Comment on gmd-2021-263

This manuscript is presenting a version of JULES that include a vertically discretize accumulation of organic and mineral soil. This version is also capable of simulating soil carbon aging. Capability of the soil organic accumulation scheme is demonstrated for various simulation set up that aim at showing sensitivity of the model to soil properties. Simulation results are compared with the standard JULES version and to peatland profiles at 216 sites. The paper also discusses the capability of the model to reproduce processes specific of peatlands formation, and stability. The novelty of this scheme is to be able to consider the resilience of peatland soil to increase atmospheric temperature and the instability of peatland soil to drainage. Thus, this manuscript represents a significant contribution to modelling science and is within the scope of Geoscientific Model Development. Model description (section 2) is well structured and the model is sorely described. Further than developing a new model for peatlands, this scheme has been designed for global scale simulations using Earth System Model (ESM). Therefore, I am convinced that the new features proposed in JULES-Peat will provide a complementary view on peatland functioning in the literature.

various simulation set up that aim at showing sensitivity of the model to soil properties.
Simulation results are compared with the standard JULES version and to peatland profiles at 216 sites. The paper also discusses the capability of the model to reproduce processes specific of peatlands formation, and stability. The novelty of this scheme is to be able to consider the resilience of peatland soil to increase atmospheric temperature and the instability of peatland soil to drainage. Thus, this manuscript represents a significant contribution to modelling science and is within the scope of Geoscientific Model Development. Model description (section 2) is well structured and the model is sorely described. Further than developing a new model for peatlands, this scheme has been designed for global scale simulations using Earth System Model (ESM). Therefore, I am convinced that the new features proposed in JULES-Peat will provide a complementary view on peatland functioning in the literature.
I have some comments that I think will help to ease the reading of the paper and I hope render the study even more appealing. Those modifications do not require to run some more simulations but aim at emphasis results that demonstrate the capability of the model to simulate resilience behaviors of peatland soil and the faster degradation of the peat soil due to a significant decrease in soil humidity.
I hope the authors find my suggestions useful and I am looking forward to read a revised version of the manuscript.
General comments: -The present land surface model JULES-peat in this manuscript is evaluated for tundra, boreal and temperate regions however there are no evaluation in tropical regions. Therefore, the title should be modified to mention that it is only for peat accumulation in the North hemisphere or also named northern peatlands. This should also be specified in the introduction and in the other parts of the manuscript.
-Section 3 present a series of simulations and the dataset that is employed for model evaluation. Almost all simulations defined in Table 2 are described in this section except for JULES-Peat-W-drain and JULES-Peat-W10-drain. Few sentences about these two simulations are needed to fully discuss Table 2 and the whole set of simulations.
-Section 4 describes and discusses simulation results. While results are very interesting the structure/organization of this section is a little confusing: -I feel like those sections 4.1 and 4.2, are more like a results part and section 4.3 to 4.5 a discussion part. Therefore, I would recommend to split this section 4 results and discuss in two sections. The first one with the results described in sections 4.1 and 4.2 on the accumulation scheme and soil aging. The second section that will discuss results described in section 4.3, 4.4 and 4.5.
-At first, it has been surprising to me to start the result section of a model on peat (organic rich soil) by a paragraph on the representation of mineral soil. In addition, the following section is titles "JULES-Peat evaluation" which could mislead the reader in thinking that results in section 4.1 are not from JULES-Peat but only from JULES therefore current section 4.2 needs a different title.
-Although, the authors should add a paragraph explaining how the different parts of this new scheme serves in representing vertical profiles of organic and mineral soils and their response to environment changes. For instance, how is the model performed a transition from mineral soil to organic soil or the other way around and which part(s) of the model is involved in this process. This paragraph can be placed either as introduction or as a subsection of section 4: results and will aim at presenting the different components (refereeing to section 2 and equations) of the scheme. Emphasis needs to be made on the new functionality of the model. For example, line 407 to 412 can be moved to this new part.
-Results could be organized into more sub-sections such as "Evaluation of intrinsic conditions" that will include results on carbon profile, age and bulk density and "Evaluation of environmental conditions" that will include results on JULES-Peat-W and initial conditions.
-In section 4.1, at the place of discussing the validation of each vertical profile, I would rather argue that in the top soil JULES-PEAT version provide more realistic carbon density. However, in soil layers deeper than 0.5m JULES-PEAT largely overestimate carbon content compare to JULES base case. Surprisingly while considering all the soil profile JULES_PEAT generate a carbon profile that fit better the observation. May be a RMSE or R2 will help quantifying the difference between observations and simulations? -I like stand-alone figures it helps me capture the overall content and structure of a paper therefore additional information need to be added to all figure captions. Figure captions should define all the element of the figure in order for the reader to be able to understand the aim and content of the figure without relying on the main discussion in the manuscript (see details comments for each figure and table).
