.\nSpecific comments:\n1. Lines 51-52: The magnitude of future Nr reduction with carbon neutrality is still\nquite uncertain. I don’t think it is proper to cite the numbers here, especially where\na questionable 50% reduction in NH is assumed.\npreceding, the pathway leading to NH reduction is described in the revised manuscript.\n2. Line 54: N not Nr. I suggest to revise it as ‘N is one of the essential nutrients for\nplant growth’\nResponse: Accepted. We replace Nr with N here. Please see Line 56 in the revised\nmanuscript.\n3. Line 59: Remove ‘evolving’?\nResponse: Accepted. Please see Line 61 in the revised manuscript.\n4. Line 65: Authors should guide readers to the descriptions of these two models here.\nResponse: Accepted. We provided explicit guidance directing readers to Methods and\nSI where they can find detailed descriptions of these models. Please see Lines 63–65\nand 67–68 in the revised manuscript.\n5. Lines 70-71: It reads not clear why authors use these two scenarios? What are the\nimplications for these two scenarios?\nResponse: We clarify that these two emission scenarios represent a range of terrestrial\ncarbon sink changes under moderate and ambitious climate/clean-air policies.\nSpecifically, the EndCtrol scenario was designed for achieving near-term climate goals\nand meeting current air quality standards, while the CNeutr scenario for long-term\nclimate targets of carbon neutrality and reducing air pollution to WHO guideline levels.\nWe provided the motivation and policy implications in the main text. Please see Lines\n72–78 and 296–310 in the revised manuscript.\n6. Line 75: Strange sub-title. Nr deposition is not ‘driven’ by carbon neutrality.\nResponse: Accepted. We clarify that Nr deposition evolution is linked to nitrogen\nmanagement under the of both pollution and carbon policy. We modified\nthe subtitle as “Reductions in Nr deposition from the present to 2060” to avoid any\nmisunderstanding. Please see Line 82 in the revised manuscript.\n7. Line 77: WRF-Chem significantly underestimates NO -N wet deposition (Fig. S2e),\n3\nwhich should be pointed out in the main text.\nResponse: Accepted. We pointed out the underestimation of NO -N wet deposition in\n3\nthe main text. This bias may be related to the fact that the bulk deposition observations\nactually involve a part of unknow fraction of dry deposition(Liu et al. 2013). Please see\nLines 83–85 in the revised manuscript and Lines 66–69 in the revised SI.\n8. Lines 84-98: It would be good if authors can clarify what variables and which year\nare these percentage numbers relative to.\nResponse: Accepted. We specified the variables and year these percentage numbers\nrefer to. Please see Lines 92–97 in the revised manuscript.\n9. Lines 107-119: The whole paragraph reads quite confusing to me. I guess authors\nthink particle-phase N is harder to be deposited than gas-phase N? Are there any\nevidences on that? Why authors say ‘this is contrast to our recent study’? In addition,\nI don’t understand why authors need to discuss the NH impacts on the response of\ny\nthe main conclusion. I suggest to move it into discussion and rephrase the statement.\nResponse: Accepted. We removed the description of NH impacts on the response of\ny\nwas to emphasize the role of atmospheric chemical transformations in regulating\nnitrogen deposition changes. Please see the refined paragraph in Lines 115–121 in the\nrevised manuscript.\n10. Line 124: NPP is not equal to vegetation biomass accumulation due to varied\nlitterfall responses. Please just use ‘net primary production’.\nResponse: Accepted. We revised it to ‘net primary production’. Please see Lines 125–\n126 in the revised manuscript.\n11. Lines 126-130: I suggest to make the ‘other carbon fluxes’ more specific, e.g.\nwildfire and harvest. Besides, the definition of NEP is NPP-Rh, where harvest is\nalready excluded. There is no ‘NEP from agricultural harvest’. Meanwhile, I don’t\nthink harvest will be constant under the carbon neutrality scenario. Although NEP\nis not the exact terrestrial carbon sink, I think authors only need to acknowledge\nand clarify that as parts of discussions.\nResponse: Accepted. Crop harvest is already excluded. And abiotic carbon losses from\nwildfire and fuelwood combustion are not accounted for in this study. We described the\nmeaning of “other carbon flux” in the discussion part. Please see Lines 124–127 and\n209–213 in the revised manuscript.\n12. Table S5: Nice table. However, I suggest authors to make the methods more specific,\nespecially for those data-driven results. For example, they can be ‘remote sensing estimates’,\n’ground-level biomass investigation’, ’up-scaling of eddy covariance measurement with statics/machine learning method’ and so on.\nResponse: Accepted. We described the specific methods for those results in Table S5.\nPlease refer to the revised SI Table S5 for details.\n13. Lines 144-146: As CLM5 results are used to interpret NEP-Nr response, why\nauthors use numbers from CoLM in Fig. 2? Why not using the average?\nResponse: We clarify that the numbers in Fig.2 represent the results range of our two\nmodels following the projected carbon-neutral (CNeutr) scenario. We apologize for any\nconfusion caused and have rephrased these sentences to avoiding any misunderstanding.