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https://doi.org/10.5194/egusphere-2026-296
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2026-296
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
03 Feb 2026
| 03 Feb 2026
Status: this preprint is open for discussion and under review for Biogeosciences (BG).
Absence of nitrogen threshold effect on soil respiration leads to an underestimation of global soil carbon sequestration in model
Abstract. Soil respiration (Rs), a key component of the global carbon cycle, plays a pivotal role in regulating atmospheric CO2 concentrations and climate. Yet, the global responses of Rs and its components, including heterotrophic respiration (Rh) and autotrophic respiration (Ra) to varying levels of nitrogen (N) deposition remain poorly understood. By synthesizing global data from 931 paired observations of 226 experimental sites, this study indicated that Rs responses generally decline with increasing N inputs: low to moderate additions exert negligible or slightly positive effects, while higher additions consistently suppress Rs, mainly through reductions in Rh. Based on an N addition threshold (Nth) that indicates whether Rs increases or decreases, we incorporated this response pattern into the Community Land Model version 5 (CLM5). We demonstrate that ignoring N's variable impacts may cause Rs to be overestimated or soil carbon sequestration to be underestimated by up to 20 %. Our results provide a robust global assessment of Rs-N relationships and demonstrate the importance of representing N-induced reductions in soil respiration in Earth system models, to improve predictions of terrestrial carbon-climate feedbacks.
How to cite. Wang, D., Yang, R., Cai, L., Gentine, P., Terrer, C., Niu, S., Migliavacca, M., Yuan, W., Tateno, R., Xiao, J., Peñuelas, J., Tang, C., Fu, Y. H., and Shi, W.: Absence of nitrogen threshold effect on soil respiration leads to an underestimation of global soil carbon sequestration in model, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-296, 2026.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Biogeosciences.
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Latest update: 10 Jun 2026
Daju Wang
Chongqing Jinfo Mountain Karst Ecosystem National Observation and Research Station, School of Geographical Sciences, Southwest University, Chongqing, 400700, China
School of Atmospheric Sciences, Guangdong Province Data Center of Terrestrial and Marine Ecosystems Carbon Cycle, Sun Yat-sen University, Zhuhai, 519000, China
Ruowen Yang
Department of Atmospheric Sciences, Yunnan University, Kunming, 650000, China
Department of Atmospheric Sciences, Yunnan University, Kunming, 650000, China
Pierre Gentine
Department of Earth and Environmental Engineering, Columbia University, New York, 10027, USA
Center for Learning the Earth with Artificial Intelligence and Physics (LEAP), Columbia University, 10027, New York, USA
César Terrer
Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, 02139, USA
Shuli Niu
Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, China
College of Resources and Environment, University of Chinese Academy of Sciences, Beijing, 101408, China
Mirco Migliavacca
European Commission, Joint Research Centre (JRC), Ispra, 21027, Italy
Wenping Yuan
School of Atmospheric Sciences, Guangdong Province Data Center of Terrestrial and Marine Ecosystems Carbon Cycle, Sun Yat-sen University, Zhuhai, 519000, China
Sino-French Institute for Earth System Science, College of Urban and Environmental Sciences, Peking University, Beijing, 100871, China
Ryunosuke Tateno
Field Science Education and Research Center, Kyoto University, Kyoto, 113-8656, Japan
Junlan Xiao
Chongqing Jinfo Mountain Karst Ecosystem National Observation and Research Station, School of Geographical Sciences, Southwest University, Chongqing, 400700, China
Josep Peñuelas
CREAF, Cerdanyola del Vallès, Catalonia, 08290, Spain
CSIC, Global Ecology Unit, CREAF-CSIC-UAB, Cerdanyola del Vallès, 08290, Spain
Caixian Tang
La Trobe Institute for Sustainable Agriculture and Food, Department of Animal, Plant & Soil Sciences, Bundoora, VIC, 3083, Australia
Yongshuo H. Fu
College of Water Sciences, Beijing Normal University, Beijing, 100875, China
Weiyu Shi
CORRESPONDING AUTHOR
Chongqing Jinfo Mountain Karst Ecosystem National Observation and Research Station, School of Geographical Sciences, Southwest University, Chongqing, 400700, China
Short summary
Soil respiration is a major CO2 source, yet its response to vary nitrogen addition levels remains unclear. Using global data from 226 sites, we found moderate N addition has negligible or slightly positive effects, whereas high N addition consistently suppresses total and microbial respiration. These findings were incorporated into the CLM5 model, improving predictions of soil respiration and carbon storage. Our work elucidates how N deposition alters soil carbon processes and climate feedbacks.
