The study uses CTSM v5.2 to simulate soil hydrology under different SOC sequestration scenarios across global cropland and grassland landscapes, focusing on how increased SOC influences plant-available water and water stress.

Simulations were conducted at a 0.5° resolution using prescribed land cover and climate forcing, comparing a control (BAU) with three SOC enhancement scenarios (High, Medium, and 4p1000) applied to the top 30 cm of soil. Soil properties were derived from the WISE30sec dataset, and changes in water holding capacity, saturation, and water stress were analyzed over a 20-year period to assess hydrological impacts.

The modeling results will be primarily dependent on how the CTSM model uses SOC to modify water flow. From the methods, the effect of SOC on water retention  was simulated by modifying the SOM fraction, which directly influences saturated water content (theta_s) and subsequently the water retention via the Campbell model: theta/theta_s = (psi/psi_b)^(-1/b). Note that psi_b and b are only affected by soil texture. The model also affects organic soils. 

Thus some issues need to be addressed:

• the methods need to clarify how SOC affects water retention and water flow in the soil, as these deternine the results. A plot of how the model simulates SOC increase with AWC would be beneficial
• The modeling of SOC sequestration potential on organic soils which is simply a blanket increase is applied to the top 30 cm of the soil column  using scenarios based on mineral soils (e.g., Zomer et al. 2017), assuming a fixed bulk density and SOC conversion factor. The model uses a maximum organic matter density of 130 kg/m³, corresponding to 100% organic matter . The blanket sequestration rate is mostly unlikely as it depends on climatic conditions and regime, dryer climates could not sequester more carbon as compared to colder and wetter climates.
• SOC increase is not linear with time.
• the Methods section does not specify how SOC sequestration is handled on organic soils. We know that SOM in peatlands are unlikely to sequester more carbon, And thus the model should not simulate sequestration on organic soils. This is especially true for soils high latitudes.
• It is unclear how CTSM simulates water infiltration and how SOC affects water flow.
• See more recent studies

Bagnall, et al, 2022. Carbon‐sensitive pedotransfer functions for plant available water. Soil Science Society of America Journal, 86(3), pp.612-629.

Panagea, Ioanna S., et al. "Soil water retention as affected by management induced changes of soil organic carbon: analysis of long-term experiments in Europe." Land 10.12 (2021): 1362.

Results

Soil carbon sequestration in the High scenario (+0.55% SOC by mass) leads to a modest but widespread increase in water holding capacity and volumetric water content in the upper 30 cm of soil, especially over croplands. This increased upper-layer moisture increases vegetation transpiration, particularly in clay-rich regions, and leads to small reductions in annual water stress and surface runoff, though with regional variation and minimal impact below 32 cm soil depth. The Medium sequestration scenario (+2.7 gC kg⁻¹ or 0.27%) causes small, consistent improvements in topsoil water retention and slight reductions in water stress and runoff. While effects are small, the model suggests that even modest carbon gains can improve plant water availability and hydrological resilience in certain environments.

The 4 per 1000 scenario causes regionally variable changes in soil moisture, depending on baseline SOC. While it improves water retention in upper layers, the downward redistribution of moisture is reduced, and in some cases, overall soil water content declines. This scenario shows the importance of initial SOC levels and local conditions

The authors should also calculate water storage change (delta_s in water balance) to determine the effect of SOC increase.

Discussion

The study adds value by exploring the upper-bound potential of SOC sequestration on soil hydrology, its conclusions are constrained by model limitations, assumptions, and a lack of integration with management, crop response, and local-scale feedbacks.

The claim that a 0.55% SOC increase leads to a ~2% increase in water holding capacity and volumetric water content depends on how CTSM modelled the effect of SOC. It is not a reality. It also has not been validated (the water retention model). And thus the authors should first clarify how SOC affects AWC though the calculation of the water retention of the Campbell’s model. Discuss with regards to recent literature (Bagnall and Pangea). And clarify that the model has not been validated with real data as opposed to meta analysis and other statistical approaches.

As above, I believe the effect of SOC on soil water could be in terms of water balance or soil moisture storage (delta_s in water balance). The AWC may not be influenced significantly, but delta_s could be significant.

The authors could also discuss in terms of other simulation studies

Araya, Samuel N., et al. "Long-term impact of cover crop and reduced disturbance tillage on soil pore size distribution and soil water storage." Soil 8.1 (2022): 177-198.

Limitations should be discussed. There was no dynamic feedback is modeled between SOC and soil structure, aggregation, macroporosity, or infiltration capacity. This limits the model’s ability to capture nonlinear or process-based SOC-water interactions, particularly under management changes or climate stress. As a result, the model may underestimate both the positive potential (e.g. in improving infiltration, reducing runoff) and negative trade-offs (e.g. reduced deep drainage or waterlogging under saturation) of real-world SOC accumulation.