\nPlease see Lines 138–140 in the revised manuscript.\n14. Line 150: The decrease of N uptake might not directly link to reduction in N\ndeposition. A more complete pathway should be not only lower N deposition ->\nlower inorganic N -> lower N uptake, but also lower N deposition -> lower plant\nbiomass and also higher C:N ratio -> lower litterfall N-> lower soil organic N ->\nlower N mineralization -> lower inorganic N -> lower N uptake. It is related to the\nwhole biosphere interactions and thus it would be good if authors can clarity it a bit.\nResponse: Accepted. Following the reviewer’s comments, we acknowledge and\ndescribe that whole biosphere interactions regarding lower N deposition. In particular,\nthe lower N deposition -> lower plant biomass and also higher C:N ratio -> lower\nlitterfall N-> lower soil organic N -> lower N mineralization -> lower inorganic N ->\nlower N uptake. Please see Lines 152–154 in the revised manuscript.\n15. Line 152: ‘of’ not ‘pf’\nResponse: Accepted. We apologize for this typo. This sentence was deleted.\n16. Lines 181-183: It reads strange to claim NEP changes are dominated by NPP or\nNuptake rather than Rh. Vegetation and soil are strongly connected. Another\nexample of such expression is in lines 214-216. I suggest this manuscript should be\nrevised thoroughly by experts in ecological modeling to avoid such expressions.\nResponse: Accepted. We acknowledge the interconnections between vegetation and\nsoil processes. We carefully reviewed, and then removed or revised the related\nstatements. Please see Lines 179–182 in the revised manuscript. This manuscript was\nreviewed thoroughly by an ecological expert (Dr. Lingli Liu, CAS).\n17. Lines 202-204: why excluding crops makes NPP and Rh almost equal?\nResponse: We clarify that the NPP values in Fig.4 represents only natural vegetation.\nR includes the decomposition of residues from both natural vegetation and crops, with\nH\ncrop vegetation residue transferred to the same litter C pool as natural vegetation in our\nmodel. We have removed this sentence and re-clarified it in the caption of Fig. 4. Please\nsee Lines 291–293 in the revised manuscript.\n18. Line 220: again, why only using CoLM results here?\nResponse: Actually, we presented both CLM and CoLM results as the model range of\n16–20%. Please see Line 206–208 in the revised manuscript.\n19. Line 237: why this uncertainty range is not shown in Fig. 2 as well as other\nconclusion sentences? How to compare it with the uncertainty ranges obtained by\ndeviations in N deposition and different land surface models?\nResponse: This range was the estimation based on existing literature, in which N\ndeposition scenarios and environmental conditions were different from our modelling\nexperiments. Figure 2 illustrates the values produced from our case-specific simulations,\nnot from previous studies. We have created Table S7 to specifically compare our CO -\nPlease see Lines 224–230 in the revised manuscript and Table S7 in the revised SI.\n20. Lines 246-249: It is quite controversial to state CO fertilization effect is mediated\n2\nby N constraints, and I don’t understand the following statement of N uptakes and\nN efficient use.\nResponse: Accepted. Here, we would like to clarify that our intention was to investigate\nhow plants would respond to N limitation under increased CO levels. Specifically, our\n2\nmodel results and other observational studies indicate that plants tend to increase their\nnitrogen uptake (reflecting a more pronounced N limitation impact) rather than improve\ntheir nitrogen use efficiency to increase C to N ratio under increased CO levels (Finzi\n2\net al. 2007). We deleted the controversial statements and refined these sentences to\nmake the point clearer. Please refer to Lines 197–199 in the revised manuscript.\n21. Lines 251-253: P deposition is not important, it makes sense, but it doesn’t mean P\nlimitation is not important. With continuous N inputs, it could be possible that N\nreaches saturation and thus turns from N limitation to P limitation. Here, please just\nacknowledge that P limitation is not considered in this study, which may bring\nuncertainties.\nResponse: Accepted. We discussed that P limitation should be considered in future land\nmodel simulations. Please see Lines 237–238 in the revised manuscript.\n22. Lines 255-260. It should be REALLY careful with the call for supplementation of\nN for natural vegetation. The N inputs may not only provide nutrient to plants, but\nalso acidify soil or threaten biodiversity. Therefore, the N fertilization effect may\nnot as strong as we expected now. More importantly, with the continuous N inputs,\nmany ecosystems have already reached N saturation, which not only means\nterrestrial carbon sequestration will no longer benefit from N inputs, but also\nindicates higher N loss into atmosphere or water body, further leading air pollution\nor water eutrophication. Instead of the current implication, calling for improved\nmanagement practice with higher N use efficiency might be a better way to increase\nterrestrial C sinks with the Nr reduction.