-It is sometimes difficult in the manuscript to distinguish between JULES the model and the simulation therefore the name of the simulation can me changed. It can be called after the reference number of the JULES version employed or JULES-BaseCase or JULES-ReferenceRun.
-In a few places, the authors are confusing ESM and land surface models. For instance, in line 7, 59 and 520 whereas in line 82 they explained that "JULES includes a verticallyresolved soil carbon scheme (Burke et al., 2017a), although this hasn't yet been used in the Earth System Model configuration." Since this study was conducted on stand-alone version (offline conditions), ESMs line 7, 59 and 520 should be replaced by land surface model.
-Citations "in prep.": I do think it is not appropriate to have among references papers in preparation e.i. Gao et al., in prep., Smith et al., in prep. In a manuscript in preparation many things can change from the reference authors, title, journal that later on, it gets difficult to figure which paper you were refereeing to. In addition, these references are not part of references listed at the end of the paper. Please remove these citations that appear in table 1, line 535 and line 290.  For instance, crosses and diamond markers are not defined as well as in panel entitled "All", the black solid line and the grey area. Also, it will be easier for the reader whether in the figure caption there will be a brief description of the main difference between JULES-Peat and JULES-Peat10 such as at line 347-348 and of JULES-Peat-W and JULES-Peat-W10 such as in line362-363. I would prefer to have the legend outside of any panels so we can see the full vertical profile for Svalbard_Ny site even though it is easy to guess. It would be interesting to display in each panel the RMSD value of each site. These values will provide a quantitative evaluation of the fit for each simulation. It will be easier to refer to specific panels in each figure if each panel is labeled with a letter. Figure 4, 5 and 6: The grey area is not defined neither in the caption nor in the legend. Here the same, I would rather have a full view of the vertical profile for Tundra sites and have the legend outside of panels. In the panel title boreal and temperate are displayed in capital letter and no tundra, I guess all region name should be displayed in the same manner. If I understand correctly figure 4, for example each red line corresponds to the simulation using JULES at a different site within the considered region then site names should be given in the figure caption. Also, I do not understand why there are no simulation of JULES in the Tundra region? In the figure caption "against typical profile for peat cores" typical can be removed resulting in "against profile for peat cores".  Table 4: There is no need of the second column "Colour in Fig.7" since there is a legend in Figure 7 that already define that.
Detailed comments for the manuscript: Line17-21: "peatlands. In particular the best performing configurations had root mean squared error (RMSE) in carbon density for peat sites of 7.7-16.7 kgC m−3 depending on climate zone, when compared against typical peat profiles based on 216 sites from a global dataset of peat cores. This error is considerably smaller than the soil carbon itself (around 30-60 kgC m−3) and reduced by 35-80% compared with standard JULES. The RMSE at mineral soil sites is also smaller in JULES-Peat than JULES itself (reduced by 30-50%)." This part is not clear when it is read for the first time Line 23-24: "This provides a new approach for improving the simulation of organic and peatland soils, and associated carbon-cycle feedbacks in ESMs, which other land surface models could follow." I believe that it is wiser to let the reader decide whether or not he want to use the same or a different implementation. I would remove the last part of the sentence "which other land surface models could follow" Line 90-94: "These changes were made based on well-known principles: Firstly, that microbial activity drops to zero in completely dry conditions (Yan et al., 2018); secondly that respiration in anaerobic conditions is known to be no higher than 20% of the maximum rate in aerobic conditions (Schuur et al., 2015); and finally that when microbes lack nitrogen, they tend to decompose plant litter faster in order to 'mine' for nitrogen (Craine et al., 2007) in contrast to the original scheme introduced by Wiltshire et al. (2021) in which the decomposition of litter is inhibited when nitrogen is in short supply. " This sentence is very long and can be split in multiple sentences by removing the semicolumn and word "that" after firstly, secondly and finally.
Line 95: "as follow:" can be removed. Line 202: "if there is a 'corner' in the function" I believe that it is name a 'discontinuity' in a function rather than a 'corner'. Line 260: "firstly" can be removed. ." Why did you add these sites for the present study? Are they more recent measurements? Are they complementary of your initial sites? If yes in which way? Do they add some representation of boreal or tundra sites?