|I appreciate the revisions by vonBloh and co-authors. Remaining concerns are largely technical in nature, but it seems appropriate in the introduction and methods to contextualize the approach for representing N limitation in a global model compared to a growing list of ESMs that tackle the same challenge using a variety of approaches.|
I'm still uncertain how N limitation actually occurs in the model, this refers to Section 2.4 of the text and Fig 2. The flow chart for Fig 2 seems to describe how GPP is calculated, but the text for the section is all about NPP, please clarify. The description of calculating a water stress, but N unlimited Vmax and photosynthesis rate and subsequently calculating the N limited Vmax seems very similar to CLM4cn (Thornton et al. 2007, referenced in the text). In CLM, however, this resulted in a decoupling of leaf-level water and C exchange. Is the same approach being taken here, or does leaf gas exchange based on the N limited Vmax calculated after N limitation is accounted for?
Minor and Editorial concerns
Page 2 line 7-9 As with the major concern, above, this sentence still seems awkward. This strikes me as an opportunity to clarify the approach taken with LPJmL5. For example, if the current 'implementation is based on previous model implementations', what are they? How is the LPJmL5 approach similar and how is it different. There are several ways to simulate N limitation in global scale models (referenced on the previous page), but the sentence here only casually describes what's being done without giving much information to the reader (e.g., 'N limitation occurs, not by XXX…', or something that briefly and accurately describe how plant and soil N availability effects the terrestrial C cycle in the model). A few sentences providing a broad overview the N approach taken here will help clarify the contributions made here.
The list of references in Table 2 is pretty overwhelming, and not terribly helpful. If the purpose of these references is for data reproducibility, it's not at all clear what data were used for particular parameter values or PFTs. Are these the citation needed to satisfy the TRY data use requirements, or were they collected in addition to the (Kattge et al., 2011) reference?
Table S1 should have a heading above, not below the table.
Section 2.6 & Fig. 3. I should have noticed this earlier, but in I'm not clear how the decomposition of SOM directly liberates NO3 during (Knit on Fig. 3, 'Fraction of mineralized N nitrified to NO−3, which = 0.2 in Table S1). It seems this value should be 0 by definition, and not shown on the figure. For example, in the Schimel and Bennett (2004) model of inorganic N transformations, or Davidson's 1991 Leaky pipe suggest that NO3 formation only occurs through nitrification. Is the direct flux from SOM to NO3 ecologically justified, or is commonly it represented in other models? Looking at Parton et al. 2001, it does seem like 20% of mineralized N is sent to the NO3 pool in DAYCENT, but is this just a mathematical artifact of the sequential solver used in the models that allows NO3 losses to occur? It seems like an odd 'feature' of the model to perpetuate?
Section 2.6.2 Gerber didn't work on CLM, to my knowledge.
Section 2.6.4 What is SWAT? The acronym should be defined in the text.
Section 2.6.5 If NO3 leaching is calculated sequentially (after denitrification) why does the abstract and main text focus on the hydrologic losses of inorganic N and not the gaseous losses simulated by the model? (See also table 4).
The colors, symbols and acronyms included in Fig 10 are so complicated as to preclude any meaningful insight from the display item. The yellow star we're supposed to compare to red circle cross is nearly impossible to find. More, the caption doesn't help explain the complexity in a way to aid readers in interpretation of the figure, thus as presented I'd recommend removing it from the main text. Alternatively, the display item can stand, but more information beyond the Müller citation is needed for readers to understand the information communicated in the context of the work presented.