\nResponse: Accepted. We acknowledged that supply more N for vegetation growth\nmight bring soil acidification and biodiversity loss. As suggested, we deleted original\nstatement and emphasize improving management practice and N use efficiency in\nregulating terrestrial carbon sinks. Please see Lines 243–249 in the revised manuscript.\n23. Figure 2: Besides the numbers of C fluxes as mentioned above, why the N\ndeposition has no uncertainty range?\nResponse: Accepted. We perform N deposition simulations using two atmospheric\nchemistry models. Their results are closed. Considering the finer grid resolution of\nWRF-Chem, we adopted its model result as input of land ecosystem models. As\nsuggested, we also included the uncertainty range for N deposition from the two\natmospheric chemistry models. Please see the new Figure 2 and its caption in Lines\n269–270 in the revised manuscript.\n24. Figure 3 and Figs. S3-S4: I am curious if the positive NPP and NEP grids are\ninduced by the increased N deposition? Could authors change the color bars in Figs.\nS3-S4 to show the positive values?\nResponse: Accepted. As shown in the new Figs. S3-S4 in the revised SI, N deposition\ndecreases nationwide with no areas showing an increase. Only reduced N deposition\ncases are considered in our simulations. The cause of the sporadic positive NPP and\nNEP values may be the interactive effects of Nr decrease with other environment factors\non vegetation, such as inter-annual variation of precipitation.\n25. Lines 325: What is the reference of the 50% reduction in NH ? How realistic it is\n3\nwith the carbon neutrality scenario? What kind of actions could reach such an\nambitious goal, while at the same time to meet the human food demands? These\nquestions should be clarified in the manuscript.\nResponse: Accepted. The consideration of the 50% reduction in NH is based on the\n3\nprevious studies on NH mitigation potential in China (Liu et al. 2019; Zhang et al.\n3\n2020). They demonstrate the feasibility of effective strategies to mitigate NH\n3\nemissions, including rational fertilization practices and optimizing animal manure\nmanagement. The implementation of these measures could collectively yield a notable\nreduction, up to 50%, in China's NH emissions. Please see the details at Lines 311–\n3\n319 in the revised manuscript and the revised Supplementary Text 1.\nReferences:\nFinzi, A. C., et al. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency\nsupport higher rates of temperate forest productivity under elevated CO , PNAS,\n2\n104: 14014-19, doi:10.1073/pnas.0706518104.\nGu, F. X., et al. 2015. Nitrogen deposition and its effect on carbon storage in Chinese\nforests during 1981–2010, Atmos. Environ., 123: 171-79,\ndoi:10.1016/j.atmosenv.2015.10.081.\nGurmesa, G. A., et al. 2022. Retention of deposited ammonium and nitrate and its\nimpact on the global forest carbon sink, Nat. Commun., 13, doi:10.1038/s41467-\n022-28345-1.\nKang, Y. N., et al. 2016. High-resolution ammonia emissions inventories in China from\n1980 to 2012, Atmos. Chem. Phys., 16: 2043-58, doi:10.5194/acp-16-2043-\n2016.\nLiu, M. X., et al. 2019. Ammonia emission control in China would mitigate haze\npollution and nitrogen deposition, but worsen acid rain, PNAS, 116: 7760-65,\ndoi:10.1073/pnas.1814880116.\nLiu, X. J., et al. 2013. Enhanced nitrogen deposition over China, Nature, 494: 459-62,\ndoi:10.1038/nature11917.\nLu, C. Q., et al. 2012. Effect of nitrogen deposition on China's terrestrial carbon uptake\nin the context of multifactor environmental changes, Ecol. Appl., 22: 53-75,\ndoi:10.1890/10-1685.1.\nLu, X. H., et al. 2016. Relationship between nitrogen deposition and LUCC and its\nimpact on terrestrial ecosystem carbon budgets in China, SCI CHINA EARTH,\n59: 2285-94, doi:10.1007/s11430-015-5277-0.\nTian, H. Q., et al. 2011. China's terrestrial carbon balance: Contributions from multiple\nglobal change factors, Global Biogeochem. Cycles, 25,\ndoi:10.1029/2010gb003838.\nYu, Z., et al. 2022. Forest expansion dominates China's land carbon sink since 1980,\nNat. Commun., 13, doi:10.1038/s41467-022-32961-2.\nZhang, X. M., et al. 2020. Societal benefits of halving agricultural ammonia emissions\nin China far exceed the abatement costs, Nat. Commun., 11,\ndoi:10.1038/s41467-020-18196-z.\nZhu, J. X., et al. 2021. Effect of atmospheric nitrogen deposition and its components on\ncarbon flux in terrestrial ecosystems in China, Environ. Res., 202,\ndoi:10.1016/j.envres.2021.111787.\nReviewer #2 (Remarks to the Author):\nThank you, the authors, for carefully responding to my suggestions made in the\nprevious review. I have noticed that the author has increased the length of the article in\nthis version and provided more detailed results. I still have some additional concerns\nand minor suggestions listed below.\nResponse: We greatly thank the reviewer for helpful comments and carefully address\nthem as follows.\n1. Perhaps the author should consider adding some subtitles to better organize the\narticle, so that readers can better keep up with the results and discussion.\nResponse: Accepted. We organized the manuscript with modified and new subtitles.\nPlease see Lines 82, 122 and 183 in the revised manuscript.