
==== Front
Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

39232004
52152
10.1038/s41467-024-52152-5
Article
Substantial nitrogen abatement accompanying decarbonization suppresses terrestrial carbon sinks in China
Shang Fang 12
Liu Mingxu 13
http://orcid.org/0000-0002-2455-2999
Song Yu songyu@pku.edu.cn

1
Lu Xingjie luxingj@mail.sysu.edu.cn

4
http://orcid.org/0000-0002-8376-131X
Zhang Qiang 5
http://orcid.org/0000-0002-0376-0879
Matsui Hitoshi 3
http://orcid.org/0000-0002-5696-3151
Liu Lingli 6
http://orcid.org/0000-0003-4481-5386
Ding Aijun 7
http://orcid.org/0000-0003-0922-5014
Huang Xin 7
http://orcid.org/0000-0002-8367-5833
Liu Xuejun 8
Cao Junji 2
http://orcid.org/0000-0002-7062-6012
Wang Zifa 2
http://orcid.org/0000-0002-3588-6644
Dai Yongjiu 4
Kang Ling 1
Cai Xuhui 1
http://orcid.org/0000-0003-2602-2235
Zhang Hongsheng 9
http://orcid.org/0000-0002-2752-7924
Zhu Tong tzhu@pku.edu.cn

1
1 grid.11135.37 0000 0001 2256 9319 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, 100871 Beijing, China
2 grid.9227.e 0000000119573309 Institute of Atmospheric Physics, Chinese Academy of Sciences, 100029 Beijing, China
3 https://ror.org/04chrp450 grid.27476.30 0000 0001 0943 978X Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
4 https://ror.org/0064kty71 grid.12981.33 0000 0001 2360 039X School of Atmospheric Sciences, Sun Yat-sen University, 510275 Guangzhou, China
5 https://ror.org/03cve4549 grid.12527.33 0000 0001 0662 3178 Ministry of Education Key Laboratory for Earth System Modeling, Department of Earth System Science, Tsinghua University, 100084 Beijing, China
6 grid.9227.e 0000000119573309 State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, 100093 Beijing, China
7 https://ror.org/01rxvg760 grid.41156.37 0000 0001 2314 964X School of Atmospheric Sciences, Nanjing University, 210023 Nanjing, China
8 https://ror.org/04v3ywz14 grid.22935.3f 0000 0004 0530 8290 College of Resources and Environmental Sciences, China Agricultural University, 100193 Beijing, China
9 https://ror.org/02v51f717 grid.11135.37 0000 0001 2256 9319 Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Science, School of Physics, Peking University, 100871 Beijing, China
4 9 2024
4 9 2024
2024
15 773826 3 2024
28 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
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 set back progress toward carbon neutrality. This unintended impact calls for great concern about the trade-offs between nitrogen management and carbon neutrality.

This study demonstrates that driven by China’s pollution control and carbon reduction policies, projected reduction of reactive nitrogen inputs from the atmosphere into terrestrial ecosystems can suppress terrestrial carbon sinks by 20%.

Subject terms

Carbon cycle
Atmospheric science
https://doi.org/10.13039/501100011002 National Science Foundation of China | National Natural Science Foundation of China-Yunnan Joint Fund (NSFC-Yunnan Joint Fund) 42075180 Song Yu issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

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 neutrality7,8. 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 NO3-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.Fig. 1 Present and future reactive nitrogen (Nr) deposition over China.

a Comparison of simulated Nr deposition over China in the Base run (present emissions scenario), EndCtrol run (projected end-of-pipe pollution control emissions scenario in 2060), and CNeutr run (projected carbon-neutral emissions scenario in 2060). The red bars represent NOy-N (NOy = NOx + its oxidation products) deposition, and the gray bars represent NHx-N (NHx = NH3 + NH4+) deposition. The black dashed lines and corresponding numbers indicate the reductions in total Nr deposition. b–d Simulated spatial distribution of annual total Nr deposition flux over China in the Base run, the EndCtrol run, and the CNeutr run. EC, eastern China; SC, southern China; SWC, southwestern China. The results presented here are derived from our simulations of WRF-Chem model.

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 = NOx + 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 technologies 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 emission scenarios.Fig. 2 Conceptual overview of terrestrial carbon budget in response to changes in anthropogenic reactive nitrogen (Nr) emissions in China following the projected carbon-neutral (CNeutr) scenario in 2060.

NPP here refers to net primary production of natural vegetation, and net CO2 sink refers to natural vegetation net ecosystem production (NEP). The blue process represents the nitrogen cycle, the green process represents the carbon cycle, and the yellow process represents the impact of plant N uptake on NPP. The red arrow marks a decrease in emission, N deposition, or ecosystem processes. The changes in N deposition presented here are derived from our simulations of two atmospheric chemical models, and the changes in carbon budgets are derived from our simulations of two process-based terrestrial ecosystem models. All calculations are performed annually for the entire China’s mainland. Credit: icons from Rawpixel (https://www.rawpixel.com/).

