the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Feb 2026
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Biogeosciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-296', Anonymous Referee #1, 28 May 2026
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RC2: 'Comment on egusphere-2026-296', Anonymous Referee #2, 17 Sep 2026
In this article the authors have conducted a meta-analysis on the response of soil respiration to N addition. The purpose was to investigate if an emerging behavior could be captured so as to implement it in a land surface model. This is a very interesting idea. The objective of the article is very clear however the methodology description lacks of precision which hinders comprehension. Specific comments:
- Line 68: You cite Chen an Chen (2023), who also have found a non linear effect of increasing N deposition on Rs. How do your results compare to their results?
- Section 2.2: Could you expand more on why you are using RR++ instead of RR? What are the groups, and how many? Also line 143 you mention the RR++ series, what do you mean by "series"?
- Fig.2: This is linked with the above commentary. I don't understand what the dots represent. The color description is unclear, "dark blue dots", "semi transparent dots" "shade of dots". I don't understand what it is referring to. What is RRs? Is it the same RR as in eq.1? Then why is there a "s"?
- Fig.3: Same comment as for Fig.2. I have the feeling that the subscript 's' 'a' 'h' is to indicate what is the variable X of eq.1. Maybe it would be clearer if, in eq.1, RR has also a subscript RR_x, and then clearly state somewhere what the subscripts are referring to.
- Eq.4: seems unused?
- Line 183: Could you precise the spinup-procedure: Is it carried out using pre-industrial N deposition and atmospheric CO2? What is the criterion to end the spinup? Do you repeat 2000 times the year 1950 for the meteorological forcing?
- Eq.13: About the reduction factor n_dep_scalar: Is this a multiplicative factor? To which variable are you applying it? I don't understand the statement line 204 "alpha = 0 indicates that the N_th is ineffective" since, in my understanding, it multiplies the decomposition rate by 0.
- Line 208: Calibration of alpha: I don't understand the calculation of R^2, what is the empirical curve? Fig.S2: What is the red line?
- Line 224: What do these scenarios mean for Ndep? Could you show the evolution of Ndep?
- Line 258: Please sepcify how you define "high N addition" and "low N addition".
- Line 261-262: I don't understand what the two numbers 8.5 % and 14.2 % are referring to. There is only one bar for the forest.
- Line 316: Could you comment on the shift in the soil carbon at low-latitudes and explain more this sentence: "This change also reflects the pattern of soil CO2 emissions across a N addition gradient".
- Line 322: What did you observe in your simulation about the change in N2O emissions between control and when using the N threshold ?
- Line 337-338: This should be in the Methods section.
- Fig4.: The Figure is intense; it would make it easier to read if you write on the top, near the letters what is the variable represented on the graph. a - Rs, b - Rh, and c - Ra. To avoid re-reading the caption.
- Section 3.3: You show results for two scenarios but do not confront the two. Could you comment on the similarities or discrepancies observed between the two scenarii.- Fig.S7: A part of the caption is an analysis of the results, which should be in the main text.
Technical corrections:
- The N threshold "Nth" is a variable and should be in italics to avoid confusion with the element N. Also, the spelling varies in the manuscript: NTh or Nth. It should be corrected for consistency.
- Fig.2: In the caption "lnwi", is it the logarithm of wi ? If it should be in Roman upright font.
- Line 261-262: Please be consistent and always write the interval the same way, beginning with the lower bound.
- Line 352: A space is missing.
Citation: https://doi.org/10.5194/egusphere-2026-296-RC2
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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.