\n2. To me, it is not a surprised result. Decreased Nr emission -> Decreased N\ndeposition-> Decreased NPP and R -> Decreased NEP, this cascade relationship is\nH\nalready described and restrained by the model structure. Therefore, the originality\nand value of the article reduced. The scenarios for predicting the impact of reduced\nnitrogen deposition on carbon sequestration should be more diverse, and\ncorresponding solutions should be provided.\nResponse: This study aimed to uncover the impact of nitrogen management driven by\nChina’s sustainable targets on NEP, which is unknown yet important to forest\nmanagement in China in the context of carbon neutrality. We point out that:\n(1) In China, the coordinated reductions of pollution and carbon emissions are now\nconsidered to benefit mitigation of climate change and air pollution simultaneously.\nHowever, our study reveals that such policy could be less effective than previously\nexpected for achieving the carbon neutral target, because the nitrogen emission\ncontrols during the decarbonization and clean-air actions would be a drag on carbon\nreductions. This penalty (or negative feedback) is non-negligible and deserves\nattention. Please see our discussion at Lines 204–209 in the revised manuscript.\n(2) We selected two scenarios that are closely linked with China's emission pathways\nfor achieve near-term (i.e., EndCtrol) and long-term (i.e., CNeutr) climate/clean-\nair targets, respectively (Lines 72–78 and 296–310 in the revised manuscript).\nFollowing the reviewer’s suggestion, we conducted additional sets of simulations\nto reveal the dependence of carbon response to reduced nitrogen on various\nenvironmental factors (Lines 184–189 in the revised manuscript). In addition, we\nbriefly discussed solutions to address this unintended negative feedback (Lines\n243–249 in the revised manuscript).\n3. Currently, predictions of natural carbon sink in China in the context of carbon\nneutrality tend to focus more on the impacts of natural succession and tree age,\nwhile lacking comprehensive consideration of the influence of global change\nfactors on future carbon sink. The author may consider enriching these contents to\nenhance the quality of the article. The additional sets of simulations, such as eCO2\nand increasing temperature, are described very little in the article.\nResponse: Accepted. We conducted another set of simulations in which all climate and\nCO factors are kept constant throughout the period. The simulations were designed to\n2\ninvestigate the isolated effects of reduced nitrogen and its interaction with\nenvironmental changes. Then, we provided more analysis on the global change effects\nincluding changing CO and temperature along with reduced nitrogen fertilization. In\n2\nshort, our results show that the NEP decline solely due to the Nr deposition reduction\nis less than the combined effect of Nr deposition and climate conditions. N limitation\neffect on NEP is also more pronounced upon increasing temperature and eCO . Please\n2\nsee the added section “Intensified N limitation effects under elevated temperature and\nCO ” for details (Lines 183–202 in the revised manuscript).\n2\n4. Is it possible to distinguish between the effects of NH + and NO – reduction on\n4 3\ncarbon sink? NH + and NO – have different sources, emission reduction potentials\n4 3\nand costs. Meanwhile, they also have different physicochemical property. For\nexample, NH + is more energy efficient than NO – because NH + is a ubiquitous\n4 3 4\nintermediate compound during such physiological processes. NO – is a mobile\n3\nnutrient in soil solution, which is beneficial for plant uptake and more easily lost\nthrough leaching, while NH + is more likely to experience absorption and fixation\n4\nby soil particulates, resulting in soil NH + accumulation. Such effects may be\n4\ninfluenced by both the deposition flux of total N and by components of N deposition\nand its ratio of NH + and NO –.\n4 3\nResponse: Accepted. We acknowledge that NH + and NO – have different effects on\n4 3\nplant physiological processes (Gurmesa et al. 2022). To our knowledge, existing land\nbiogeochemical models cannot treat them separately. We discuss their effects and\npropose future research and model development needs. Please see Lines 233–237 in the\nrevised manuscript.\n5. Line 63-64: delete “the nationwide Nr deposition and carbon dioxide exchanges\nmeasurement dataset”\nResponse: Deleted. Please see Lines 65–66 in the revised manuscript.\n6. Line 152: revise “pf” to “of”\nResponse: Accepted. We apologize for this typo. This sentence was deleted.\n7. Line 198-199: “regions with widely distributed natural vegetation are generally\nmore N-limited, as found in southern China”. Need to be careful with this statement.\nSome forests in southern China are N-rich ecosystems.\nResponse: Accepted. We refined the paragraphs with this statement removed. Please\nrefer to Lines 157–182 in the revised manuscript.\nReferences:\nGurmesa, G. A., et al. 2022. Retention of deposited ammonium and nitrate and its\nimpact on the global forest carbon sink, Nat. Commun., 13, doi:10.1038/s41467-\n022-28345-1.