The manuscript shows the impact of nutrient limitation on estimating the remaining carbon budget in Earth system models. The C cycle and its interaction with nutrients play a crucial role in determining the amount of C that terrestrial vegetation can take up. However, the representation of nutrient cycles in modeled terrestrial ecosystems is often overlooked, despite its influence on the C cycle feedback in future climate projections. The study estimates the C budget and the remaining C budget using an Earth system model (UVic ESCM) that incorporates N and P cycles to limit vegetation productivity and biomass. The results demonstrate that nutrient limitation reduces the remaining C budget in comparison to models without nutrient limitation, with the N-P limitation resulting in the lowest remaining carbon budget. The findings highlight the significance of considering nutrient limitations when estimating and interpreting remaining C budgets, highlighting the importance of accurately representing these limitations in Earth system model simulations. 

This research topic is very important due to its implications for understanding the C cycle and predicting future climate change. Accurately estimating the remaining C budget is key for developing effective climate mitigation strategies. By incorporating nutrient cycles into ESMs, this study shows that neglecting to account for nutrient limitations can lead to uncertain projections of C uptake and release by terrestrial vegetation. Therefore, understanding the role of nutrient limitation in the C cycle and considering it in modeling frameworks is crucial for improving the accuracy of climate change predictions and guiding appropriate mitigation efforts.  

The manuscript would benefit from an overall improvement in writing for clarity. References must be checked, as I have found at least one missing reference; the use of a reference manager is advised. Moreover, often, the source of a method or approach is mentioned in the method's section without further explanation. It would be helpful for the reader to provide a short explanation of the method/approach or some context. Specifically, the description of the nutrient cycles, which are the focal point of the manuscript, could be highly improved so the reader does not have to go for a second publication to learn the basics about what the model is doing. Also important, the manuscript lacks a more robust discussion on the uncertainties of the assumptions of the model. What is missing, and what could be improved in the future? So the reader can properly assess and interpret the results. 

Based on the above, I would not recommend the publication of this manuscript without further major revisions. 

See specific comments below. 

Line 9: Add SSPs between parentheses to introduce the acronym. 

Line 30: Missing reference at the end of the sentence. "Since year 1850, the cumulative CO2 land sink has been estimated to be 210±45 PgC, which represents 31% of all anthropogenic carbon emissions". 

Lines 58: Consider reading Fleischer et al., 2019, which might be a good reference there. 

Very long first paragraph in the introduction. Consider splitting it. 

Lines 69-70: "The remaining carbon budget framework used in this study follows"… Sounds like a sentence that should be in the methods and could be rephrased. 

Line 94: "budgets budgets" appears repeatedly along the manuscript. Please clarify. 

Line 121: What does the process of biological nitrogen fixation and mineralization of organic nitrogen depend on in the model? 

Lines 133-134: "The soil litter decomposed is transferred to the soil organic P pool. The dynamics of P organic matter are adapted from Wang et al. (2007)." It would be helpful for the reader to provide a short description of the dynamics. The same is true for line 130, where Goll et al. (2017) are cited. What do these dynamics depend on?  

How are N and P together influencing SOM decomposition in the model? 

Description of the terrestrial model: The methods do not mention leaf nutrient resorption. That's an important aspect of nutrient demand. Although the full description of the model can be found elsewhere, a more complete summary of the terrestrial N and P model would be helpful for the reader. 

Line 146: The mention of tuning climate sensitivity using an equilibrium climate sensitivity parameter designed by Zickfeld et al. (2009) could benefit from further explanation. Explaining how the tuning process alters the flow of long-wave radiation back to space and how it impacts the climate sensitivity in the model would improve understanding. 

Line 160: The mention of "Tokarska et al. (2018) approach" is not explained. It would be helpful to provide a brief description of the approach. 

Line 161: The acronym in the sentence "SSP 5-8.5 a high emission scenario" is introduced without any prior explanation. It would be helpful to provide a brief description of the SSPs scenarios and what the 5-8.5 scenario represents. 

Line 165: The term "Zero Emission Commitment Model Intercomparison Project (ZECMIP) protocol" is introduced without prior explanation. It would be beneficial to provide a brief description of what the ZECMIP protocol entails. 

Line 166: The statement "the 1pctCO2 experiment is followed until diagnosed cumulative emissions of CO2 reaches 1000 PgC thereafter emissions are set to zero further CO2 emissions" could be clarified. It is not clear what is meant by "diagnosed cumulative emissions". Providing more details on the methodology would improve understanding. 

Lines 175-178: The SSP scenarios are again mentioned without explanation, a brief general description of what they mean would be helpful for the readers. 

Line 179: Throughout the manuscript, I am confused with how you use the term "land use change." Please, clearly define it. It seems the term is being used to describe natural changes in vegetation cover. But land use change is related to anthropogenic transformations of the natural landscape.  

Line 183: punctuation missing 

Line 191: Term GISS introduced without further clarification. Additionally, was this mentioned in the methods? 

Line 204: I believe the correct wording should be "less carbon is taken up from the land." Please, check the rest of the manuscript for the same issue (e.g., line 220, replace uptake for take up). 