SOC gains in real systems are tightly linked to land management practices, e,g, no-till, cover cropping, etc.which influence soil compaction, infiltration rates, rooting depth, and microbial activity. These management pathways are not modeled in CTSM. As such, the study simulates the effect of added carbon, not the processes or trade-offs involved in achieving that carbon gain. And the model (especially the efffect of SOC on hydraulic paraneters) has not been validated.

General Comments

This manuscript presents a comprehensive global assessment of the eco-hydrological impacts of soil organic carbon (SOC) sequestration using the Community Terrestrial Systems Model (CTSM). The study is timely and highly relevant given the increasing attention to nature-based climate solutions, particularly soil carbon sequestration, where potetials and limitations should be addressed. The work explores the co-benefits of SOC increase on soil water retention, evapotranspiration, runoff, and water stress.

The paper demonstrates that SOC additions can modestly enhance water retention and plant-available water in many regions, resulting in increased evapotranspiration and reduced water stress. These findings add nuance to discussions on soil-based mitigation by highlighting co-benefits for drought resilience and water use efficiency.

However, some key issues require clarification or expansion. First, the hydrological effects are generally modest and must be contextualized in terms of model sensitivity and uncertainty. In this sense, I agree with Referee #1 that a representation of the effect of increasing SOC to the soil hydraulic properties is recommended. Is the effect linear? Does adding SOC impact differently soils with distinct textures? Is the effect similar in the wet and dry ends of the soil hydraulic properties? Answering these questions can enlighten the model results and give a more comprehensive outlook in the observed effects. Second, the discussion would benefit from more critical evaluation of model assumptions and limitations, particularly regarding plant response (phenology), soil physics, and regional heterogeneity. The absence of a model validation has to be acknowledged, pointing this exercise as a model sensitivity evaluation.

Finally, the authors should provide more consistent terminology (e.g., soil water content vs. soil moisture vs. liquid water content; soil carbon sequestration vs. sequestration, etc.) and improve clarity around the time averaging of results, especially in the figure captions.

Specific Comments

SOC “increases the size of the bucket,” but...

• Increasing water holding capacity only improves resilience when water is present. In drought scenarios, especially in very dry regions, no extra water may be available to be held. This should be emphasized in the discussion to temper expectations about SOC’s effectiveness as a drought mitigation tool unless paired with other strategies like irrigation. The authors also present conflicting information on the effect of SOC increase in sandy/arid regions (see detailed comment below).

Waterlogging/oxygen stress:

• In wet regions, increasing water retention can increase the risk of oxygen stress. This potential trade-off is worth mentioning, even if not captured by the current model setup.

Vegetation dynamics:

• The model uses prescribed vegetation phenology and does not allow dynamic feedback from changes in water availability. This limitation should be clearly stated in the methods. It likely leads to over/underestimation of the full eco-hydrological co-benefits. The separation of the results between croplands and grasslands are also not clear. Is it an average over everything? It could be beneficial to see if effects are different for each of those.

Water stress:

• The simulations showed that transpiration of the plants is increased with enhanced SOC, but water stress is only slightly affected. As mentioned in the manuscript, this is related to the very conservative definition of water stress and cannot be put in the context of irrigation. No one would wait until the soil reaches PWP to start irrigation. Since the manuscript is written in terms of cropland use, it would be beneficial to see if other drought criteria are more realistic with its needs, such as 50% of FC, or a some critical soil matric potential that is not as low as PWP.
• It is also not clear in which conditions irrigation is applied. Is it in all regions?

Recent literature in effects of SOC to be considered in the discussion

Skadell, L.E., Dettmann, U., Guggenberger, G. and Don, A. (2025), Effects of Agricultural Management on Water Retention via Changes in Organic Carbon in Topsoil and Subsoil. J. Plant Nutr. Soil Sci.. https://doi.org/10.1002/jpln.70004

Technical corrections 

L. 9 – how much SOC?

L. 82, 95, 143 and elsewhere – references missing

L. 85 – what is meant by soil energy? Matric potential? 

L. 116 – porosity is not always the same as soil water content at saturation. 

L. 124 – equation 4 must be reviewed. The matric potential at saturation is zero. 

L. 151 – constant only in year 2000? 

L. 172 – per cent or per mile?

Table 1 – I find it confusing the mixture of % and gC/kg soil in the same table. I would also remove the + sign from the medium and high scenarios, it gives the impression that this values was added to something, when in fact it is just constant everywhere. I would also recommend more clarity to the scenario description, I read it many times and could not be sure of what they mean: constant value over the whole globe or constant addition to current values over the whole globe? If the first, then how are the model representing organic soils, that have more  SOC than that?

L. 194 – how do you define FC? Do you really need to evaluate this water holding capacity since your results derive from a Richards-based model? 