\nReviewer #3 (Remarks to the Author):\nGeneral Comments:\nThe manuscript under review addresses a critical and timely topic regarding the\ninterplay between nitrogen abatement and carbon sequestration within the context of\nChina's decarbonization efforts. The study employs a multi-model approach to assess\nfuture nitrogen deposition and its effects on terrestrial carbon sinks, which is a robust\nand commendable methodological choice. The author has provided a detailed response\nand made revisions to the previous peer review comments. However, the manuscript\nrequires significant revisions to enhance clarity, comprehensiveness, and scientific\nrigor before it can be considered for publication in Nature Communications.\nResponse: We appreciate the reviewer’s comments and address them carefully as\nfollows.\nMajor Comments:\n1. Model Selection and Validation: The authors have employed two atmospheric\nchemistry models (WRF-Chem and CAM-Chem) and two terrestrial ecosystem\nmodels (CLM5 and CoLM). It is crucial to discuss the rationale behind selecting\nthese specific models and the extent to which their results are consistent or divergent.\nAdditionally, the manuscript would benefit from a more detailed comparison of the\nmodel outputs, including any discrepancies and the potential reasons behind them.\nResponse: Accepted. We provided more description on the model selection and\nvalidation.\n(1) This study employed multiple numerical models for atmospheric chemistry and\nland biogeochemistry to ensure the reliability of our major analysis. WRF-Chem\nand CAM-Chem are sophisticated atmospheric chemistry models and have been\nwidely used for regional- and global-scale modelling of tropospheric chemistry\n(Lamarque et al. 2012; Grell et al. 2005).(Lines 337–341 in the revised manuscript).\nCLM5 and CoLM are configured with advanced nitrogen-carbon coupling\nprocesses (Lawrence et al. 2019), which make them quite suitable to ascertain\ncarbon sink response to nitrogen input. Please see Lines 369–371 in the revised\nmanuscript for the rationale behind model selection and Supplementary Text 3 in\nthe revised Supplementary Information (SI) for detailed model descriptions.\n(2) We made comprehensive model validation using nationwide nitrogen deposition,\naerosol compositions, and carbon fluxes across China. These validations were put\nin Supplementary Information Text 2 and 4, which affirmed that our models are\ncapable to represent N deposition patterns and the reduced-nitrogen effects\nreasonably. While the detailed inter-model comparison is not the purpose of this\nstudy, we show their major differences regarding N deposition patterns, carbon\nfluxes and CO -N response, which could be attributable to diversities in model\n2\nstructure and parameterization. Please see Lines 231–233 in the revised manuscript\nand Lines 172–186, Fig. S2-7 in the revised SI.\n2. Scenario Justification: The scenarios chosen for the study, CNeutr and EndCtrol,\nneed to be better justified and aligned with China's current and projected\nenvironmental policies and climate commitments. The authors should elaborate on\nhow these scenarios reflect realistic future conditions and the rationale behind their\nselection.\nResponse: Accepted. The linkage of these two scenarios to China’s environmental\npolicies and climate targets is described in details. The EndCtrol scenario aims at\nattaining near-term climate goals and air quality standards, while the CNeutr scenario\naims at long-term climate targets of carbon neutrality and reducing air pollution to\nWHO guideline levels. These two emission scenarios are driven by targeted\nenvironmental policies and thus have the realistic implications in our study. Please see\nthe added information in Lines 72–78 and 296–310 in the revised manuscript.\n3. Impact of Nitrogen Deposition on Carbon Sinks: The manuscript suggests a\nsignificant reduction in terrestrial carbon sequestration due to decreased nitrogen\ndeposition. It is important to discuss the potential ecological implications of this\nfinding, including the long-term effects on biodiversity, soil fertility, and ecosystem\nservices.\nResponse: Accepted. In addition to carbon sinks, other ecological implications in\nresponse to reduced nitrogen, including long-term effects on ecosystem services, such\nas wood harvest, as well as potential complex effects on soil fertility and biodiversity,\nare mentioned in the discussion section. Please see Lines 249–253 in the revised\nmanuscript.\n4. Interactions with Other Environmental Factors: While the focus is on nitrogen and\ncarbon cycles, the authors should consider and discuss potential interactions with\nother environmental factors such as phosphorus availability, land use changes, and\nclimate change (e.g., temperature and precipitation patterns) that could also\ninfluence carbon sequestration.\nResponse: Accepted. To understand the interactions with other environmental changes,\ne.g., climate and CO concentration, we conducted another set of simulations in which\n2\nall climate and CO factors are kept constant throughout the simulation period. We\n2\nprovided more analysis and discussion on it. Please refer to the added section\n“Intensified N limitation effects under elevated temperature and CO ” in the Results for\n2\ndetails (Lines 183–202 in the revised manuscript). The existing models do not account\nfor phosphorus limitation yet. We briefly discussed that phosphorus limitation should\nbe considered in future land models. Please see Lines 237–238 in the revised\nmanuscript.