Review of “A global reference database in FAOSTAT of cropland nutrient budgets and nutrient use efficiency: nitrogen, phosphorus and potassium, 1961-2020” by Ludemann et al.

Essd-2023-206

I enjoyed the paper and I am confident it will be highly influential in both the research and management communities. I have some suggestions for improving the readability of the paper, enhancing the description of the methodology, and refining interpretation of the results . I recommend acceptance pending minor revision, as detailed below.

Major comments:

• I think the readability of the paper can be improved by maintaining consistentterminology regarding nutrient balance, nutrient surplus, and nutrient deficit. It would be helpful to provide a clear definition at the beginning of the paper. For instance, nutrient surplus in line 38 denotes nutrient balance, which can be either positive or negative. In lines 58-59, nutrient surplus is directly defined as balance, and deficit is defined as negative surplus. However, in the following content, the surplus and deficit are explained as positive and negative values respectively. I would suggest to give a definitive clarification in Abstract (line 22) and 38, such as: “The nutrient balance (surplus if positive or deficit if negative)” and eliminate the verbose and inconsistent explanations for surplus and deficit elsewhere.
• The paragraph between line 56 and 60 explains the componentsof nutrient inputs and outputs, and calculation of nutrient balance and nutrient use efficiency in this study. It would be better to  merge this paragraph to Method section, especially for the calculation of nutrient use efficiency, which is lacking in Method.
• In the paper, it is mentioned that N amountsin manure are estimated by multiplying live animal weights by N excretion coefficients (line 119-121), which represents the amount of N livestock produces (N excretion). However, according to the methodological note of “Manure applied to soils” domain under “Climate Change-Emissions-Farm gate” by FAOSTAT, it is the N in treated manure in manure management systems that is applied to soils. I assume the manure N applied to soil in this study is consistent to that from FAOSTAT, I would suggest to revise the description regarding manure N to avoid confusion and ensure consistency.
• The spatial patterns of nutrient budget and nutrient use efficiency are very interesting. As a reader, I expect to compare the spatial pattern of single nutrient input and output with the nutrient budget and nutrient use efficiency. I would suggest to incorporate these maps in supplementary.
• The authors point out that the N outputs are generally greater than those from other studies, resulting in a greater NUE in line 359-362.To help readers interpret the findings appropriately and assess potential uncertainties associated with using this data, I would suggest to explain the factors contributing to this difference.

Minor comments:

• The paper lacks reference information for Buckley et al. 2018 in Figure 1.
• Many of the reference are difficult to find, such as the reference of FAO. Please update valid reference for readers to access
• Line 187: typo “to cropland to cropland”
• Line 370: incomplete sentence.
• Table 6: The estimates given in the first row were by Zhang et al. (2023) but not from current study.  

Comments to the Author

This manuscript focuses on nutrient budgets and nutrient use efficiency, present a global database of country-level budget estimates for nitrogen (N), phosphorus (P) and potassium (K) in cropland. This study introduces improvements over previous work in relation to key nutrient coefficients affecting nutrient budgets and use efficiency. Results highlight the wide range in nutrient use and use efficiencies across geographic regions, nutrients, and time. However, before acceptance, several issues deserve attention, as outlined below.

General:

(1) The necessity and innovation of the article should be presented to the introduction.

(2) This study is not explicitly addressed in its exploration of the global farmland nutrient budget versus nutrient use efficiency, as many studies have been conducted in this area;

(3) In the process of calculating cropland nutrient budgets and nutrient use efficiency, many coefficients are used consistently, which may lead to great uncertainty;

(4) For the input of organic nitrogen, the CF value of organic fertilizers is the same as that of synthetic fertilizers, which may cause certain deviations in the results;

(5) Whether to consider adding a part, compared with other research methods, the necessity and innovation of this study.

(6) The discussion section lacks sufficient elaboration on key findings and the content appears too vague.

(7) Show more self-criticism towards your methods, discuss all limitations of your findings.

Specific:

• Line 50-55 “We see two main rationales for estimating nutrient budgets on cropland. First, cropland is typically where nutrient flows and related environmental impacts are highest, and cropland budgets and derived indicators such as the surplus are therefore more likely to capture potential pollution hotspots. Second, permanent meadows and pastures present some particular method challenges, primarily due to lack of global data on productivity and biological N fixation” Please add relevant references.

(9) Line 100-115 “For the majority of countries, due to lack of specific information, default cropland fraction estimates of 100% were used for N, P, and K, thereby assuming all fertilizers were applied on cropland area.” Is there evidence to support this hypothesis?

(10) Line 115-120 It is assumed the same CF values for SF are used to apportion nutrients from manure from livestock to cropland to cropland. For example, the proportion of animal manure returning to the field like horses seems to be very low, and the rationality of this parameter is doubted.

(11) Line175-180 Fraction of livestock manure applied to cropland, The uncertain of livestock manure should also be considered.

(12) Line360-365 N inputs from the current study were ‘mid-range’ compared with the other studies but N outputs were generally greater than those estimated from other studies, This result requires careful interpretation.

(13) Line 360-375 Compared with previous studies, only nitrogen analysis, lack of phosphorus, potassium analysis

(14) Line405-410 NUE values were generally greater than those made by other studies, this result requires careful interpretation.