Soil respiration is a major CO2 source, yet its response to vary nitrogen addition levels...
General Comments:
This paper conducts a meta-analysis of soil respiration responses to experimental nitrogen deposition and uses the results of that analysis to determine a relationship between N deposition and soil respiration response. That relationship is then used to calibrate a new parameter that is implemented in the soil module of the Community Land Model v5. The parameter is calibrated using the meta-analysis data and then used in climate change projections. The authors find that soil respiration increases in response to low N deposition but decreases in response to high deposition and this is largely associated with heterotrophic, rather than autotrophic, respiration. When they add this relationship to CLM5, they find considerable changes in soil C storage. I applaud the large amount of work that was carried out for this study and think it is a worthwhile exercise to add our empirical understanding to process-based models. I also believe the focus on N deposition dose effects is worthwhile, given the amount of N deposition is often found as a controlling factor in carbon cycle responses. However, the lack of information in the methods and the methodological choices in this work make it difficult for me to assess the robustness of this study. In particular, the choice to use soil mineral N as the threshold determinant in the CLM simulations needs much more explanation and justification, given the empirical threshold was determined based on N deposition rates. Further, the discussion needs to be adjusted to better align with the results of the study. Significant reorganization of the results and discussion sections is needed. I really appreciate the goals and extent of this work but believe it needs more attention for me to be able to judge the robustness of the study.
Specific Comments:
Lines 39-40: I struggle a bit with this comparison in the first line here. Fossil fuels are an unbalanced flux whereas respiration is largely balanced by photosynthesis. Is there another way to highlight the importance of soil respiration for the carbon cycle?
Line 43: I would appreciate a more thoughtful introduction of N deposition here. N deposition is no longer increasing over the entire world and so some other justification of the importance of studying this phenomenon would be helpful here (e.g., N is a major limiting nutrient).
First paragraph of introduction: There has been a lot of work on N deposition effects on Rs (as the authors cite), so I think highlighting instead the findings of dose dependence and how we have yet to incorporate this finding into process-based models would be a better way to highlight the gap this paper is filling, rather than vague phrases about the response being “unclear”.
Second paragraph of the introduction: I am not quite following the relation of the N saturation concept to the N threshold concept. The N saturation concept I am familiar with (Aber work) is quite specific in the responses that are expected, but this threshold idea seems to be the dose at which responses change from positive to negative. The authors cite some work here that doesn’t really reference N saturation but rather just varying rates of N deposition (e.g., Treseder, 2008). I think more explanation is needed here about the relationship between N saturation and this N threshold idea. Alternatively, I am not sure N saturation needs to be invoked here, and the authors could instead just focus on the threshold idea, which seems to be a sort of dose-response curve.
Line 88: What does “simulated N deposition” mean in this context? I would assume modeled N deposition but only field studies were included.
Figure 1: Could points be made transparent? Also, could the colors of grasslands, wetlands, and shrublands be altered? Is it hard to tell these apart from boreal forests given the similar colors and size of points.
Section 2.2: Did the authors perform sensitivity tests and tests of publication bias on their meta-analysis data? If not, could these be performed and reported on? These metrics will help determine how reliable the threshold they calculated is. Additionally, I am curious if the authors performed any analysis on other drivers beyond amount of N deposition in driving the response of soil respiration to ensure there were no confounding variables? For instance, perhaps there are generally higher N fertilizations in grasslands and grasslands also have negative Rs responses. Further, there might be unique thresholds for different ecosystems, climate regimes, etc. Did the authors consider this in their analysis? Based on the results, it seems the authors did consider these covariates and do find different responses across ecosystems, which makes me wonder why they did not implement the thresholds on a per-ecosystem basis in CLM5 by calculating different thresholds for each plant functional type.