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).Fig. 3 Responses of terrestrial nitrogen and carbon budget to the CNeutr run in China.

a Simulated spatial pattern of changes in natural vegetation N uptake (NUPTAKE) in CNeutr run compared to Base run (present emissions scenario). b–d The same as (a) but for natural vegetation net primary production (NPP), total heterotrophic respiration (RH), and natural vegetation net ecosystem production (NEP). EC and SC in (a) indicate eastern China and southern China, respectively. CNeutr denotes the projected carbon-neutral emissions scenario for 2060.

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 dominates the annual carbon sinks. Following the carbon-neutrality scenario by 2060, the NEP in warm seasons is reduced by 37.6–41.3Tg 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.Fig. 4 Seasonal carbon budget and its responses to N reductions in China.

a Natural vegetation net primary production (NPP), heterotrophic respiration (RH), and net ecosystem production (NEP) in the Base run. b Changes in natural vegetation NPP, RH, and NEP (Tg C) in the CNeutr run compared to the Base run. The bounds of the I-shaped boxplots represent the range of the seasonal carbon budget or changes in the carbon budget due to nitrogen reduction simulated by the two terrestrial ecosystem models. The green boxes are for the warm season, including spring (March, April, and May) and summer (June, July, and August), while orange boxes are for the cold season, including autumn (September, October, and November) and winter (December, January, and February). Note that the NPP values represented here are for natural vegetation NPP, while RH includes the decomposition of residues from both natural vegetation and crops. Base run denotes the present emissions scenario; CNeutr denotes the projected carbon-neutral emissions scenario for 2060.

Intensified N limitation effects under elevated temperature and CO2

This study focuses on the role of Nr deposition reduction in driving land carbon sinks, while its interactions with other environmental factors might also be important, such as increasing atmospheric CO2 and temperature. We performed additional sets of simulations for the Base and CNeutr scenarios: (1) keep all other factors constant throughout the simulation period, (2) running with an elevated CO2 (eCO2) concentration, and (3) running with elevated temperature (see Methods and SI Fig. S15 for details).

Our simulations show that the NEP in China will be reduced by 42.6 Tg C yr–1 (14.7%) solely due to the Nr deposition reduction under the carbon neutrality scenario, which is 5.9 Tg C yr–1 less than the combined effect of Nr deposition and climate change (48.5 Tg C yr–1 or 15.8%). This suggests that the ongoing increase of temperature and CO2 in the future will intensify the response of carbon sinks to the reduction in Nr deposition (SI Fig. S15). We find that in the areas where Nr deposition reduction exceeds 10 Kg N ha yr–1, the negative impact on the regional NEP is strengthened by eCO2, with an additional NEP decrease of 0.6 Tg C yr–1. In addition, the decrease in NEP becomes larger under increasing temperature conditions (21.7% vs. 20.2%). The N deposition reduction combined with climate warming is projected to exert a negative synergistic effect on terrestrial carbon sequestration22.

Discussion

The policies aiming at reducing air pollutants and CO2 synergistically are projected to substantially decrease both fossil-fuel and agricultural nitrogen emissions in China. This study demonstrates that the consequent decrease in Nr deposition would diminish the land carbon sinks from natural ecosystems by 48.5–52.9 Tg C yr–1 (the range of the two models) following the carbon neutrality emission scenario, which in turn impedes the progress toward carbon neutrality. It should be noted that the change in terrestrial carbon sinks is calculated as that of NEP. Abiotic carbon losses from wildfire and harvest are not accounted for in this calculation due to the minor contribution of wildfire in China23, and little knowledge of fuelwood combustion emissions in harvest. Nevertheless, NEP itself is a key metric to assess the response of the ecosystem’s carbon sequestration capacity to changing Nr input16.

Our results of carbon responses to future Nr deposition decreases are put into the context of previous investigations. In-field experiments reveal nitrogen limitation to NPP in China, including in planted young forests over large areas in southern China24–26. This accords with a decline in NPP by nitrogen deposition reductions from our simulations. Specifically, the CLM5 simulations of plant growth responses at the plant functional type (PFT) level to N deposition changes, which reflect the effect of nitrogen availability on net primary production, are broadly comparable with the ranges observed in field studies reported by meta-analysis (SI Table S8)27. NPP to NUPTAKE ratios also fall within the ranges derived from the stoichiometric scaling study at the PFT level (SI Table S8), indicating our simulations can reproduce the vegetation production to N responses reasonably. We also estimate the response efficiency of NEP to Nr deposition reduction of 7.6–8.8 g C g−1 N in China, which are broadly consistent with other modeling results of historical N deposition impact on carbon sequestration in China (6.0–13 g C g−1 N) (SI Table S7). We note that uncertainties could arise from the disparities in N deposition patterns (e.g., historical annual increases vs. projected emission reduction scenarios for 2060), as well as from other environmental conditions (e.g., temperature and CO2 level) between our study and others.