\n5. Economic and Policy Analysis: The brief mention of the economic cost of nitrogen\nfertilizer application is a valuable point, but the manuscript would benefit from a\nmore in-depth analysis of the economic implications and policy considerations. This\ncould include a discussion on the trade-offs between nitrogen management and\ncarbon neutrality.\nResponse: This study highlights that the loss of natural carbon sinks due to reduced\nnitrogen by carbon neutrality and clean air goals, should be involved in nitrogen\nmanagement policy and Earth system modeling. The economic valuation of nitrogen\nemission control is associated with its multi-faceted impacts on the environments, such\nas air quality, nitrogen fertilization to ecosystems, and aquatic/soil eutrophication. Such\nassessment can be highly region-specific depending on the responses of these\nenvironmental issues, which is not the focus of this study. Please see our brief\ndiscussion at Line 253–255 in the revised manuscript.\n6. Figures and Data Presentation: There are critical issues with the representation of\nChina's geography in the figures, specifically Figure 1 and Figure 3, which do not\ndisplay the complete map of China as required by official cartographic standards.\nGiven the importance of accurate geographical representation in scientific\npublications, especially those that are likely to be accessed by a global audience, I\nrecommend that the authors make these revisions before the manuscript can proceed\nto the next stage of review.\nResponse: Accepted. We have corrected the map of China in each panel of Figures 1\nand 3, as well as all the figures in SI. Please refer to the revised manuscript and SI for\nthese updates.\n7. Technical Details and Uncertainty Analysis: The methods section is detailed, but\nadditional information on model calibration, validation, and uncertainty analysis is\nneeded. The authors should provide more details on the input data used for the\nsimulations and conduct a thorough uncertainty analysis.\nResponse: Accepted. We have included additional details regarding model calibration,\nvalidation, input data used for simulations, and an uncertainty analysis in\nSupplementary Information (SI). Please refer to the Supplementary Text 2, 3 and 4 in\nSI for comprehensive information on these technical and uncertainty aspects.\n8. Language and Grammar: The manuscript is generally well-written, but there are\nminor grammatical errors and awkward phrasings that should be corrected. It is\nrecommended that the authors engage a professional language editing service to\nrefine the manuscript.\nResponse: Accepted. An ecological expert, as a new co-author (Dr. Lingli Liu, CAS),\nhelped to polish the manuscript. We also reviewed the manuscript several times to avoid\nany grammatical errors.\nRecommendation:\nGiven the importance of the topic and the potential contributions of the study to the\nfield, I recommend a \"Major Revision\" followed by a resubmission for further review.\nThe authors should address all the points raised in this review to ensure that the\nmanuscript meets the high standards of Nature Communications.\nResponse: We hope our above response can well address the reviewer’s concerns.\nReferences:\nGrell, G. A., et al. 2005. Fully coupled \"online\" chemistry within the WRF model,\nAtmos. Environ., 39: 6957-75, doi:10.1016/j.atmosenv.2005.04.027.\nLamarque, J. F., et al. 2012. CAM-chem: description and evaluation of interactive\natmospheric chemistry in the Community Earth System Model, Geosci. Model\nDev., 5: 369-411, doi:10.5194/gmd-5-369-2012.\nLawrence, D. M., et al. 2019. The Community Land Model Version 5: Description of\nNew Features, Benchmarking, and Impact of Forcing Uncertainty, J. Adv. Model.\nEarth Syst., 11: 4245-87, doi:10.1029/2018ms001583.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nI appreciate authors’ efforts to improve this manuscript. In particular, I am grateful for the\ncomprehensive model evaluation. The current version is much easier to follow. I can in general\nrecommend this work to be published at Nature Communications, with addressing the following\ntext-phrasing points and clarifications.\n1. Line 39: The ‘CO2’ needs full name here. Also check the manuscript thoroughly to use\nabbreviation below.\n2. Line 44: I don’t see how reductions in agricultural NH3 meets the ambitious C neutrality goal. I\nsuggest authors adding one more sentence before line 42 to describe the current severe air\npollution in China, which makes it more sense to ‘synergize the mitigation of air pollution and CO2\nemissions’\n3. Line 48: remove ‘pool’\n4. Lines 61-80: I feel these two paragraphs are more like methods. Please make it short as one\nparagraph, while put details into the method section.\n5. Line 74-75: Scenarios’ descriptions are still confusing. I don’t understand what are ‘near-term\nclimate goals’ and ‘long-term climate target’ referring to? Do they mean the climate targets of SSP2-\n4.5 scenario and the 1.5-degree scenario, respectively? Meanwhile, which are the ‘WHO guidelines’\nand the ‘current air quality standards’? Which one is stricter?