Figure 1 – differing scales

L. 214 – wilting point soil moisture = soil water content at wilting point. Also, use either soil moisture or soil water content for consistent terminology. Is soil liquid water the same thing as soil moisture?

L. 214 – I would connect differences in soil porosity to the saturated fraction, but not necessarily to the FC. Maybe change it to “improved soil pore distribution” from the addition of SOC. 

L. 217-218 – very confusing. What is the actual water content? An average over all evaluated years?  

L. 235 – isn’t it a result of water being hold in the topsoil, therefore less input is going to deeper soils? 

L. 242-243 – this go against your abstract: “Our results show that soil organic carbon redistributes water within the soil profile, retaining moisture in the rooting zone and limiting percolation into deeper layers, which is particularly pronounced in arid regions with sandy soils.”

L. 259 – Which soil texture effects? 

L. 265 – soil water storage capacity is not the same as saturation. 

L. 274 – SOC sequestration 

L. 275 – could it be that increase in SOC makes the water to be “trapped” in the topsoil and susceptible to evaporation, and therefore not available to the plant roots?

L. 283-285 – the two sentences say the same thing?

L. 375-381 – is that in the correct place?

L. 387 – ? ¨

The research presented in the manuscript is extremely relevant to evaluate and support ongoing policy discussion and NBS implementations of soil carbon sequestration as climate change mitigation. Specifically, to verify and validate eco-hydrological co-benefits of soil carbon sequestration at global scale as climate change mitigation following some policy-relevant scenarios. The assessment follows an implementation through earth system models, which provides a global evaluation but implies some simplification and generalization both in terms of vegetation systems under scrutiny (unmanaged C3 crop), spatial resolution (0.5 degrees) and through empirical pedo-transfer functions. However, this generalization also grants an harmonized assessment to compare results across different regions worldwide.   

-    The understanding of results would definitively benefit by a more specific and detailed description of some modelling characteristics and especially in relation to vegetation. I would emphasize for instance a more detailed linkage to the original model and technical description of the current version used in this paper. I could not find manual for CSTM v5.2 and is not clear if the CSTM is embedding the CLM v5   (line 82).

-    Would be useful to know certain characteristics of the crop PFT like rooting depth, or crop development characteristics, in order to evaluate results. The extent of rooting depth would be relevant to evaluate eco-hydrological effects at depths below and above 32 cm. 

-    Moreover, authors should clarify what constitutes vegetation evaporation … I can only think of evaporation from vegetation rainfall interception, which is not linked to soil water content. This could explain why there is such a limited effect of SOC increase on vegetation evaporation. Figure 9 also may be moved to methodology, and there to better explain these concepts.

-    The concept behind and comparison and result across cumulative water stress and days with water stress (Page 15) require some reconciliation in the methods and explanations in the results. While n of days with water stressed refers to days with extreme water stress (theta below wilting point), cumulative water stress refers to the cumulated value when theta is lower than theta at wilting point (as specified in the methods). The latter seem quite counterintuitive and lead to difficult understanding in results (Figure 6a) where cumulative water stress can go up to 2-3 meters. Please clarify. 

-    The results and in general research question are quite relevant and could bring forwards a richer elaboration in the discussion

-    There are several typos, double punctuations, and often use of long sentences which could be shortened or made more direct for more clear understanding. Please consider a revision. 

More specific recommendations:

Line 10 -- Under a scenario with a uniform SOC increase of gC kg-1 soil, globally averaged total global soil liquid water content increases by 4 mm in the first 30 cm. -> How much gC?

Line 10 consider rephrasing as following …. Global average soil liquid water content increases …

Line 60-65 : better to provide also some quantitative results in terms of the effect of SOC, from literature

Line 140: Theta at Wp is calculated from Theta at sat through empirical functions. Such empirical functions may explain how sometimes increase of SOC may induced an estimated decrease of water holding capacity (higher theta wp??), downplaying the role of enhanced SOC especially in some more prone areas to drought (fig 2a and 2c). Wonder how water content at field capacity may decrease with increasing SOC … fig A4 (page 25)

Line 180. Would be relevant to know how it was aggregated: WISE30sec and aggregated to the horizontal resolution of 0.5° by 0.5°

Line 190-195 Some of these variables (like water holding capacity) varies through the scenarios and control, but still climate invariant

Line 263-264 … I thought that carbon sequestration in upper layer limits percolation to deeper layers, also from previous sentence, while here it states that it promotes water percolation into deeper layers. Please clarify any misunderstanding

Figure 1. a and b seem to report SOC and increase in SOC, while in the caption indicate weighted sand percentage and clay percentage. Please verify.

279: It is odd to think that carbon sequestration boost vegetation transpiration in tropical areas, because of enhanced water availability … where water should not be a limiting factor. Would be possible that enhanced OM can favour the biogeochemical cycle rather than hydrological cycle.