Section 2.3: Did the threshold identification only use Rs and not the Ra and Rh data that were collected?
Line 176: Given the papers focus on improving ESMs, it would be good to point out that CLM5 is the land component of CESM2.
Line 179: It would be helpful to clarify whether the authors are talking about their simulations or CLM5 simulations more broadly, as CLM can use a variety of atmospheric inputs, but it would be good to know if the authors forced their simulations with GSWP3 data. Using first person language here could help clarify (e.g., “We conducted our simulations using GSWP3…”)
Line 182: Why this length of time for the N deposition experiments? I imagine few in the dataset were that long.
Line 184: What did the authors use as their metric of equilibrium for spin up?
Lines 185-186: I am not sure about this – what specific processes does CLM5 represent related to how Ra changes with N availability?
Paragraph at line 185: It would be good to clarify here for the reader that the default soil model in CLM5 is a Century-like model, and this has implicit microbial activity. Given this, could the authors describe what flux they are then reducing? The flux arising from the respiration fraction multiplied by the pool size? I would appreciate a more explicit description of how the authors changed the CLM5 soil module.
Line 198: Why did the authors choose soil mineral N when their data-based threshold is based on N addition to mimic N deposition? Soil mineral N is much more variable and affected by many other processes. This needs much more justification.
Equation 13: Is the Greek character (sorry not sure which one that is!) meant to be essentially a translation between N deposition and soil mineral N concentration?
Paragraph at line 222: I am having trouble squaring up the line at the beginning of the section about the N deposition experiments being run from 1951-2014 with these projections. Was the transient period (1951-2014) used for calibration and now that the model is calibrated, the projections described in this section are evaluating how including the threshold would influence Rs responses to climate change? Please also stylize the projections correctly (e.g., SSP 2-4.5) and describe the differences in N deposition projected for these two scenarios.
Section 3.2: What are “high” and “low” N in this paragraph? How are those defined?
Line 291: I don’t understand this conflation of N deposition and soil mineral N. I would not expect these to act the same in the ecosystem and so using the exact same threshold as found empirically doesn’t make sense to me. It would make more sense if the Greek letter in equation 13 was being used as some sort of conversion between the two. I believe this aspect of the work needs to be strongly reconsidered.
Section 3.3: Please clarify for the reader that all of the statements in this section refer to results using SSP projections.
Lines 292-298: This text feels like discussion, and I suggest it is moved there.
Line 318: Again, this feels like discussion text.
Section 4.1 up to line 371: This part of the paper feels out of place and out of scope for the paper. I think it is worthwhile to try to understand the drivers for this threshold response but that feels like an entirely different paper than this one. Further, there are a lot of the meta-analysis results here that should be in a results section and not a discussion section. I suggest this section and Figure 6 are removed.
Paragraphs at lines 372 and 383: These paragraphs are more aligned with the text and could stay with additional discussion related to the results presented in the paper. The discussion text that us currently in the results section could move down here.
Paragraph at line 395: This is some of the justification that is needed much earlier in the paper. Further, I think there needs to be more convincing evidence that we should expect the effect of N deposition rate and soil mineral N concentration to have comparable effects on soil respiration. I am not sure we do and that is a major assumption of the method in this paper.
Paragraph at line 416: This justification is also needed higher for not having ecosystem-specific thresholds.
Discussion, overall: I would appreciate more discussion specifically about the results presented in the results section and why we are seeing them and how they influence our understanding of carbon cycle responses to global change. In particular, more discussion about changes in soil respiration over space and time and with the different projections would be valuable here.
Lines 437-438: I don’t agree with this and suggest this language be softened. The results with this new parameter were not compared to real world data to show that soil C predictions are more accurate with this new parameter and so this statement is not fully borne out here.