\n6. Line 78: It is better to clarify the meanings of ‘lower and upper bounds’, e.g. ‘Lower and upper\nbounds of anthropogenic Nr reduction to meet the carbon neutrality goal’\n7. Line 142: which simulation is adopted? It is not clear what the ‘its’ refers to.\n8. Lines 194-199: the argument with NUE v.s. N uptake is still confusing. It is an expected model\nresult that the NEP changes induced by Ndep reduction are higher under eCO2. However, only\nrelying on model is quite weak to support the argument that Nuptake is more important than NUE\nfor plants under eCO2. Such statement also makes the texts hard to follow. Therefore, I suggest to\nremove it.\n9. Line 214: ‘Multi-model’ usually refer to model members greater than 3. Please revise it and also\nthroughout the paper.\n10. Lines 249-253: Authors should point it out that these long-term effects may influence the\nestimates of this study.\n11. The 50% reduction in NH3 is an upper-bounder estimate, which is still a challenging goal for real\nagricultural management. As a result, authors should also mention it in the discussions, especially\nthe associated overestimates of Nr-induced NEP changes.\n12. Table S7. The table is nice but some numbers are confusing. Clarification is needed to make\nsure the N-induced NEP changes in this study is comparable to previous results:\na). Please unify the time periods of ‘environmental conditions’, ‘N deposition change’, ‘baseline\nNEP’ and ‘N-induced NEP change’.\nb). For Lu et al.75, where is the number of 140 Tg C yr-1 from? They concluded about 6.73 Pg C\nNdep-induced changes in ecosystem C storage over 1901-2005, and 4.69 Pg C over 1961-2005.\nThen the averaged number should be around 62(6.73/105) and 104(4.69/45) Tg C yr-1, respectively.\nReviewer #3 (Remarks to the Author):\nI have carefully reviewed the revised manuscript entitled \"Substantial Nitrogen Abatement\nAccompanying Decarbonization Suppresses Terrestrial Carbon Sinks in China,\" and I am pleased to\nsee that the authors have addressed the previous concerns with substantial evidence and clear\nexplanations. The revisions have significantly strengthened the manuscript, and I am now confident\nin recommending this paper for acceptance in Nature Communications. Below are my specific\ncomments and recommendations:\n1.Comprehensive Response to Reviewer Comments: The authors have thoroughly responded to\neach point raised by the reviewers. They have provided additional analyses, simulations, and data\nto support their arguments, which has clarified the findings and improved the overall quality of the\nresearch.\n2.Data and Methodological Rigor: The authors have included detailed descriptions of their\nmethodologies, data sources, and model simulations. This transparency is commendable and\nallows for the reproducibility of their results, which is a cornerstone of scientific integrity.\n3.Addressing Nitrogen Limitation: The additional simulations and analysis provided by the authors\nhave effectively addressed the potential impacts of nitrogen limitation on terrestrial carbon sinks.\nThe inclusion of elevated CO2 and temperature scenarios in their models has been particularly\ninsightful.\n4.Implications for Climate Policy: The discussion on the trade-offs between nitrogen management\nand carbon neutrality is now more robust, with clear implications for policy-making. The authors\nhave successfully linked their findings to broader environmental and climate policy discussions.\n5.Broader Context and Future Research: The authors have expanded the discussion to include the\nbroader context of their findings and have suggested directions for future research. This includes\nthe need to consider phosphorus limitation and the potential for differentiating the effects of NH4+\nand NO3– in ecosystem models.\nIn conclusion, the authors have made significant improvements to the manuscript, addressing all\nthe concerns raised in the initial review process. The paper now presents a compelling case for the\nunintended impacts of nitrogen abatement on terrestrial carbon sinks in China, offering valuable\ninsights for both scientific and policy communities working towards carbon neutrality.\nI recommend accepting this manuscript for publication in Nature Communications.\nReviewer #1 (Remarks to the Author):\n(Comments are marked in blue)\nReferee comments for NCOMMS-24-18211-T: “Substantial Nitrogen Abatement\nAccompanying Decarbonization Suppresses Terrestrial Carbon Sinks in China.”\nI appreciate authors’ efforts to improve this manuscript. In particular, I am grateful for\nthe comprehensive model evaluation. The current version is much easier to follow. I\ncan in general recommend this work to be published at Nature Communications, with\naddressing the following text-phrasing points and clarifications.\nResponse: We thank the Reviewer’s helpful comments and make minor revisions on\nthe manuscript based on the following points and clarifications.\nSpecific comments:\n1. Line 39: The ‘CO ’ needs full name here. Also check the manuscript thoroughly to\n2\nuse abbreviation below.