Line 213: It would be helpful to remind the reader what the TCR means and how to interpret it. For instance: "the amount of global warming expected to occur when atmospheric CO2 concentrations double from their pre-industrial levels, while all other factors remain constant", or what you think is more appropriate. 

Line 218: Sentence could be improved; it is not complete, should be “vegetation uptake of carbon” I believe. 

Line 222: Replace: "terrestrial vegetation is constrained by nutrient…" 

Line 235: Remind the reader of what is and how to interpret ZEC values. For instance: "values indicate the estimated temperature increase resulting from the ZEC after 50 years of no further emissions. Higher ZEC value suggest… (continue)." 

Line 262: Replace "Figures 2-8." 

Lines 273-274: "As the model reduces vegetation due to nutrient limitation and trees are replaced by grassed, the land surface albedo is increased." This needs further clarification. In what parts of the world is this happening, and what ecosystems are being replaced by grass? Additionally, replace grassed by grasslands. 

Lines 272-274: Here, you are talking about land use change, that trees are being replaced by grasses due to nutrient limitation. Is this land use change? I would say anthropogenic activities, not nutrient limitation, drive land use change. Nutrient limitations might cause natural changes in vegetation cover. 

Lines 282-283: "When the diagnosed C:N or C:P leaf ratios are higher than the set maximum leaf ratio, the vegetation biomass dies so that the leaf ratios decrease back to the maximum ratio threshold." This sentence needs further explanation. If there's a maximum set, how can it go higher? I don't fully understand what is the assumption here with the biomass dying and the ratios being reset. 

Lines 283-284: First time the acronym PFT is used, it is not defined, and no further clarification is given—the relevant differences among PFTs were not summarized in the methods or there. Also, be consistent with the use of all acronyms throughout the text; either use them or not. 

Lines 284: "nutrient limiting stressors such as nitrogen and phosphorus should be applied carefully as a high limitation of phosphorus can easily underestimate the land sink capacity of tropical vegetation". This needs additional clarification. What in the model's representation of the P cycle could be causing P to be too limiting, causing this underestimation? Considering you do not account for the C costs of P uptake, I would expect the other way around. What are the limitations of the P cycle in the model? Couldn't other assumptions in the model, not related to the P cycle, be the problem in tropical regions? 

Lines 305-306: "These estimations are larger than the rough estimate of 27 PgC reduction of carbon budgets due to unrepresented carbon feedbacks (Rojeli et al., 2018), suggesting that this value may have been underestimated in the IPCC 1.5◦C report." What did Rojeli et al. do differently? Rojeli is not on the reference list; is that Rogelj? Please check the references and reference list for such mistakes. The use of a reference manager is recommended. 

Line 320: Revise the sentence; it is unclear. "Although, different using ECS might assess some variability shown in other models, there is a clear uncertainty in how variable the impacts is in other model structures." 

Discussion: the manuscript lacks an important aspect, which is a more robust discussion of the model's limitations. What are the most important limitations and assumptions of the model (Land surface model or otherwise)? What needs to be improved? You briefly speak of the lack of data, some uncertainties, but what uncertainties, and what data specifically? (Starting on line 315).  

Line 315: Not clear, nutrient concentration in what? Soil? Leaf? Biomass? 

Line 325: "If the objective is to improve the carbon cycle accuracy, the inclusion of P is advisable for its limiting role in tropical regions. From a carbon budget estimations view, we observed similar results for CN and CNP." I find this to be a very overstated conclusion. There are several studies pointing to the very uncertain representation, particularly of the P cycle in land-surface/vegetation models. Several C costs of acquiring N and P are not accounted for in these models (e.g., C allocation to mycorrhizas, enzymes, and other root exudates). These could highly impact projections of biomass growth in tropical forests (e.g., Fleischer et al., 2019; Braghiere et al., 2022). Additionally, the vegetation models poorly represent the links between N and P cycles (e.g., N cost for phosphatase enzyme production for P acquisition from leaves and soil, or a P cost for N fixation), which could also affect global projections. Moreover, there are several uncertainties about P pool dynamics in models and their fixed ratios. This is to say biomass growth in some vegetation models could be overestimated, particularly in tropical regions, especially in future scenarios. Based on your CNP vegetation/terrestrial model, could such limitations, if overcome, change your results and affect the remaining C budget, for instance? In other words, is it true the P cycle does not make a difference, or the problem is that it is loosely represented due to a lack of data? This should be discussed. 

Literature 

Braghiere, R.K., Fisher, J.B., Allen, K., Brzostek, E., Shi, M., Yang, X., Ricciuto, D.M., Fisher, R.A., Zhu, Q. and Phillips, R.P., 2022. Modeling global carbon costs of plant nitrogen and phosphorus acquisition. Journal of Advances in modeling earth systems, 14(8), p.e2022MS003204. 

Fleischer, K., Rammig, A., De Kauwe, M.G., Walker, A.P., Domingues, T.F., Fuchslueger, L., Garcia, S., Goll, D.S., Grandis, A., Jiang, M. and Haverd, V., 2019. Amazon forest response to CO2 fertilization dependent on plant phosphorus acquisition. Nature Geoscience, 12(9), pp.736-741.