Additionally, while the main conclusions are consistent between our models, the differences in the two land model structures and parameterization schemes lead to variations in carbon fluxes and CO2-N response (see Supplementary Text 3 and Table S9 for details). We note that NH4+ and NO3– could have different effects on plant and soil processes, potentially influencing biomass carbon sink28. Future developments that treat them separately in modeling are necessary and could improve our understanding of N-use efficiency for plant growth. Phosphorus limitation of plant growth is not considered and should be included in future ecosystem land models29. The long-term effects of nitrogen on ecosystem services, such as wood harvest, as well as potential complex effects on soil fertility and biodiversity, may also influence the estimates of carbon fluxes.

Deep mitigation of nitrogen oxide emissions is imperative for the continuous improvement of air quality, but the trade-offs between N deposition reduction and its resulting shrinking of the land carbon sinks should be noted and considered in climate policies. The human reliance on nature-based carbon sinks to achieve carbon neutrality may be undermined. To achieve the carbon neutrality goal as scheduled, first, we call for greater efforts in enhancing natural carbon sinks through improved management practices, such as selecting suitable plant species30, habitat protection and restoration31, and enhancing N use efficiency by improving soil N retention and cycling. Second, aside from reducing carbon emissions through energy transition, carbon capture, utilization, and storage (CCUS) methods (e.g., direct air capture and negative-emissions bioenergy technologies) could serve as a supplement to natural carbon sinks in efforts to mitigate climate change32. In the current century, as climate and clean air policies in many countries will likely induce large reductions in anthropogenic N emissions and deposition33, the resulting impacts on carbon sinks and ecosystem services, are globally important. Earth system modeling should take fully account these feedbacks of anthropogenic nitrogen management on natural ecosystems.

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/m3 4. 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 SSP 2 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/m3 4.

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 Emissions 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 levels 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).

Supplementary information

Supplementary Information

Peer Review File

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-52152-5.

Acknowledgements

This study was funded by the National Natural Science Foundation of China (NSFC) (42075180, and 92044302). H. Matsui was funded by the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Japan Society for the Promotion of Science (MEXT/JSPS) KAKENHI Grant Numbers JP20H00196, JP22H03722, JP23H00515, JP23H00523, JP23K18519, JP23K24976, and JP24H02225, by the MEXT Arctic Challenge for Sustainability phase II (ArCS-II; JPMXD1420318865) project, and by the Environment Research and Technology Development Fund 2–2301 (JPMEERF20232001) of the Environmental Restoration and Conservation Agency. Thank Dr. Yongjie Huang (IAP/CAS) for providing map database (https://github.com/huangynj/NCL-Chinamap.git).

Author contributions

Y.S. and T.Z. conceived and led the study. F.S. performed WRF-Chem and CLM5 simulations. M.L. and H.M. performed CAM-Chem simulations. X.Lu. and Y.D. conducted the CLM5 and CoLM simulations. F.S., M.L., Q.Z., A.D., X.H., X.Liu., L.K., X.C., H.Z., J.C., and Z.W. analyzed data and interpreted the results. F.S. and M.L. wrote the draft. F.S., M.L., and L.L. contributed to data analysis and interpreted the results during the revision process.

Peer review

Peer review information

Nature Communications thanks Guirui Yu, Yanyu Lu and the other, anonymous, reviewer for their contribution to the peer review of this work. A peer review file is available.

Data availability

The nitrogen deposition results data, ecosystem carbon flux results data and processed data generated in this study have been deposited in the Zenodo database under accession code 10.5281/zenodo.13305127. The present and future nitrogen emission data used in this study are available in the MEIC and DPEC database under accession code http://meicmodel.org.cn/?p = 1579&lang=en and http://meicmodel.org.cn/?page_id=1901&lang=en. The Nationwide Nitrogen Deposition Monitoring Network data are publicly available at 10.1038/s41597-019-0061-2. The map database for China was created by Dr. Yongjie Huang (IAP/CAS) based on the National Catalogue Service for Geographic Information (https://github.com/huangynj/NCL-Chinamap.git and https://www.webmap.cn/commres.do?method=result100W).

Code availability

The WRF-Chem and CAM-Chem source codes are freely available for download at https://ruc.noaa.gov/wrf/wrf-chem/ and https://www.cesm.ucar.edu/models/cam, respectively. The CLM5 and CoLM source codes are freely available at https://github.com/ESCOMP/CTSM/tree/release-clm5.0 and https://github.com/CoLM-SYSU, respectively. Figures were created mainly based on NCAR Command Language (NCL, https://www.ncl.ucar.edu/) and the codes are available from the corresponding authors upon request.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

These authors contributed equally: Fang Shang, Mingxu Liu.
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