\nResponse: Accepted. We have added the full name \"carbon dioxide\" at its first mention\nand carefully verified the correct use of its abbreviation throughout the manuscript.\nPlease see Lines 39 in the revised manuscript.\n2. Line 44: I don’t see how reductions in agricultural NH meets the ambitious C\n3\nneutrality goal. I suggest authors adding one more sentence before line 42 to\ndescribe the current severe air pollution in China, which makes it more sense to\n‘synergize the mitigation of air pollution and CO emissions’\nbeen dedicated to curbing severe air pollution.” (Zhang et al. 2019). Please see Line 42-\n43 in the revised manuscript.\n3. Line 48: remove ‘pool’\nResponse: Accepted. Please see Line 49 in the revised manuscript.\n4. Lines 61-80: I feel these two paragraphs are more like methods. Please make it short\nas one paragraph, while put details into the method section.\nResponse: Accepted. We have combined these two paragraphs into one and shortened\nthe description, providing only necessary information for readers to understand the\nsubsequent results and analysis. The details have been moved to the Methods section.\nPlease see Lines 61-76 and 250-268 in the revised manuscript.\n5. Line 74-75: Scenarios’ descriptions are still confusing. I don’t understand what are\n‘near-term climate goals’ and ‘long-term climate target’ referring to? Do they mean\nthe climate targets of SSP2-4.5 scenario and the 1.5-degree scenario, respectively?\nMeanwhile, which are the ‘WHO guidelines’ and the ‘current air quality standards’?\nWhich one is stricter?\nResponse: Accepted. We have provided detailed descriptions of these scenarios in the\nMethod section to clarify the meanings of them. The original description at Line 74-75\nis removed to shorten the paragraph following the reviewer’s suggestion. Please see\nLines 258-262 and 266-268 in the revised manuscript.\n6. Line 78: It is better to clarify the meanings of ‘lower and upper bounds’, e.g. ‘Lower\nand upper bounds of anthropogenic Nr reduction to meet the carbon neutrality goal’\nResponse: Accepted. We have clarified this as the reviewer suggested. The original\nsentence has been moved into Method. Please see Lines 253-254 in the revised\nmanuscript.\n7. Line 142: which simulation is adopted? It is not clear what the ‘its’ refers to.\nResponse: Accepted. We have replaced ‘its’ to CLM5. Please Line 138 in the revised\nmanuscript.\n8. Lines 194-199: the argument with NUE v.s. N uptake is still confusing. It is an\nexpected model result that the NEP changes induced by Ndep reduction are higher\nunder eCO2. However, only relying on model is quite weak to support the argument\nthat Nuptake is more important than NUE for plants under eCO2. Such statement\nalso makes the texts hard to follow. Therefore, I suggest to remove it.\nResponse: Accepted. We have removed the statements related to NUE v.s. N uptake.\nPlease see Line 190-193.\n9. Line 214: ‘Multi-model’ usually refer to model members greater than 3. Please\nrevise it and also throughout the paper.\nResponse: Accepted. We have corrected this word throughout the paper. Please see\nLines 169 and 200 in the revised manuscript as examples.\n10. Lines 249-253: Authors should point it out that these long-term effects may\ninfluence the estimates of this study.\nResponse: Accepted. We have mentioned this point in the Discussion section. Please\nsee Line 230-232 in the revised manuscript.\n11. The 50% reduction in NH is an upper-bounder estimate, which is still a\n3\nchallenging goal for real agricultural management. As a result, authors should also\nmention it in the discussions, especially the associated overestimates of Nr-induced\nNEP changes.\nResponse: Accepted. It is known that China has potential to achieve its 50%-reduction\ngoal for NH emission, though great effort is needed. We have mentioned its potential\n3\nfor overestimating the influence on NEP changes. Please see Line 277-278 in the\nrevised manuscript.\n12. Table S7. The table is nice but some numbers are confusing. Clarification is needed\nto make sure the N-induced NEP changes in this study is comparable to previous\nresults:\na) Please unify the time periods of ‘environmental conditions’, ‘N deposition change’,\n‘baseline NEP’ and ‘N-induced NEP change’.\nb). For Lu et al.75, where is the number of 140 Tg C yr-1 from? They concluded\nabout 6.73 Pg C Ndep-induced changes in ecosystem C storage over 1901-2005,\nand 4.69 Pg C over 1961-2005. Then the averaged number should be around\n62(6.73/105) and 104(4.69/45) Tg C yr-1, respectively.\nResponse: Following the reviewer’s comment, we have corrected the number from 140\nto 104 derived from Lu et al. The time periods of those variables cannot be unified due\nto a lack of available data from previous studies. Please see revised Table S7 in\nSupplementary Information.\nReference:\nZhang, Q. et al. Drivers of improved PM air quality in China from 2013 to 2017.\n2.5\nPNAS 116, 24463-24469, doi:10.1073/pnas.1907956116 (2019).\nLu, C. Q. et al. Effect of nitrogen deposition on China's terrestrial carbon uptake in the\ncontext of multifactor environmental changes. Ecol. Appl. 22, 53-75, doi:10.1890/10-\n1685.1 (2012)"}]

Decision letter